Australian physicists have programmed a quantum computer half a globe away to create, or at least mimic, a record-size time crystal—a system of quantum particles that locks into an eternal cycle in time, similar to the repeating spatial pattern of atoms in an actual crystal.
The new
time crystal has 57 quantum particles, which is more than double the size of a
20-particle time crystal that Google scientists simulated last year. According
to Chetan Nayak, a condensed matter physicist at Microsoft who was not involved
in the research, "no normal computer could mimic it." "So that's
a significant step forward." The research demonstrates quantum computers'
ability to model complicated systems that would otherwise only exist in
physicists' theories.
When Frank
Wilczek, a Nobel Prize–winning theoretical physicist at the Massachusetts
Institute of Technology, pondered the stunning spatial pattern of atoms in a
conventional crystal ten years ago, he came up with the concept of a time
crystal. What is the source of the pattern? The equations governing the forces
between the atoms don't clearly state it, but it appears to allow any atom to
be anywhere with equal chance. Rather, if the atoms cool sufficiently, it
develops spontaneously. Once a few atoms have nestled next to one another, the
next one's position becomes predictable, and a pattern forms that is simply
implicit in the forces.
Wilczek
worried if something similar will happen in the future. He imagined a system of
quantum particles interacting through constant-force interactions that managed
to perform some cyclic evolution even in its most intense state. This proved to
be impossible. In 2016, two independent organization’s revisited the idea by
imagining a system that is continually nudged by an external signal. They
discovered that under the correct circumstances, it might lock into a pattern
of change over time that repeats at a lower frequency than the stimulus. A time
crystal's signature is a lower frequency response.
The system
is made up of a chain of small quantum mechanical magnets that can point up,
down, or, thanks to quantum physics' peculiar principles, both ways at the same
time. Neighboring magnets in the chain tend to align in opposite directions to
minimise their energy, however a locally determined magnetic field causes each
magnet to point more one way or the other. The magnets are periodically flipped
up and down by a constant stream of magnetic pulses. The notion is that under
the correct circumstances, any combination of magnets will rotate once every
two pulses. Researchers have demonstrated the concept in a variety of systems,
including electrons in a diamond, ions trapped in a trap, and quantum bits, or
qubits, in a quantum computer.
Now,
University of Melbourne theorists Philipp Frey and Stephan Rachel have created
a far larger qubit example. They used quantum computers manufactured and run by
IBM in the United States to run the simulation remotely. The qubits, which may
be set to 0, 1, or 1 and 0 at the same time, can be programmed to interact in
the same way that magnets do. Any initial configuration of the 57 qubits, such
as 01101101110..., remains stable for specific settings of their interactions,
the researchers discovered, returning to its original state every two pulses,
the researchers write Yesterday in Science Advances.
On the
surface, the observation may appear to be a little underwhelming. If the
magnets didn't interact, each pulse would flip them 180 degrees, resulting in
the half-frequency response. According to Dominic Else, a condensed matter
theorist at Harvard University, what makes the system a time crystal is the way
the interactions among the magnets stabilise the pattern. This makes the system
impervious to flaws such pulses that aren't long enough to completely flip the
spins. Else explains, "It's essentially a phase of matter that's
stabilised by multiple body interactions."
Surprisingly,
simply increasing the strength of the magnets' interactions is insufficient.
Rachel explains that the interactions must be random from one set of neighbours
to the next. He says that if all the magnets engage with the same strength, if
one goes wrong, it could influence others along the chain to flip the wrong way
as well. According to Rachel, the randomization prevents such errors from
spreading and stabilises the time crystal.
Unlike the
Google simulation, which involved over 100 researchers, Frey and Rachel worked
alone to create their larger demonstration, which they then submitted to IBM
computers through the internet. Rachel states, "It was just me, my
doctoral student, and a laptop," adding, "Philipp is great!" He
believes that the entire project took roughly 6 months.
Rachel
admits that the demonstration isn't ideal. He claims that the flipping pattern
should continue indefinitely, while qubits in IBM's machines can only keep
their states for roughly 50 cycles. He observes that the stabilising impact of
the interactions could eventually be exploited to store the state of a string
of qubits in a quantum computer's memory, but that such a breakthrough will
take time.
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