Researchers create 'time crystal' with quantum computers
The versatility of quantum computers have helped physicists create a "time crystal," a new phase of matter.
A handout picture from October 2019 shows Sundar Pichai and Daniel
Sank (R) with one of Google's Quantum Computers in the Santa Barbara
lab, California, US (photo credit: GOOGLE/HANDOUT VIA REUTERS)
Researchers have successfully created a "time crystal," a new phase of matter, with the help of a quantum computer, according to a new study published in the peer-reviewed journal Nature on Tuesday.
A
time crystal, first proposed by physicist Frank Wilczek in 2012, is a
phase of matter which repeats in time, similar to how a regular
crystal's structure repeats in space. What that means is that the
particles in the crystal perpetually switch between two states without
requiring the input of more energy and without losing any energy.
These
crystals are the first objects to break what is known as
"time-translation symmetry," a rule in physics that says that a stable
object will remain unchanged throughout time. Time crystals avoid this
rule, being both stable and ever-changing.
So, for example, ice when stable will remain ice and will only
change when temperature or another factor makes it unstable. A time
crystal would change even when in its ground state, acting differently
than all other phases of matter.
But
scientists still needed to figure out how to create this phase of
matter. They turned to a phenomenon called many-body localization.
Many-body-localization
is when a one-dimensional chain of quantum particles gets stuck in a
fixed state. Each particle in the chain has a magnetic orientation
(known as "spin") that points up, down, or a certain probability of both
directions, according to Quanta Magazine.
For example, imagine the particles are set up so that the first one
points up, the next one points down, the one after that points down and
the one after that points up. Usually, the spins would quantum
mechanically fluctuate and align, if possible. But with random
interference between the particles confining their activity, the row of
particles can get stuck in a particular configuration, unable to
rearrange or settle into thermal equilibrium. They'll point in that
configuration indefinitely.
In
2014, Vedika Khemani, a condensed matter physicist who is now at
Stanford, and Shivaji Sondhi who was her doctoral advisor at Princeton
at the time, found that many-body localized systems could exhibit a
special kind of order: if all of the spins in the system were flipped,
it would be another stable, many-body localized state, according to
Quanta Magazine.
What
this means is that if the system were to be prodded with a laser, it
would forever cycle between the two states without absorbing or
releasing energy from the laser.
Khemani
and Sondhi, together with Achilleas Lazarides and Roderich Moessner at
the Max Planck Institute for Physics of Complex Systems, were able to
find such a system where the spins of the particles flipped between
patterns that repeated forever, at a period twice that of the period of
the laser.
This
system is unique because it is a system of millions of things that
oscillate between two states, only completing a cycle when prodded twice
by the laser, and doing so without absorbing or releasing energy.
An article on Stanford's website stressed that while this may sound like a "perpetual motion machine," which would break the laws of physics by allowing perpetual motion without any external energy source, this is not the case.
Entropy
- a measure of the disorder in the system - remains stationary, not
increasing, but not decreasing either, which means it still fits into
the second law of thermodynamics which rules that disorder cannot
decrease, but allows for the disorder to remain at a constant level as
long as the process is reversible.
An
example of a reversible process is a gas flowing through a pipe that is
tight in the middle. As the flow moves through the constricted part of
the pipe, its pressure, temperature and velocity change, but these
values return to their original conditions after they enter the widened
part of the pipe, meaning that the change in entropy is zero.
While
other attempts have gotten close to making a time crystal, the crystal
described in the study published in Nature is the first to meet all the
requirements needed to make a truly infinitely stable time crystal.
The access to Google’s Sycamore quantum computing hardware was what allowed the researchers to make their breakthrough.
Although
the hardware is still imperfect, meaning that the experiment was still
limited in size and duration, the researchers created a number of
protocols that allowed them to assess the stability of the time crystal,
including running the simulation forward and backward in time and
scaling the size.
“We
managed to use the versatility of the quantum computer to help us
analyze its own limitations,” said Roderich Moessner, co-author of the
paper and director at the Max Planck Institute for Physics of Complex
Systems. “It essentially told us how to correct for its own errors, so
that the fingerprint of ideal time-crystalline behavior could be
ascertained from finite time observations.”
The
researchers used a chip with 20 qubits - controllable quantum particles
which maintain two possible states, 0 and 1, at the same time - made
out of superconducting aluminum strips, according to Quanta Magazine.
The states were programmed to represent up or down spins.
The
programmers were able to randomize the interaction strengths of the
qubits, creating the interference needed to lock the particles into a
set pattern of spins instead of letting them align.
The
researchers tested a large number of initial configurations to see if
they could all be locked into an eternally oscillating pattern of spins
which would oscillate at twice the period of the prodding, probing over a
million states in just a single run of the machine.
They
were also able to extrapolate trends from the relatively small systems
that could be created on the Sycamore hardware to much large systems.
The researchers were also able to show that, except for decoherence in
the processor itself, there was no increasing entropy in the simulated
system itself.
All
of these findings together substantially bolstered the case for the
existence of time crystals more so than past experiments were able to.
“I
am optimistic that with more and better qubits, our approach can become
the main method in studying non-equilibrium dynamics,” said Pedram
Roushan, a researcher at Google and senior author of the paper.
“We
think that the most exciting use for quantum computers right now is as
platforms for fundamental quantum physics,” said Matteo Ippoliti, a
postdoctoral scholar at Stanford and co-lead author of the work. “With
the unique capabilities of these systems, there’s hope that you might
discover some new phenomenon that you hadn’t predicted.”
On Saturday, another team at QuTech,
a collaboration between the Delft University of Technology and the
Netherlands Organisation for Applied Scientific Research (TNO),
published their findings on a time crystal they had created with a
quantum processor which lasted about eight seconds.
“While
a perfectly isolated time crystal can, in principle, live forever, any
real experimental implementation will decay due to interactions with the
environment,” said Joe Randall on QuTech's website. “Further extending
the lifetime is the next frontier.”
The
QuTech team used nine quantum bits and manipulated them to lock their
spins into a periodically inverting pattern which formed from a variety
of different initial states.
The team referred to the research conducted on the Google Sycamore
quantum computer, with Tim Taminiau, lead investigator at QuTech, saying
that "It is extremely exciting that multiple experimental breakthroughs
are happening simultaneously.
"All
these different platforms complement each other. The Google experiment
uses two times more qubits, our time crystal lives about ten times
longer," said Taminiau.
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