A circuit QED stands for circuit
quantum electrodynamics. It's the study
of electrical circuits that
operate at microwave frequencies.
Frequencies
comparable to those used in
cellphones for example
and they're superconducting circuits, they're
cooled very close to absolute zero
so that there's no resistance. Electrical
resistance or dissipation of energy and
the goal
is to make these electrical circuits
behave like
quantum mechanically, to behave like
atoms that have
quantized energy levels. When we study
the interaction of
those atoms, artificial atoms
with quanta of light, with photons
but it's not ordinary light, visible
light
it's microwave
frequency, electromagnetic waves,
and we're sort of used to the idea that
visible light
comes in photons in discrete lumps of
energy
but even microwave photons with
10,000 times less energy
also are particles and come in discrete lumps.
We know from the theory of quantum
mechanics
that atoms, the electrons in atoms orbit in
discrete energy level
orbits and if they make a transition
from
one orbit to another they can do that by
omitting or absorbing
photon like so we'd like to make
electrical circuits which
have quantized energy levels. They, 
the energy takes on only certain
discrete values
in quantum mechanics the
size of the energy change when you go
from one level to another
is determined, or determines a frequency
and in Adams those frequencies
correspond to
typically visible light or even
ultraviolet light
in these electrical circuits it's a much
lower
energy change and it corresponds to a
microwave frequencies where
the light, the microwaves are oscillating
relatively slowly only 5 to 10 billion
times per second
as opposed to 10 to the 15 times per second
as occurs in visible light.
So in circuit QED the
circuit element that plays the role of
the
artificial atom is
a small antenna about
a millimeter long thats split into
two halves and the two halves are
connected by a small
circuit element called the Josephson
junction. In a superconductor,
the electrons travel together in
pairs flowing without any electrical
resistance
and a josephson injunction is a
small barrier that
you would think would prevent electrons
from traveling from one half of the antenna
to the other but because electrons are
also
waves they're able to tunnel quantum
mechanically through this barrier
and slosh back and forth between
the two halves of the antenna.
In the ground state of these
artificial atoms, there is very little of this
sloshing of charge back and forth in the
excited states there's
more and more. So that corresponds to if
you're familiar with let's say the
picture of the,
the Boer model of the hydrogen atom
electron is in some
orbit like a planet around the Sun and then it
can be in a bigger orbit and a bigger orbit
and I'm that corresponds to the
different
acetation levels of the the real atom,
the hydrogen and in this
artificial atom in this electrical
circuit it's the
sloshing of the charge harder and harder
back and forth across this antenna that
corresponds to the acetations.
Now when you have an antenna and
charges moving back and forth current is
flowing in the antenna then it can radiate 
radio waves just like an antenna on
your cell phone
for example and those radio waves are
photons. They can travel to another part
of the circuit
and be absorbed by another
artificial atom, another one of these
antennas with a josephson
junction. So in circuit QED,
you get these artificial atoms talking
to each other by exchanging
microwave photons and so you can
communicate
quantum information from one place to
another.
In particular, I said that
quantized energy levels are one of
the hallmarks of
something behaving quantum mechanically
but
even more importantly is the fact that
you can be, it's
this the superposition principle you can
be in a superposition of the ground
state and the excited-state
at the same time so in a quantum computer
the quantum bit can be
both 0 and 1 which is where the,
the power of a quantum computer comes
from and you can
transfer that superposition
of 0 and 1 to another
atom in the circuit, artificial atom
by use of these exchange via these
emission and absorption of photons.
So real atoms are pretty small they're
hard to see they're hard to hold onto
If you do hold on to them you
perturbed their properties. On the other
hand, they have a lot of great properties
that's why they're used in atomic clocks,
they have a very precise frequencies
they have,
you can put them in superposition states
for a long time so they have many
advantages. These
artificial atoms are made
by evaporating
aluminum which is the superconductor
onto
substrate by a process that's very similar,
electron beam lithography, very similar
to the same process that's used to make
the computer chips that are in your
laptop. So in principle
not yet in practice but in principle you
can imagine scaling up
this process and manufacturing
structures with first tens and then
hundreds and then thousands
someday f these qubits by
manufacturing methods that are
relatively straightforward. Qubits are
sort of glued down on the substrate they
don't, they don't
fall under the influence of
gravity the way atoms do they're,
they stay there. You can
engineer their properties if you need
them to be
bigger or the antenna have a different
shape you can just
change your design so there are a number
of are advantages in that sense that
they're more
engineerable than ordinary
atoms or ions. Nobody knows yet what is the
optimal hardware to use to try to build
a quantum computer people are using
atoms, trapped ions, spins, and
semiconductors. There are many different
ideas and there's no clear winner yet
but
the rate of progress
in using superconducting qubits over the
last thirteen years since the first one
was built by the NEC group in 1999
has been really fantastic.
The length, the coherence time
the length of time that you can maintain
a quantum superposition of the ground
state and the excited state
started at at essentially zero in 1999
with the first qubit. It may be one
nanosecond let's say, and today
we have coherence times that are
a hundred and fifty thousand times
longer than that.
So there's been a real exponential
growth
in the ability to engineer a quantum
states of these
systems and each
order of magnitude longer time is harder
than the last
order of magnitude to achieve but so far
it's been
showing really exponential
progress so it's still very early stages
were still
in some sense we're not trying to build
the quantum
version of the pentium processor we're
still trying to discover the
quantum version of the vacuum,
then we'll get to the transistor and
then we'll get to the integrated circuit
that's still very
early days but we think this
circuit QED architecture
looks promising in terms of being able
to
eventually scale up to large sizes.
So the thing which gives a quantum
computer
its great power is the ability to
put the bits, the quantum bits into
superposition states
and into fancier superposition states
called entangled states
and that same feature that gives
the computer it's great power is also its
achilles heel,
its quantum system
will remain in a coherent superposition
state only if you don't look at it only
if you don't
measure it and it doesn't have to be you
personally the experimenter that
measures it. It could be some
coupling of the qubit to a
noise or source or to the environment
to some other part of the circuit where
you
lose the quantum information. So
there's, this is a kind of new field of
quantum
electrical engineering in which we have
to understand how to get
it, how to take a device which is
exquisitely sensitive to tiny
noise in sources and perturbations
and isolate it to the greatest extent
possible from the environment so this
five orders of magnitude progress that
we've made in the last decade
came from
rethinking the design of these
artificial atoms and
changing it and going to different
kinds of structures.
So, in order to keep, to make those
structures
as insensitive to the external
perturbations as possible,
but the engineering challenges that
you can't,
you want the system to be completely
isolated from its environment
during the computation but you want it
to be strongly coupled to your control
device when you
program the computer when you give it
the data. Then you want it to be
completely decoupled from everything and
then at the end you have to measure the
final state of the computer to get your result
so now you have to be very strongly
coupled to a measurement apparatus
and so you have these conflicting
engineering requirements of extreme isolation
and extreme coupling to your measurement
apparatus
and getting that
enormous on/off ratio and the size of a
couple axis
is a big challenge.
