Hi, everyone.
Welcome to the MIT Museum.
My name is Jennifer Novotny,
and I'm our public programs
coordinator here.
I'm really excited that you
guys chose to come and join us
tonight for our first of
three evenings in our Quantum
Quandaries and Other Heavy
Matters soapbox series.
Some of you may have attended
some of our other soapboxes
in the past.
Tonight will be a
little bit different,
but of course, you
can always expect
some interesting conversation
and a good opportunity
for you guys to engage
in the conversation.
We don't want this to be
just another passive talk.
We want you guys to
have the opportunity
to get your questions
asked, answered,
and hopefully to
dream up and come up
with some other great
questions as well.
But before we get
started, I wanted
to encourage you guys
to come and attend
some of our other
events at the MIT Museum
because we care about a wide
variety of topics, not just
quantum physics.
So hopefully, you'll consider
joining us for our Girls Day
coming up on this Saturday.
It's our math March
Madness, so we'll
be exploring
mathematics and sports
and trying to encourage a
future generation of scientists
and mathematicians.
We also have, for a
slightly older audience,
our holography evening
coming up in April,
where we're going to
be pulling holograms
from our collection that have
never been displayed before.
So you might consider joining
us for this adult evening.
We also have our
annual Nautical Night
where we celebrate our
nautical collection.
And of course, since the topic
of tonight's conversation
is getting into quantum
physics and some other physics
weirdness perhaps, I wanted to
encourage you to come and join
us for the premiere of Both/And,
a quantum physics play that
has been written in
collaboration with MIT
and the Central Square Theater
as part of the catalyst
collaborative.
So we'll be premiering
that here on Saturday,
April 15th, which is the first
Saturday of the Cambridge
Science Festival.
So hopefully, some of
these other options
are appealing to you,
and hopefully you'll
consider joining us for more
events at the MIT Museum.
But now, let's dive
on in, and I would
like to welcome our moderator
for the evening, Professor
David Kaiser
[APPLAUSE]
Any better?
That sounds better.
OK, thank you.
So thank you to
Jennifer for arranging
this and, indeed, a
whole series of events
I'm looking forward to.
I hope you'll
enjoy them as well.
I'm really delighted to start
our soapbox series this evening
with two really wonderful
speakers, members of the MIT
community.
I'm very delighted
that they're going
to talk with us about their own
work and what motivates them,
and then we get to
interrogate them shamelessly,
so think of all the hardest
questions you can think
of and put them on the spot.
Not really.
So, we've asked
each of our speakers
to come up and present,
informally, about aspects
of their work for, on
the order of, 15 minutes
or so to get a whole swirl
of ideas on the table for us.
And then, I'll moderate a
brief discussion with them,
and then it really will be
open to all of us to chime in.
So let me introduce
our speakers.
We have two of them.
The first speaker is
Professor Brad Skow.
Brad is the Laurance
Rockefeller associate professor
of philosophy here at MIT.
He completed his
undergraduate degree
in philosophy and English
at Oberlin College,
his master's degree
in philosophy
at the University
of Sydney, and then
his PhD in philosophy
at New York University.
He joined the MIT
faculty in 2007.
He's the author of
many, many articles
on the philosophy of physics and
metaphysics in all the leading
journals in the field.
He's also-- All, every
single one of them.
I counted.
He's also the
author of two books.
His first book was entitled
Objective Becoming,
published by Oxford
University Press in 2015.
His second book
was called Reasons
Why, published by Oxford
University Press in 2016, which
means I think by
the end of tonight,
he will have published his
third book if that rate is
to continue, which
is pretty cool.
His investigations have
focused pretty broadly.
He's published on the
interpretation of quantum
theory, on varieties of
scientific explanation,
and on the nature of time.
Our other speaker
is Paola Cappellaro.
Paola is the Esther and
Harold Edgerton associate
professor of nuclear science
and engineering here at MIT.
She's also a member of
MIT'S research laboratory
for electronics, where she leads
the quantum engineering group.
She completed her undergraduate
and masters degrees
in nuclear engineering in
Milan, a master's degree
in applied physics at the
Ecole Centrale in Paris,
and then her PhD in nuclear
science and engineering
here at MIT.
Following a postdoctoral
fellowship at Harvard,
she joined the MIT
faculty in 2009.
Her research articles on
quantum information processing
and quantum enabled
precision measurements
that she'll tell us about have
appeared in all the leading
journals on the
face of the planet,
including Nature Precedings, the
National Academy of Sciences,
and many more.
Her work has recently
been honored with a Young
Investigator Award
from the Air Force
Office of Scientific Research.
So I'm really delighted to
have Brad and Paola here.
I'm going to ask Brad
up first, and he'll
talk with us about
some of his ideas
about how a philosopher
looks at quantum theory,
and then we'll have Paola
talk about an engineering
perspective.
So, Brad.
[APPLAUSE]
All right.
Can you all hear me?
Is the microphone on?
No?
Jennifer, what should I do?
The green light is on.
I think you're actually good.
Oh!
All right.
Well, thank you, David.
I wanted to talk just
for a few minutes
about Einstein's contention
that quantum mechanics is
incomplete.
And this will connect
with Einstein's complaints
about spooky action at a
distance in quantum mechanics.
So Einstein thought that
quantum mechanical descriptions
of the world were incomplete.
What did he mean by that?
I mean, he meant if you
have some physical system,
like that chair or the
screen behind me, anything,
and you ask to be told
everything quantum
mechanics has to say about
that physical system,
Einstein's view was that
even after you'd written down
everything quantum mechanics
had to say about it,
there would be facts about
that physical system that
will have been left out.
That's what it would mean to
say that quantum mechanical
descriptions are incomplete.
And an analogy I keep in
mind to help me understand
what this view
amounts to is, think
about what we all believe, or
anyone who knows enough physics
believes, about thermodynamics.
So if you said, well
tell me everything
that thermodynamics has
to say about the state
of the air in this room, right?
The answer would be
something like, well, you
would say that the temperature
is, I don't know, 70.
It's kind of a little bit chilly
in here, 68 degrees Fahrenheit.
The pressure is what
atmosphere, the volume
of the gas of the
air in this room
is so and so many
cubic meters, but that
would be the end of the story.
That would be more or less
everything thermodynamics had
to say about the state
of the air in this room.
And clearly, that
description is incomplete.
It leaves out all sorts
of facts about the state
of the air in this room.
It doesn't say anything
about what kinds of molecules
make up the air, how many
air molecules there are,
where each one of them is,
how fast they're moving.
All of that information
is stuff that
the thermodynamic
description leaves out.
And Einstein's view was,
quantum mechanics is like that.
Even if you know everything
quantum mechanics has to say,
there's information
you're missing.
Now this is
connected to the fact
that, in quantum
mechanics, you can only
make chancy predictions for
the outcomes of experiments.
So I could have a box--
I didn't bring my props,
but imagine I have a box,
and I put a particle in the box,
an atom, silver atom, whatever.
Anything you like.
And I close the box.
And I say, I'm going to
look in the box in a minute.
What can you tell me about
where the particle will
be when I look?
OK, now, given what I've
said, you'll probably say,
well I can't really
say anything.
It's not very interesting.
What's more interesting is, even
if you know everything quantum
mechanics has to
say about the state
of that particle in that
box, even then you can only--
If things are set up right--
Even then, you can only
make chancy predictions
about where you'll
find the particle.
Like maybe quantum
mechanics will tell you,
well there's a 50% chance
you'll find it on the left,
and a 50% chance you'll
find it on the right.
That's the most you could say.
And Einstein's
attitude was, well,
the reason you'd only make
chancy predictions, even based
on everything quantum mechanics
has to say about the particle,
that's because there's stuff the
quantum mechanics doesn't say.
There's information
that's being left out.
If you knew all
that, maybe you could
say with more certainty where
you'll find the particle.
But the understanding
of quantum mechanics
that Einstein opposed,
the standard or orthodox
interpretation of
quantum mechanics,
it went beyond the
claim that you can only
make chancy predictions.
And to get a grip on the other
aspect to that interpretation
that I wanted to
say something about,
we need to distinguish
between two questions you
might ask about the particle.
One is the question
I just discussed.
Where will I find
it if I look for it?
Open the box and look.
There's another question.
Where is the particle
right now, before I look?
It's clearly a different
question that we tend to think,
or at least people who
haven't been exposed
to quantum mechanics
tend to think,
the questions are going
to have the same answer.
The answer to the question
of where the particle will
be when I look is going to
be the same as where is it
right before I look.
Because you think,
well, by looking
in the box, what
happens is the particle
reveals to me the location
it had right before I looked.
The orthodox interpretation
of quantum mechanics,
though, says that these
questions come apart.
It says, well you can make only
chancy predictions about where
you will find it, though it
does say that whenever you look,
you'll find it to be
somewhere in particular,
either on the left or the
right half of the box.
Or even more specifically,
like exactly where it'll be.
You'll find it
somewhere specific.
But according to the
orthodox interpretation,
before you look, at
least in the usual case,
there is just going to be no
fact of the matter as to where
the particle is.
I mean, maybe it'll be
definitely inside the box,
but nothing more
specific is true as
to where it is inside the box.
It's somehow
objectively indefinite,
or objectively indeterminate,
where in the box
the particle is.
It's not that you
don't know where it is.
There is nothing to
be known about where
it is beyond it's in
the box, according
to the standard interpretation.
So that, I think, was an aspect
of the orthodox interpretation
that Einstein opposed even more
strongly, the idea that it only
makes chancy predictions.
OK, so just to quickly
summarize these two
ways of thinking about
quantum mechanics,
both parties agree
that quantum mechanics
doesn't say anything definite
about where the particle is
before you look.
Einstein's idea was, well that's
just because it's leaving out--
It's got-- Quantum mechanical
descriptions are incomplete.
It's leaving out facts
about where the particle is.
The orthodox interpretation
says, no, to the extent--
If the-- If quantum mechanics
doesn't say anything definite
about where the
particle is, that's
because there is just no
definite fact of the matter
as to where the particle is.
All right.
The orthodox view
is really weird.
I think it's really weird.
What could it mean for it to
be objectively indefinite where
the particle is?
Or for there just to be no
fact of the matter as to where
the particle is.
The standard interpretation
of quantum mechanics
is asking you to think
there's something
defective about the question,
where is the particle?
You should somehow learn to
just stop asking that question.
But Einstein didn't just find
the orthodox interpretation
weird and complain about it.
He didn't just assert that
he thought it was wrong.
He gave an argument
that it was wrong.
He provided reasons
why he thought
everyone should agree
with him that the orthodox
interpretation is wrong.
So I want to say something
about Einstein's argument.
So we have to imagine a set up,
maybe a little bit like the one
I just had, where I was
talking about this box
with a particle in it.
And we need to imagine that
there are two boxes, each
with its own particle.
And the argument is going
to proceed from some claims
about the predictions
quantum mechanics makes
about what you'll find
if you look in the boxes.
And those predictions are
going to be predictions
that no one disputes that
quantum mechanics makes
these predictions.
Einstein's starting
point, these predictions--
That's something that he
and his opponents agreed on.
And there are two claims
about those predictions that
play a role in his argument.
The first claim is a claim
about what predictions
you can make from a complete--
From everything
quantum mechanics
has to say about the boxes.
If you look in a single box.
Well, you can set things up so
that if you look in one box,
say box number one, there's
a 50-50 chance you'll find
the particle on the right--
The right?
There's like stage right
and like audience right.
50-50 chance you'll find it
on the right, 50-50 chance
you'll find it on the left.
And the same goes for box two.
You look in box
two, you can only
make that chancy prediction.
And that's the first prediction.
The second prediction
quantum mechanics
makes, if you set
things up right,
is a prediction about
what would happen
if you look at both boxes
and compare the results.
Quantum mechanics says, if you
set the boxes up right again,
that if you look in both
boxes, you're guaranteed 100%.
With certainty, you can predict
that you'll find the particles
on opposite sides of the boxes.
If you find the particle
in box one on the left,
you're guaranteed to find
the particle in box two
on the right, and vice versa.
OK.
So that's where
Einstein starts from.
How does his argument from
there, from those premises,
proceed to the conclusion
that quantum mechanics
is incomplete?
Well first, we need to
separate these boxes,
move them very far apart.
So you imagine that we've
got one of the boxes
here, in the MIT
Museum, and we've
got to move box two
somewhere far away.
I mean, we could drive
it down to Harvard
and put it in some
Harvard Museum.
We could put it on a
train and take it over
to some museum in Los Angeles
or California, somewhere else.
Or even put it on
a rocket, send it
off to one of those
exoplanets that they just
discovered, 40 light years
away, really far away.
The claims about the
predictions quantum
mechanics makes about
what would happen
if you look in the
boxes, those still
hold even if you move
box two very far away.
OK.
And Einstein reasoned like this.
All right, now suppose
I look in box one,
and I find the particle
on the left hand side.
Now I know for certain that
if someone looks in box two,
they'll find the particle
in box two on the right.
And Einstein
concluded from that,
that even if no one
does look in box two,
the particle in box
two is on the right.
Even if no one looks,
even before anyone looks.
And that's a claim,
actually, that his opponents,
the believers in the orthodox
interpretation, will accept.
It'll say, well--
The orthodox interpretation
says that it's often the case
that there's just no fact
of the matter as to where
the particle is.
But, if we know with
certainty that you'll
find it on the right
if you look, then
you can say even before you
look, it's on the right.
So that-- That first
conclusion he reaches,
that's not one his
opponents disagree with.
It's the second one that they're
going to start complaining.
So Einstein says, well look.
After I look in box one, I
know the particle in box two
is on the right.
Furthermore, when I looked
in box one, my looking in box
one didn't in any way
influence or disturb box two.
It had no effect on what
was going on in box two.
Why is that?
Well, box two is 40
light years away.
Now maybe, doing
something with box one
can disturb what's going on in
box two, 40 light years away,
but that influence is going
to take a while to get there.
After all, it would take a
light 40 years to get there.
So at least in the short
term, at least immediately
after I look in
box one, I can know
that box two is in
the same state it
was before I looked in box one.
OK, so if box two is
in the same state right
after I look in box
one, as it was right
before I looked in box one,
and after I look in box one,
I know that the particle in
box two was on the right,
it follows that before
I looked in box one,
the particle in box
2 was on the right.
And there, that, if you
accept the reasoning that's
led to this
conclusion, Einstein's
got you where he wants you.
It follows that quantum
mechanical descriptions
are incomplete because
remember, the original quantum
mechanical description
of the boxes
as they were before anyone
looked in either one of them--
That was the
description that said,
there is just no fact of
the matter as to where
the particle in box two is.
And Einstein said, well maybe
that's what quantum mechanics
tells you, but that just means
quantum mechanics is leaving
out the fact that the
particle in box two
was on the right, which was
the conclusion it reached,
his argument.
So that was Einstein's
argument, that quantum mechanics
is incomplete.
It didn't convince anybody.
A couple of decades
after he published it,
the physicist John Bell
started thinking about it.
He got very focused
on this assumption
Einstein made that when I look
in box one, I don't in any way
change box two.
It's got-- It can be
called Einstein's--
The assumption of locality,
the locality assumption.
It's the assumption
that nothing I do here
can immediately,
instantaneously affect things
that are happening far away.
The only way for
something I do here
to affect things that
are happening far away
is for it to affect
things nearby
and for there to be a chain
of causes, each of which
is near its effect, and
for that to propagate out
to that distant thing,
and that process
is going to have
to take some time.
That was Einstein's
locality assumption.
Now if Einstein is right
that quantum mechanics is
incomplete, then there must
be some more complete theory.
Sort of, you could replace
quantum mechanics with
or supplement quantum
mechanics with.
And that more complete
theory, since Einstein
was assuming that
the world is local,
that more complete
theory would be
a theory in which this
assumption of locality
was true.
So that's sort of what
you would conclude
if you agreed with Einstein.
You'd think, my job is
to find that theory.
What Bell proved--
Bell proved a theorem.
He proved that given a couple of
seemingly innocent assumptions,
there cannot be a local theory
that reproduces the predictions
of quantum mechanics.
No local theory, given
certain assumptions,
can reproduce the predictions
of quantum mechanics.
That's Bell's Theorem,
and it's pretty.
Shocking It seems like Bell's
shown, if you think his--
if you accept his theorem
and accept his assumptions,
he's shown that what
Einstein called spooky action
at a distance, that what I
do right here in this room
can immediately and
instantaneously and directly
affect things that are happening
far away, 40 light years away,
arbitrarily far away--
That's true.
That is the way the world works.
So if Bell's right, Einstein was
wrong to assume that locality
is true when he was arguing
that quantum mechanics is
incomplete.
And moreover, the standard
way of understanding quantum
mechanics seems to accept
this idea that there
is spooky action at a distance.
Like the orthodox
interpretation seems to say,
when I look in box one,
before I look in box one,
there's just no
fact of the matter
as to where the particle is.
But the instant I
look in box one,
immediately the
particle in box two
acquires a definite position.
So, I don't have much time left.
In fact, I'm probably over
time, but let me just--
I'm just going to conclude with
a couple of quick thoughts.
One thing I think
is important is
to distinguish between
Einstein's conclusion, quantum
mechanics is incomplete,
and his argument
for that conclusion,
which relied
on the locality assumption.
Bell may have shown that
Einstein's argument was bad,
but that does not mean that
he showed that Einstein's
conclusion was false.
You can have bad arguments
for true conclusions.
And in fact, people have
developed alternatives
to quantum mechanics or
interpretations of quantum
mechanics, they're
usually called,
which do attribute
definite positions
to every particle at all times.
So those interpretations
of quantum mechanics--
If you think they're right,
you will agree with Einstein.
Quantum mechanics as usually
understood is incomplete.
What's interesting
about these alternatives
to quantum mechanics, or other
interpretations of quantum
mechanics, is they make
the same predictions
that quantum mechanics
makes about the outcomes
on any experiments we've
done or are likely to do.
And this is a puzzling
situation to be in.
What should you do when you
have many theories which
make all the same predictions?
What should you believe?
Which one of them
should you believe?
Should you think, well
I should be agnostic?
I should think they're all
equally likely to be true.
I shouldn't believe
any one of them
more strongly than I believe
in any of the others.
You might have that attitude.
Or you might think, well
even though they all
make the same predictions,
somehow one of them
is more worthy of our
belief than the others.
There was some
principle by which
you can select one theory
as more worthy of belief
than another, even if they
make the same predictions.
You might have that attitude.
Now sociologically,
what's interesting--
I mean, I don't know much
about the sociology here,
but the impression I get
is that, the community
of physicists
continue to believe
the orthodox interpretation
in the face of the fact
that there are other
interpretations of quantum
mechanics to make
the same predictions.
An interesting question
about that fact
is, well does that
mean there really
is some principle for
choosing theories that
are equivalent in the sense that
they make the same predictions?
Which would say
you should believe
the orthodox interpretation
under those circumstances
and that what the
physicists are doing
is thereby the rational,
or reasonable, thing to do.
Or is the fact that the
community of physicists
prefers the orthodox
interpretation
of these alternatives
just a sign that they're
prejudiced in some way.
All right, I'll leave
that question hanging
as the end of my comments.
Thank you.
[APPLAUSE]
You see immediately
the difference
between the engineer
and the philosopher.
I come with slides.
[LAUGHTER]
OK.
Very well.
Yeah, so what I'd like
to do is to briefly--
I think is almost too--
That the volume too high.
I don't know.
I just want to briefly introduce
a little bit some [INAUDIBLE]
about quantum technology
more generally,
and I will speak mostly
about quantum computers.
And my perspective is,
as you can imagine,
slightly different from Brad.
And it starts
somehow like these.
Just got a notification.
But even if what you want to
do is manipulate information,
so this is what a computer,
either classical or quantum,
does.
What it does is to
take some information,
manipulate it, and give you
some other answer, which
is still information.
So, information is typically
a very abstract concept,
but for an engineer,
what you need
is really to encode
with information
in some physical system.
Otherwise, how can you do a
manipulation the information
itself.
And so it becomes really an
engineer or a physical problem.
And so, typically what
has been done until now
is to encode with information
into a system which
would behave classically.
But really, quantum physics
is a more fundamental theory
than classical
mechanics, if you want.
And so the idea is to
say well, can we actually
try to exploit this habit?
Physics is, after all, quantum.
And trying to use the
quantum weirdness that Brad
has mention to our advantage.
And as the size of the
components of a computer become
smaller and smaller,
you would have a way
to take into account
quantum effects,
and so why don't we turn
them to our advantage?
And really, all
these weird things
about quantum
mechanics that you have
to try to understand, or at
least wrap your mind around,
actually in the end
maybe are the things
which makes quantum
computer really powerful.
And so even if the person
somehow, if you want,
had headache to deal with when
you try to really understand
them deeply from an
engineering point of view,
is actually what is
you're trying to exploit.
So let's see some of these
weirdness of quantum mechanics.
So the first thing
that you can try to do
is really a very,
very simple experiment
which will show how when you
are trying to measure something
in quantum mechanics, the
outcome, or the measurement,
is going to be some random
number, some random outcome.
So for example, what we can do
is a very simple experiment.
OK, so we used just
a beam splitter
which takes an initial beam--
I'm in the nuclear
engineering department,
so I will consider a
source of single neutrons.
This could be, as well, done for
example with photons of light.
And so I sent an initial beam
of neutrons, which come one
by one, onto beam splitter.
A beam splitter is an
apparatus which can either
let the neutron go
in the same direction
or change the
direction by 90 degree.
OK.
And it does so in a way
which is somehow random.
So for the moment,
let's say that this
is done in a random way.
So if I have two detectors,
I have a 50% probability
of seeing my neutron arriving
here, or instead, arriving
at this other detector here.
OK.
So if I have a quantum
object, a single neutron,
and I try to measure where
it's position, as Brad say,
this question can only be
answered by a random outcome
or the measurement itself.
So this is something which is
weird in quantum mechanics,
You measure something,
and it's a random outcome.
But is it really fundamentally
weird as I presented it?
Doesn't that also happen
in classical mechanics?
So I have a very,
very silly experiment.
Let's say that I have--
for the classical
[INAUDIBLE], let's say
that I have an explorer.
OK, so Moana here.
OK.
And she decide to sail into the
sea to see if she can find--
I don't even know exactly what
it was, some kind of stone
or something.
OK.
And so she has to sort
of leave her island
and decide if to
go west or east.
To do that, she flip a
coin to decided that action
because she doesn't
really know where to go.
And so she can go
either west or east.
And in one case, she just
lost at sea, 50% probability.
In the other case, she
actually arrive at the island
where she wants to arrive.
So randomness is not just
intrinsic to quantum mechanics,
but even in classical mechanics
and classical physics,
you also can have randomness.
So OK, random outcome of
measurement, random outcome
of measuring the
coin side are also
present in classical mechanics.
So what is really weird and
different in quantum mechanics?
The point is that,
in this travel,
we will find our explorer
either in this direction, going
let's say east, or in this
other direction going west.
OK.
At every instant in time, she's
in a well defined position.
So if I keep measuring,
I will always find her
in some of the two positions.
OK?
I can always define very
well where the position is.
However, if I consider
my neutron again,
when I say that this beam
splitter, which is like my coin
flip, decide which direction
the neutron will go,
it is different.
I'm not saying but the neutron
is either here or here,
but it's actually, at the
same time, both here or here
OK, so these are some
quantum notations.
We like these weird bracket,
which again, they're
just indicating that either on
this path, I call it zero path,
or I'm on this other path,
which I call it the one path.
And in order to get, in the
end, this 50% probability
here or here, I need to have
one over the square root of 50,
or 0.5.
So one over square root of two.
And then, of course, I can
try to find even more path
after that.
And if I have these
different position here,
I can still have a state
in which my neutron is,
at the same time, in each one
of these possible positions.
It's not either here or
here or here or here.
At the same time, it's in any
of these possible position here.
OK, so this is like a
fundamental difference
between classical behavior
and quantum behavior.
Just as an aside, since
possibly many of you
have never seen a
neutron beam splitter,
OK, this is what it looks like.
It's actually a lingot of
silicone with very sharply end,
like a fabricated planes.
And the crystal
planes themselves
decide the direction
of the neutron
based on the scattering.
It can indeed have
more blades here, so
that you will have more path.
It becomes more and
more difficult to have
larger crystals for neutrons,
so this [INAUDIBLE].
But for example, for photos,
it becomes much easier
to have many more path.
Typically, you use mirrors.
In this case actually, what we
used was a crystal of diamond,
and you have internal reflection
in the diamond crystal itself,
which creates all these path.
It's a bit dim, so I'm
not sure you can see,
but you can see a lot of
crossing, crisscrossing here
in the crystal.
So why do we want to
have many, many states,
many, many superpositions?
Again, what we want to do is to,
in the end, encode information
in some physical system.
So what we can do
is, for example,
to try to encode some
information, either in 0 1
which is a classical bit.
For example, a circuit
which is open or closed,
and we assign open to
zero and close to 1.
Or we can use quantum
bits again 0 1,
just with this weird bracket to
indicate [INAUDIBLE] quantum.
And for example, there could
be a true level physical system
which can be
pointing up or down.
For example, my path going up,
and my path going horizontal,
so these are two
possible states.
The main point is that
for a classical bit,
my circuit can either
be closed or open,
but it's not going to be close
and open at the same time.
While for a quantum
bit, they can actually
be in the so-called
superposition
state, in which you
are really both 0
and 1 at exactly the same time.
OK, then if you
measure something,
I will not get out the
superposition here.
I cannot really measure that.
What I will get out
is either 0 or 1,
with a probability,
which is given
by the square of this
coefficient here.
So, probability 0
will be a squared.
What do you think is the
probability of being in one?
Perfect.
So, it's the square,
which is supposed
to be one minus a squared.
Very good.
So this is for one
bit, or one qubit.
And I have two parameters which
indicate what is the state.
Until now, not
much has happened.
But what I can do
is then to look
when I have more
than one quantum
beat, or so-called qubit.
If I have one, as I said,
I need two parameters
to determine what is
the state, a and b.
But if I have two now,
that can be four possible,
different state
with 00, 01, 10, 11.
Also, classical bit can
have this combination
but can only be in one of
its four state at a time.
Instead now, what I can have
is also a superposition state.
And some of these combinations
correspond to these entangled
state that we already
discussed, which
really also identify
correlations
among the two possible qubit.
And so here, I need
four parameters
to describe the state.
Now let's say that I consider
three qubits, so three
of these quantum,
two-level objects.
How many parameters, how
many possible states,
do you feel that I can have?
Eight!
OK.
So I can have 000, 001, so
there are all the possible
combinations.
So it's two to the cube,
which is eight possible state,
I need eight parameters.
And now what I have is
only three physical system,
but I can encode eight
bit of information.
So you start seeing how quantum
systems are much more powerful
than classical systems.
So I need three system to
encode eight bit of information,
but I would need eight classical
system to encode the same way.
And of course, it keeps going.
So if I have 10, I have
one kilobit of information,
using my 10 qubits.
If I have 20, I have one
megabit of information.
It's megabit, not megabyte,
so it's a bit less.
You can see.
And if I have 40, I have
one terabit of information.
OK, so which means that I can
encode a lot of information,
and I can process
that information
onto this much larger
possible number
of states at the same time.
So I'm really having
a lot of advantage
because this allows me to
really do a lot of computation
in parallel.
OK, so I have all these
amount of possible states,
and each of them can
make a computation.
Instead, if I have a
classical computer,
it will have to make
that computation
one after the other,
so I would really
need two to the 40
different classical system
in order to do the same
computation in the same time.
So I have an explanation, if you
want speed up, in my resources.
However, we haven't looked at
all possible quantum weirdness.
So there are some
additional quantum weirdness
which sort of are either--
sort of destroy
this dream of doing
this competition in parallel,
but then bring it back.
So what is the first one?
So let's say that I have
a classical computer.
How does the computation work?
I have a string of
inputs, and then I
will have a string of outputs.
So these are bits, 0 and 1.
OK.
If I input some bits, and
then I will output some bits.
What about a quantum computer?
Even in quantum computer, I
will input some classical bits
because we are human.
We think in terms
of classical things,
and so I will still have some
classical bits as my input.
But then the calculation
inside this quantum computer
is actually done
on these quantum
bits, which are called qubits.
And so it will be done,
not just on the bits,
but also superpositions
of my quantum bits.
So a superposition
like this one,
which is one of
these Bell state that
has been discussed before,
or even for a larger system,
three qubits, four qubits,
and so on and so forth.
And so this computation itself
can be done much, much faster
than the classical one because
of its parellelism we just
saw in the slides before.
OK?
The problem is that when
I try to measure things,
I cannot really measure
the superposition,
and so I cannot also
measure these other one.
If I measure something, I will
get out either 011 or 110.
I cannot know both of
them at the same time.
This is the so-called state
of wave function collapse
in quantum mechanics.
So now, the weirdness
of quantum mechanics.
So out of the
computation, I still
get a string of bits,
of classical bits.
So even if I did a
lot of computation
in parallel, in
the end, I can only
get one result, one of these
possible results or computation
which I've done.
And so, while it seems that I'm
losing all the power of quantum
computers here--
I can do a lot of
computation in parallel,
but I get only one answer out.
So it seems that there
is really no increase
in the number of outputs.
So what can I do to try
to solve this problem?
Let's go back to my example
of the explorer, OK?
So I had my explorer,
and she flipped the coin,
decide to go either north
or south or east or west.
But after a day of travel,
she was still in the sea
with no island in her sight.
So she decide to flip
the coin again and try
to decide for a new
direction and hope
that this will actually
bring her to either island.
So she flips the coin
again and based on that,
she decide to go more
north, or flip back
on south, or the opposite.
But then again, she can only
arrive in these two places.
And again, the probability of
arriving at the island is 50%,
and the probability of her
being lost at sea is still 50%.
So nothing much has changed.
The probability are still
more or less the same.
What about my quantum
system, a quantum explorer,
which I took to be a neutron?
OK, so I can replace
the detectors
here with some
mirrors so that I can
increase the number of path.
And then add the second
beam splitter, which
is another flip of the coin.
This will create
more possible path
like in the same
way as the explorer,
so to bring me back into a
possible previous direction.
But there is a difference.
Here again, my
possible states are not
multiplied, but
the probabilities
of being in either
one or 0, but they're
multiplied by the square
root of a probability.
And in quantum
mechanics, that number
can even be a complex number.
Or more simply, for
example, it can have
a minus sign in front of it.
So we can arrange how this
operation, the beam splitter
and the mirror, behave
on our neutron so
that when they're
recombined together,
actually I only have
100% probability
to arrive at one detector
and 0% probability
to arrive in the other one.
But doesn't happen all the time.
I need to set up my experiment,
or if you want my quantum
computer in a particular way--
OK, so I should be able to
program my quantum computer
in a way so that the
final answer which I get
is actually the
one which I want.
So in some sense,
quantum mechanics
allow you to always arrive
to the happy ending,
to your island.
OK.
So if you're able to program
your quantum computer,
to program your
interferometer, you're
always able to arrive at a final
correct answer, the island.
So really, there
are two ingredients
which give the power
of quantum computers.
It's not simply
superposition, which
gave you this massive
parallelism which
allow you to compute
things much, much faster.
But you also need
this other ingredient,
which is interference between
different states, which
allow you to select the
correct answer as the one
that you will read out when
you do a measurement which will
collapse your state into one
of the possible superposition
paths.
So these two together really
give this exponential speedup
of the computation.
So, as I said, we can do
things exponentially faster
with quantum
computation, but you
need to be able to program
your computer in such a way you
do get out the correct answer.
So not all possible
algorithm are
allowed to be programmed
in a quantum computer
so that you do get this
exponential speedup.
So only some of them
are amenable to this,
particularly if you
want a nice program
you get to the correct answer.
But when you do, you do get
a very, very large advantage.
So when does this advantage
could be beneficial,
for what type of application?
So for example, one of
these is database search.
So you're looking
into your phone book--
I mean, you don't do
it anymore, but this
was a classical example
when quantum computer were
first proposed.
You look into your phone
book, and if you're
a classical computer, and you
had to search for one person,
you would just start
alphabetically and just
look to all possible entries.
A quantum computer can do
that much, much faster.
So this kind of
application, for example,
in bio-informatics or
even like a Google search.
Another possible algorithm
is the factorization
in prime numbers,
which sounds just
like a mathematical
application, but actually
is at the base for a
lot of cryptography.
And so being able to
factorize large numbers
could break current
cryptographic codes.
So it's another very
important application.
You might be able to do
some physical modeling.
For example, reduce a
very complex problem
like climate economy,
engineering problem,
and put them in the form
a quantum computer might
be able to solve, particularly
with some form of [INAUDIBLE].
You could be able to simulate
other quantum systems.
Quantum systems are very
complex to be simulated.
For the same reason
that you need
this large amount of information
to encode the quantum system,
it also means that if you try
to use a classical computer
to simulate the
quantum system, you
will need an exponential
amount of resources,
an exponential number of
computers, if you want.
So you can use a
quantum computer
to simulate a quantum system,
and it will be much faster.
Now this can have application
in chemistry, and for example,
material science, and
so on and so forth.
There are also other application
for secure communication
in precision measurement.
In particular, I'm working
on some of these precision
measurement.
So why don't we have quantum
computers everywhere?
OK, so we sort of
have understood,
as physicists and engineer,
how quantum computer can
give this exponential speed up
and these kinds of benefits.
But what we want to do is also
to have a physical quantum
computer based on some
particular physical system,
be it neutron, photons, atoms,
spins, et cetera et cetera.
The problem is
that quantum states
are extremely sensitive to
any outside perturbations,
to noise.
And so their quantum
property tends
to disappear as soon
as they interact
with any external perturbation.
This is sort of why we don't
see the so-called Schrodinger's
cat, the cat which is at the
same time alive and dead, which
was a famous thought
experiment of Schrodinger.
Because the larger
the quantum system,
the faster this decay of
its quantum properties
into just classical
property happen.
And so, to build a
quantum computer,
we want to build and
control and preserve
the quantumness of larger
and larger quantum systems,
and this become more
and more challenging.
So this is difficult,
but at the same time,
the fact that quantum
system extremely
sensitive to any
external perturbation
led us to think about
the thought of using them
as quantum sensor.
OK, so if you are very
good at picking up
any random disturbance, well
let's use them as sensor.
Maybe we also get an advantage.
And indeed, we have
an announcement
in the signal to noise ratio
that you can have of a sensor,
or the order of a
square root of n.
So if you have n
classical sensors,
your signal to noise
ratio typically improve
at square root of n.
But for n quantum sensor,
your SNR will improve as n,
so you have quite an advantage.
So one thing that I'm
actually working on--
And I'm also going over time,
so I will conclude rapidly--
It's actually to try to
build a magnetometer,
so a magnetic field
sensor based on spine.
So spin is an intrinsic
quantum property
that all fundamental
particle have.
And in particular,
what we are using
is the spin of an electron.
So in the same way
that a compass needle
will align with
the magnetic field
and will move if you move
a magnetic field around it.
Also, the direction
of an electron spin
will also move responding to
an external magnetic field.
And so we can
detect this movement
and use the electronic spin
as a sensor for the presence
of magnetic fields.
And actually something similar
is already used in a technique
that you might be familiar.
So somebody here
has done an MRI.
You've been using the same
property of spin, in this case,
typically a proton spins.
OK, so all the spins
which are basically
in water, which compose
a lot of our body.
So what we want to do is really
to take this technique, MRI,
magnetic resonance imaging,
which, if you want,
is a technique which is
based on quantum mechanics
because spins are quantum
mechanical properties.
But I would call
it Quantum 1.0 OK,
so the first version of quantum,
like for example, laser.
Based on quantum mechanics, but
again, now we are used to it.
And we are not exploiting
the most weird fact
about quantum mechanics.
What we want to do
is really to exploit
even more weird property
of quantum mechanics
and build a Quantum 2.0 version
where we start with detectors,
in which you are able to
really look at proteins not
as a large ensemble
of a million proteins,
but look at single proteins and
try to identify atom by atom
what is their structure.
And also, instead of
looking at the brain
and looking at just a millimeter
size feature in the brain,
we want to be able to
look at individual neuron,
again using these novel
quantum sensor based
on the electronic spin.
The electronic spin
that I'm talking about
is actually a defect, which
you can find in diamond.
And it's a defect which gives
a pink color to diamond.
So if I had this giant, I
would be very, very happy.
Typically, we don't use
this, as you can imagine.
We just use like small, diamond
chip, synthetically grown.
But the [INAUDIBLE] gives
color, so it emits light,
allows us to really see
individual electronic spin
inside the diamond itself.
And we use this fact
to be able to look
at a single, electronic spin and
its connection, its interaction
with external magnetic field.
So one of the thing
that we would like to do
is to use our electronic
spin to look at other spins,
like water, in a protein
to identify the structure
of the protein itself.
OK.
I'll leave you with
more of the things
that we do in my quantum
engineering group,
and I will stop here.
[APPLAUSE]
You want to bring
back your computer?
[NO AUDIO]
This might work better.
OK.
So thank you, both
to Brad and to Paola.
That was a fantastic tour.
We covered an enormous range of
ideas and topics and devices.
I want to make sure we have time
for questions from all of you.
I have a bunch of questions
that I can talk with them about,
basically all night.
Which I might subject them to.
But I want to make sure
you've a chance for you
to ask some questions as well.
So if anyone has
a burning question
they want to launch
into, please feel free.
Jennifer might try to come
to you with a microphone,
or I can just
repeat your question
to make sure
everyone can hear it.
So does anyone want to
jump right in before I
start interrogating them?
Yeah, Michelle?
[INAUDIBLE]
Do you want me to just
repeat the question?
Yeah.
So, you know, very
briefly, Michelle
was asking Brad to describe
his interest in time
and the philosophy
of time, and how
that intersects with other
philosophical interests,
either about quantum
theory or more broadly.
That's what your first
book was all about,
trying to understand time.
Yeah, I really like
that my first--
Like to be able to say
that, my first book.
You've got two out.
You've got to
count them somehow.
So, yeah, I could talk
about that for a long time.
I guess I'll try
to keep it brief.
One of the questions
philosophers
ask, believe it or
not, is there really
such a thing as the
passing of time?
Is passage of time
a real phenomenon?
Sounds like a dumb question,
like the answer was obviously
yes.
Unless you're some like crazy
Buddhist who thinks somehow,
it's all an illusion.
Part of the reason
philosophers ask
that question is if you
like, look seriously
at what physics says
about time, especially
theory of relativity, not so
much quantum mechanics, theory
of relativity makes
it look like well,
like from a god's eye
perspective, the past, present,
and future are all
laid out already.
And there really isn't
anything that would correspond
to the passage of time.
In fact, part of this connects
up with the idea of well,
if there is such a thing
as the passage of time,
then there must be such a thing
as the way the world is now.
So there must be a fact
of the matter of what's
happening on the moon now.
But special
relativity is supposed
to teach us that there really is
no objective facts about what's
happening on the moon right now,
and this notion of simultaneity
is, that's the relative
notion, and so, philosophers
have taken that to put
pressure on the idea
that there really is such a
thing as the passage of time.
The passage of time [INAUDIBLE]
requires some notion
of absolute simultaneity.
So part of my work
in the philosophy
is to try to navigate
through those issues.
I'm on the side of the people
who think, as they say,
there's no such thing
as the passage of time,
misleading as it may be to say
in a non-philosophical context,
and try to like explain away why
it might appear that there is.
That's sort of
what I write about.
I'm going to take
chair's prerogative
and build on that for a second.
Actually, a question
for each of you.
Now this is really on.
And I wonder if you
could think back
when you first began
encountering some
of these weirdnesses,
either about relativity
and the passage of time, or
about the quantum phenomena
that we heard about.
Can you remember what it
was like when you first
began kind of tripping on these
ideas that seem not to follow
our ordinary intuitions,
at least they don't
follow my ordinary intuitions.
And what was it?
Was it exciting?
Was it terrifying?
Was it already kind of
banal because everyone
seemed to know that?
Can you get back to your former
self, speaking about passage
of time, when you when
you began encountering
some of these
delightful weirdnesses,
quantum mechanical,
relativistic, or otherwise?
How did that--
Did that put you on a
path that led you to here,
at MIT, and the MIT Museum?
Was that not so
surprising in hindsight?
What do you think?
Maybe I'll start with Paola
and then come back to Brad.
I mean, I think
that when I first
started quantum mechanics,
it was definitely weird.
So I feel that now, I
have this acceptance
of quantum mechanics.
So I'm not sure if I
understand completely,
but it's like OK,
I use it everyday.
And so there are things
which are automatic.
So, the idea that you
have a superposition,
yeah it's fine,
because you can always
measure on some other basis, and
then your result is definite.
So, there is some like
concrete meaning for me,
what a superposition is.
But when I started thinking
about it, it was like OK,
so you might be here and
there at the same time.
What does that really means?
It doesn't really
correspond to anything
that you can experience
directly with your senses,
and so it's really puzzling.
At the same time,
because it's puzzling,
then you're like oh OK, I really
want to know more about it.
And I think that's
what sort of started
me to wonder OK, this is
really something that I
want to learn more about.
So you're kind of where it
sounds like where you can sleep
at night, which is good, right?
We want that for basic health,
but there's still a kind of,
it sounds like Paola,
there's still--
When will you kind of
pause and come up for air
in between grant writing
cycles and teaching classes,
all the things keep us busy.
But there's still a
basic weirdness there.
The weirdness hasn't
bled out, right?
Although, it might
have become more--
You're used to it maybe.
I still teach like the
fourth undergrad class
in quantum mechanics for
nuclear science and engineer,
for nuclear engineer, so
it's still like every year,
brings me back to try to
put myself in somebody
else shoes who see this.
Somehow, for the first
time, people sometime
come with already preconception.
So I try sort of demystifying
some preconception
and try to make it at least
a bit accessible while
at the same time
keeping the mystery.
Because I think that
having some of the mystery
is really what pushes you
to keep doing experiment,
keep understanding better,
and so on so forth.
Brad, can you
remember a time when
it seemed new and weird,
instead of just weird?
I don't know.
I think actually, I
understand the weirdness of it
more, as time goes
on, than I did
when I first encountered it.
I think-- As I've been trying
to think of what I might say,
there's maybe two
aspects to that.
First is, maybe I first
got exposed to some
of the strangeness of quantum
mechanics early in life,
like in high school.
You think chemistry, and you
learn about electron orbitals,
and they tell you
we're not supposed
to think that the electrons
like moving around
the nucleus in the same way that
the earth moves around the sun.
You're supposed to
somehow think of it
as like, spread out through the
orbital or something like that.
And when you're young
and impressionable
and your teacher says that,
you're sort of like OK.
You think well, that's some
deep thought that I don't fully
grasp, but it can't be that
bizarre because everyone
seems--
All the grownups
seem to believe it.
And then also, I was an English
major in college in the '90s.
I don't know what's going on in
English now, but at the time,
It was like literary critics
and literary theorists
were full of crazy,
metaphysical ideas
like oh, there's no
objective reality.
It's all constructed by our
readings of these texts.
That was another thing
that was delivered down
to me by the authorities
at an impressionable age.
And it was only after
I became a philosopher
and had been studying philosophy
for a while that I said,
these are crazy ideas.
What are these people
even talking about?
What could this mean?
I mean, I guess I find it
weirder now than [INAUDIBLE].
And like Schrodinger,
in his paper,
he's really good about this.
He says, oh well, maybe we can
make our peace with the idea
that an electron is somehow
smeared out through space, even
though whenever--
Maybe this isn't
so true for that,
and whenever you look for them,
there they're point sized.
But somehow, when
you're not looking,
they'd manage to
just be smeared out.
But what would it mean
to think about a cat,
that it could be
somehow smeared out
between being alive and dead,
like it's just a porridge,
it doesn't make any sense.
But nevertheless,
like the standard way
of thinking about
quantum mechanics says,
that's the way the
world is sometimes.
How could that be?
What does that even mean?
I don't know.
And people are
pushing the boundary
of what is between like
an electron and the cat
I mean, nowadays, people
can connect superposition
of molecules which contain
like hundred atoms of carbon,
for example like
[INAUDIBLE] and even larger.
And people are even
thinking of sort
of creating the superposition
of viruses, which
can survive vacuum, just
because it's easier to have like
isolate the system in vacuum.
So maybe we will have, one
day, like idea of superposition
of a living object.
I mean, to me, I can easily
accept my superposition
on my electron spin, but like
a superposition of a living
object.
I'd probably reviewing, still
being tough to really grasp.
Thank you.
Other questions from any of you?
Yeah, please, right in front.
How mature is the theory
of quantum mechanics?
And what should we expect to
see breakthroughs or changes
or kind of where
does that stand?
Great question.
How mature is the theory
of quantum mechanics?
Are we in for more surprises,
I think is a fair summary.
This might be
something that David
is most qualified to answer,
rather than either of us.
I'm supposed to interrogate you.
You read the rules
beforehand, Brad.
You know that, right?
No, I have a few
thoughts on that, too.
But, either Brad or
Paola want to start?
There is a lot which
is known, and if you
have time to build
a quantum computer,
probably a lot which would
go into quantum computer
is settled.
But then, there is I think
still a lot of things which
are puzzling, like something
from measurements for example,
and so I think that hopefully,
we will have more breakthrough.
Maybe this-- Maybe
here's my two cents.
If you mean how mature is
it, like how well-tested
are its predictions, it's
the most well-tested, highly
[INAUDIBLE] scientific
theory we've ever had.
It's prediction's
been checked and found
to be correct to far
more decimal places
than any other theory we've ever
had in the history of science.
So in that sense, you might
think it's quite mature.
But if you mean
how mature is it,
how well do we understand
what it's saying about what
the world is like, then I think
the theory is in its infancy.
There are dozens, or
at least a handful,
of interpretations
of what the theory is
saying about what the
world is like, which
are radically different.
Completely different.
If I told you what one of them
said and told you another,
you'd say there's no
way those theories have
anything to do with each other.
And in a sense, that's true.
The only thing they have
to do with each other
is they make the predictions
quantum mechanics makes,
but they disagree
radically about what
the world is really like.
So in that sense--
And if you say, well is there
any like rising consensus
by the experts as to which
of these ways of thinking
about the world is
the right one to have,
if quantum mechanics
is the right theory,
like there's no consensus.
I'll just add briefly, I agree
with each of these comments.
And so it is remarkable that
every single experimental test
has been consistent
with predictions
of the equations of quantum
mechanics, some of them
to 12, 13, even
14 decimal places.
It really is just--
To me, that's mind boggling.
Far more precise than tests
of any other scientific theory
that I can think of.
Even when people design
increasingly intricate kind
of baroque tests
to say, oh did we
overlook some weird
kind of thing over here?
Well wait, actually even
that checks out as well.
I've been involved with
some colleagues and some
of these kinds of tests of
entanglement, of the ideas
that Brad described
in the beginning.
But nonetheless, I think there's
another way in which the theory
might be, if not in
its infancy, then
at least maybe not
a finished product.
And that comes from more
theoretical motivations.
We've never caught it
wrong, empirically,
and now people like
Paola and her colleagues
can build on its strengths to
make new stuff in the world,
with pretty impressive
tests as well.
But we also don't have
any way to fit that
together in a really
kind of first principles
way with the other
great, beautiful edifice
of modern physics, relativity.
So relativity,
Einstein's basic theory
of the behavior of space
and time that gives
rise to things of
phenomena like gravitation,
that's also been tested a
gajillion different ways
and passed every single
test, even the ingenious
and Baroque ones.
And yet, conceptually, we
just have no particular way
to fit those puzzle
pieces together.
We have some ideas that haven't
quite been completed yet,
and they might also just
be complete dead ends.
So does that mean quantum
mechanics ultimately
has to give way
to something that
looks more like relativity?
Is it the opposite?
Is it some third way that
people haven't dreamed up yet?
So even while we continue
to test and build
on quantum mechanics
with kind of hair
raising success on
a range of scales,
there's still some kind of--
I don't want to call
it a ticking time bomb,
but there's still some
big challenge out there
that excites a number
of my colleagues
to say, can we actually
answer one of Einstein's
own great dreams and
have a theory of nature,
a self-consistent script
of nature that could help
us telescope not just from
the electrons and the viruses
and Paola's quantum
sensors, but also up
beyond the big bang
and black holes
and everything between galaxies.
And that really is elusive .
And whether that means
we have surprises coming
from quantum mechanics or
relativity or something else,
that I think, no.
We really don't know yet.
Another area to look at kind
of fissures that might open up.
Other questions?
Yeah, right up front.
What's limiting the development
of quantum computers now?
Is it the algorithm?
Or what is it that
limits the development?
What's the limiting factor for
quantum computers these days?
What the basic limitation,
most important limitation?
So I think that in
some sense, he already
eluded at the true limits.
On one side, we have a finite
number of quantum algorithms
which allow these sort of
programming things correctly
so that you get the correct
answer, which I alluded to.
So like database
searching, factorization.
People are coming up
with more of them,
but it still is not
covering everything.
So that's one limitation, if
you want, which sort of what
I call quantum computer science.
And then there is
like the software
and then there is like
the hardware part.
So we want to have
a physical system
to really run this
quantum algorithm.
And even there, like the
limitation is the fact
that we need to build,
control a large quantum system
so that we are really able to
run this large computation.
So for that, the
limitation is really
this interaction between
your quantum system
and the external environment.
Any possible
perturbation, be it like
stray electromagnetic field,
temperature variations,
like even a dust particle, et
cetera, which makes immediately
collapse you're quantum system.
And there are ways of trying
to address this, which involve
trying to do error correction.
So in the same way we
think classical computer,
you try to correct
possible errors which
occur during the
computation, you
can do it also in
quantum computers.
But if I sort of demand to
increase the number of qubits,
so if I want to have a
computation done on one qubit,
I would actually need
five physical qubits
to be able to correct
for any possible error.
Or maybe seven, if you
want to be more accurate,
and so on and so forth.
So then, you sort of like
trying to catch something
like a moving target.
You want to build
a larger computer,
but then it becomes
more and more fragile,
and you need even more qubits to
try to make it robust to errors
and so on and so forth.
At the same time, like the
technological progress,
I think especially in
these past few years,
is really sort of
improving quite a bit.
And so there is the
hope that maybe we
will not have a computer, a
quantum computer, which can
do any possible computation.
But if you want to do
like one particular task.
Let's say you want to simulate
like a chemical reaction,
you might be able to do that.
And this is I think quite
close on the horizon,
so like specific task computers.
Good.
Other questions?
Yeah, thank you.
Right up front.
Thanks.
Yeah, I'm curious with
one of you an engineer
and one a philosopher,
how much your fields
are able to talk to
one another or are
interested in talking
to one another
about quantum mechanics?
That's a great question.
They're eyeing each
other suspiciously.
Maybe that's your
answer right there.
Who's going to go first?
Arm wrestle.
I mean, so like--
I can start, since I'm speaking.
I think that like, even
while I'm building things,
I still have on my mind
these questions which are,
I think in some sense beyond
engineering, beyond physics.
And sometime, I do have
time to think about it.
Not all the time.
And these are quite interesting.
There is also part
of my research, which
might be sort of
pushed a bit more by be
these more fundamental
question, which are not just
to find an
application, but really
trying to understand,
for example, how
a quantum system
in the end becomes
something which looks a
classical thermodynamic system.
So I think that like, there
are sort of moving boundaries
between what we can do, which
are sometime unfortunately set
by all the commitments which
we have to take care of.
Ludwig Wittgenstein said that
philosophy leaves everything
as it is, so--
He was right, and
I don't think what
the engineers, their research
programs on quantum computers
or--
Seem to be very strange if I
marched over there and said you
guys are making this, you're
never going to succeed.
And here's why you're making
this terrible mistake.
I mean, there are
episodes in the history
of science of philosophers
doing that sort of thing.
Seems unlikely to me that that
is what is going to happen.
I mean, there are--
There have been famous
quantum physicists who said,
quantum computing proves that
the many world's interpretation
of quantum mechanics
has to be correct,
otherwise quantum computing
would make no sense.
There have been attempts
to say that the success
of these engineering
research programs
shows that one of the parties
in the philosophical debates
of quantum mechanics is right,
the other ones are wrong.
But I think he's just
completely confused.
Yes.
I think there are
border regions where
some of these sort
of tests of quantum
entanglement, say laboratory
tests, which are sometimes
done to inform, or in
some sense shore up
practical applications.
So if entanglement
were somehow a mirage,
if the equations of
quantum mechanics
were leading us
down a blind alley,
and there was some alternate
explanation more akin to what
Einstein had articulated
or hoped for,
the way Brad
introduced, then a lot
of the core ingredients to
things like quantum encryption
and quantum computers
would be suspect.
Maybe encryption
could be hackable.
So that could have real
world implications.
At the same time, a lot of these
fancy and fancier and fancier
experimental tests of,
say quantum entanglement,
rely on technologies
and devices that
come from efforts like
Paola's and her colleagues.
So if you can really sort
of manipulate single photons
and ask very specific
questions of them,
that comes because they
have all kinds of ideas
they have in mind for those.
And then some of
those experiences
in, say precision
instrumentation, can come back
to answer philosophically
motivated questions about,
is the world like that?
What about Einstein's
other ideas?
What about rival theory
number two or three?
And so there are meeting
grounds where sometimes you
have philosophers and
engineers and other folks
in between in this sort of
academic Schrodinger's cat
superposition who really
have a certain kind of space
to try to, if not
debate, then at least
have a kind of borrowing.
But I do think you're right.
I think universities are
big, complicated places,
and we kind of tend to--
We collectively, I
think, tend to drift
with a kind of disciplinary
momentum, more often than not.
That's my impression, at least.
Other questions?
Jennifer has a question.
So what predictions
that quantum mechanics
makes are you the most excited
about to see come to reality?
Or not come to reality, but that
you're the most excited to see
what develops with it.
Yeah.
It's a stumper.
That's good.
So, one thing is this.
If you want to divide between
quantum and classical worlds,
so as we many times, we
cannot see Schrodinger's cat,
so it's superposition of
a very microscopic object.
There's no real prediction
where the boundary between what
is quantum, in terms
of size and complexity,
and what is classical
should really be.
So, I don't think
that there's going
to be like one moment in
which we can say, OK, here's
the boundary.
I think it's going
to be more like,
we're going to push more
and more of that boundary
to a larger and larger,
more complex system.
But there are like some people
doing heroic experiments
along these lines.
I mean, I have great
respect for them
because they're very,
very tough experiments.
Like every time that we
publish something new,
I'm like they've managed to.
Now it's no longer, like
hundred is 120 atoms.
I think that that's
really exciting.
Maybe this is what I
can say about that.
I mean, it's not like I
have read the Encyclopedia
of Quantum Predictions
so I can circle
my favorite,
unrealized prediction.
You can borrow my
copy, if you want.
Can I?
Yeah.
It's--
I'll share it.
I'm sure you have one.
So, the standard view
of quantum mechanics
says like, before
you look in the box,
there's no fact of the matter
about where the particle is,
but then the act of
looking causes the particle
to choose a particular position
to be in, which is insane.
It's crazy.
Like certainly,
it wasn't the case
that all the electrons
in the universe
were waiting around for
some human to be born.
Before the evolution of
humans, no part of particle
was ever anywhere.
And then finally, when
finally, the first human
arrived on the scene, then
they all chose their positions
when he looked around.
Couldn't possibly be right.
So there's one view
about quantum mechanics
that says, well no, what happens
is, things spontaneously choose
to have definite positions.
It's not that looking
at them causes them to.
But you can fine tune
how they spontaneously
do it so that it seems
to us like looking
is what causes them to
choose a different position.
So on that view, the way
it's typically understood,
big things just can't
be in indefinite states.
Like a cat just couldn't
be, for very long,
more than a fraction of
a fraction of a fraction
of a second, you know--
It would be indefinite whether
the cat's alive or dead.
And what the people who
work on computers do,
their goal is to put
bigger and bigger systems
in states of indefiniteness
for longer and longer stretches
of time.
And the more they
succeed in doing that,
the more that casts doubt
on these interpretations
that say things localize
themselves spontaneously
in such a way that
big things couldn't be
indefinite in their position.
So I guess that I'd be
interested to see them build
something big that
managed to stay
in an indefinite state for
a reasonable amount of time.
I want to ask a variation
on Jennifer's question,
then I'll get to yours as well.
For both Paola and Brad.
Basically, if there were
some sort of friendly demon--
That maybe is the
wrong way to say it.
There was some
friendly answer giver
who granted you each one answer
to one question, what would you
most want to have answered?
Maybe it's just another
version of Jennifer's question.
You get one shot, only
one question, let's say.
What would you-- I mean,
would you want to say,
oh is it the many worlds
interpretation or not?
What would your one question
for this friendly quantum genie
be like?
You've always
wanted to know this.
The rest you can
keep working on.
Question about
quantum mechanics?
For quantum mechanics.
We've only got so
much time here, Brad.
Let's start with
quantum mechanics.
It's a quantum
genie in particular.
Well, maybe I'd put myself in
a superposition of asking him
many different questions.
That's like the-- I guess
that's asking for more wishes.
It might be, but according
to some interpretations,
you'd only be able to
recognize one outcome anyway,
at a given time.
There's no free
quantum lunch there.
It's going to--
OK, so I don't know.
But here's something
I think about.
When Paola was giving
her presentation,
she said like the neutron
hits the beam splitter,
and then she said, and then it
goes down both paths, right?
OK.
So what she said
contradicted what
I said the orthodox
interpretation of quantum
mechanics is.
The standard
interpretation says,
what happens when it hits
the beam splitter is,
there is just no fact of the
matter as to which path it goes
down.
You say, which path is it on?
We should say, there's something
wrong with that question,
and you're making
some terrible mistake.
So she was working with a
different way of understanding
quantum mechanics, more like
the many worlds interpretation
than I was trying
to present when
I talked about the
standard interpretation.
I'd like to know if the
many-worlds interpretation is
correct.
If the many-worlds
interpretation is correct,
then you know, you
could resolve--
You could stand in front of one
of these beams splitters, one
of the paths, and say well
if I see the neutron--
Maybe you'd want it
to be something bigger
than a neutron.
Anyway, if I see it,
then I'll resolve
to make my life go one way.
I'll devote myself to
being a concert pianist.
And if I don't see
it, I'll devote myself
to being a philosopher.
And then if the many-worlds
interpretation is true,
what happens is after the
neutron hits the beam splitter,
is you've lived both lives.
So I'd like to know if that
is the true understanding
of quantum mechanics, I'd want
to do this experiment a lot
because I could live a very
rich, complicated life,
or many rich, complicated
lives simultaneously.
Do you have a wish for
the quantum genie, Paola?
No.
OK, there was a question.
Yes, here.
Maybe one will come to
you in the meantime.
Yeah?
Paola, you talked about
with this quantum computing,
we can simulate chemical
reactions and things like that.
So do you think
with this computing
power, we can simulate,
or in some sense,
recreate human brain?
And a follow up question
to that is that,
if at all we can
do that, then we
can simulate every
brain on this planet,
and in some sense
predict the future?
What is going to happen?
So do you think that
is a possibility?
Maybe I should have say that
wanted to know from the demon.
It's the price you pay.
If everything is deterministic,
or if it is really free will.
Sort of would answer
your question somehow.
I mean, again, in the same way
that some algorithms work well
in a quantum computer, also
some quantum simulations
will work well.
So there might be
system which are better
adapted to be simulated
on a quantum computer.
So when I talk about
like chemical reaction,
it's because fundamentally,
our quantum systems
reacting with each other.
And so, mapping it to another
quantum system may be easy.
For a brain, I do not
know enough about brain
to tell you yes, there will
be like a quantum algorithm
for which this is efficient to
both encoding the information
and then computing
the information
so that we will be able
to simulate a brain.
There's also the question,
is the brain quantum?
Since for me,
everything is quantum,
so also the brain
should be quantum.
But there is some limit.
In terms of the future, probably
even for a quantum computer,
there are limits.
There are classes of
problems which are still
hard on a quantum computer.
So like there are problems
for classical computer
which will take an exponential
amount of time to solve.
And some of these
problem takes only
like a linear amount of
time on a quantum computer.
But there are even
harder problem
that are still exponential
in the number of sources
that you will need on
a quantum computer.
So I have no reason to say
this, but I sort of think it,
probably predicting the
future is something which
is going to be an exponentially
hard problem, even
for a quantum computer.
As much as I can guess.
There's a question here.
You can-- I'll repeat
it, If you want.
[INAUDIBLE]
So the question was,
at least in brief,
what counts as an observation
within quantum mechanics?
We very casually
say, the particle
has no definite state
until I open the box
and make an observation.
The cat is neither
alive nor dead.
It's both alive and dead
until I make an observation.
So what counts?
to the limiting
case of what could
count as an observation
in quantum theory?
That's a great question.
I mean, I guess
what I would hope--
The thing with John Bell's view
was that you just would not
have to answer that question.
So if you're working with a
version of quantum mechanics
that says, the way things
behave is different
when they're being observed
than when they're not
being observed, which is a
common way of understanding
quantum mechanics, things
jump around when you look.
Then of course, you need to
say, when is someone looking?
Is it enough for a lemur
to be looking at it?
Does the person looking
have to have a PhD?
It would really be much better
if your physical theory didn't
require you to have
answers to these questions
to be able to
figure out what you
thought was going to happen.
So my hope would be
that you wouldn't
need to say anything
particularly informative
about that because it just
wouldn't be true, fundamentally
speaking, that the
way things behaved
depended on whether someone
was observing them or not.
Yeah, go ahead.
[INAUDIBLE]
So the comment was, Richard
Feynman, a fine MIT alum--
We're very proud of
Richard Feynman--
Had said once that once
you perform an observation,
you change the
system being studied.
So what does that mean for
the future of that system,
or what does it mean for the
nature of observation, I think,
is the gist of the comment.
So, from my perspective, like
going back to the observation.
For me, you have your
system, and then you
have another system.
And the observation is
nothing as an interaction
between the two.
And so, you don't really
need to like a sentient being
to make the observation.
Just the fact of
an interaction is
sort of exchanging information
between these two systems.
Now of course,
what I would think,
is my first system
to be quantum,
and second system, my
measurement apparatus,
also to be quantum.
Then at some point, I would like
to translate it into something
which my brain can understand.
I understand mostly
classical mechanics.
In some sense, the
system which is observing
becomes larger and larger.
So let's say that I start
having like a photon,
and I want to
measure where it is.
I will make it interact
with an electron.
I can still consider
the electron is quantum,
but then it will cascade
into multiple electrons,
and then it becomes more and
more difficult to isolate
these growing measurement
apparatus, which
are all interacting.
And they're no
longer interacting
only with each other, but also
with the rest of the worlds.
And that sort of, somehow,
decided which state it will be.
And so it's sort of
like an imperfect chain.
So you could think OK, I
start with a quantum system
and measure with
a quantum system,
and nothing has
changed measurably.
[INAUDIBLE] and so
on and so forth.
You could go on forever,
but somehow along the way,
this interaction with all that
is external to this quantum
system sort of decide which
direction they'll go in.
So it's not a very good answer.
I know that this still leaves
some open question which,
to me, are very,
very interesting,
which is like this
measurement problem.
But it's direction to try
to understand where maybe
the answer could be looked for.
And people have indeed try to
do experiments along these lines
to see how this quantum
measurement apparatus can
indeed induce this
collapse of the system
to a well-defined state.
I think that's--
A hope for the future is
a fine point to end on.
So I think Jennifer has
some announcements, yes.
Fantastic.
Thank you, guys.
Let's give all of our presenters
a huge round of applause.
So, I passed out a survey.
So if you guys could
take a quick minute
to fill that out and then turn
it into Amanda over there.
If you put your name on
the bottom of the survey,
in about three minutes,
when everyone turns it in,
we're going to do a quick
drawing for 101 Quantum
Questions.
So to perhaps keep spurring
your conversations or thoughts
about quantum
quandaries and maybe
helping develop and answer
some of your own questions.
So if you can just
take a quick minute,
fill that out and
turn it in to Amanda,
and then we'll do a drawing.
Those of you who
have filled it out,
I do encourage you to
come join us next Tuesday.
We'll be moving to a
slightly larger scale
where we'll be looking that
the Higgs boson and neutrinos.
So hopefully, the same types
of questions, the same types of
conversations, but with
a slightly larger system.
Let's just take a quick
minute, turn it in,
and then we'll do a drawing.
Thank you guys for
joining us tonight.
