SERGIO BOIXO: Hi.
I am Sergio Boixo from
the Google AI Quantum team,
and today, we're going to
talk about an experiment we're
working on, which is known
as quantum supremacy.
The latest experimental
quantum processor
produced at Google, Bristlecone,
has 72 qubits or quantum bits.
We're testing quantum
circuits in Bristlecone
with the goal of
reducing errors.
By their nature, quantum gates
have a probability of errors,
and errors can cross
quantum circuits.
As we calibrate
quantum circuits,
we bring down the
probability of error.
We simulate quantum circuits
with traditional computers
to benchmark and calibrate
quantum circuits.
As we work to reduce the
probability of an error,
simulations gets
exponentially harder.
This means that it gets too
computationally intensive even
for a supercomputer to keep up.
From this, we get the
name quantum supremacy
for this experiment.
This has to do with something
called a strong Church-Turing
thesis in computer science.
Traditional computers from
the abacus to your laptop
implement equivalent
operations or classical gates,
although a modern computer is,
of course, much, much faster.
The strong Church-Turing
thesis says
that all universal computers
are equivalent in this way,
and can simulate each
other efficiently.
But according to
quantum computing,
the strong Church-Turing
thesis is false,
and quantum computers can solve
some problems exponentially
faster than other
universal computers.
So what we're
trying to do is kind
of breaking the strong
Church-Turing thesis.
You can think of a qubit
as an arrow pointing
to some direction on a sphere.
Quantum gates are
operations on qubits.
Similar to classical gates,
we often depict quantum gates
as boxes, with the
input on one side
and the output on
the opposite side.
In a quantum circuit,
we apply layers
of gates, one per clock cycle.
A measurement at the end
of the quantum circuit
produces a string of beats.
For the quantum
supremacy experiment,
we choose the quantum
gates at random.
This is a Hello World program
for quantum computers.
Crucially, in this case, we have
the strongest critical evidence
against the strong
Church-Turing thesis.
It takes exponential time
to simulate a random quantum
circuit with a
classical computer.
According to quantum
mechanics, every particle
can also act as a wave,
and this applies to qubits.
The quantum state of
a quantum computer
contains an exponential
number of waves
or computational paths.
This is the property
that we are testing.
The output state of a
random quantum circuit
looks like the
speckles of a laser.
This is a fingerprint
of the quantum circuit.
For some bit strings,
the computational paths
interfere constructively, and
the intensity of the output
probability grows.
For others, the computational
paths interfere destructively,
and the output
probability decreases.
Simulating interference
of the exponential number
of computational paths
in the quantum circuit
takes exponential time.
We can check if we obtain
the correct fingerprint
in the experiment, and measure
the probability of error.
First, we get around
a million bit strings
from the quantum computer.
This takes a few seconds.
Then we use an expensive
classical simulation
to check if those bit strings
have high probability.
If this is the case,
the error rate is low,
and the experiment
has succeeded.
The implication will be
that quantum computers
seem to be breaking the
strong Church-Turing thesis.
As we reduce errors
farther, we expect
to see a similar
exponential speed up
for a practical problem.
So what's next?
Visit the other
videos in these series
to learn more about how
a quantum computer works
and how to program it.
You can also visit OpenFermion
to learn more about how
quantum computers can be
used to solve problems
in chemistry and
material science,
or check out the
links included below.
Thank you.
