Thanks to a breakthrough by a
research team led by University of California Berkeley physicist Norman Yao,
time crystals are now a reality.
Following a blueprint created by
N.Y. Yao et al, University of Maryland physicists made the first
time crystal using a one-dimensional chain of ytterbium ions; each ion behaves
like an electron spin and exhibits long-range interactions indicated by the arrows.
Image credit: J. Zhang et al.
If crystals have an atomic structure
that repeats in space, like the carbon lattice of a diamond, why can’t crystals
also have a structure that repeats in time? That is, a time crystal?
In a paper
published in the journal Physical Review Letters, Dr. Yao and co-authors describe
how to make and measure the properties of a time crystal, and even predict what
the various phases surrounding the time crystal should be.
This is not mere speculation. Two
groups at the University of Maryland and Harvard University followed the
Berkeley team’s blueprint and have already created the first-ever time
crystals. The researchers reported their success in papers (paper #1 and paper #2) posted recently at arXiv.org.
“Time crystals repeat in time because
they are kicked periodically, sort of like tapping Jell-O repeatedly to get it
to jiggle,” Dr. Yao said.
“The big breakthrough is less that
these particular crystals repeat in time than that they are the first of a
large class of new materials that are intrinsically out of equilibrium, unable
to settle down to the motionless equilibrium of, for example, a diamond or
ruby.”
“This is a new phase of matter,
period, but it is also really cool because it is one of the first examples of
non-equilibrium matter.”
“For the last half-century, we have
been exploring equilibrium matter, like metals and insulators. We are just now
starting to explore a whole new landscape of non-equilibrium matter.”
While Dr. Yao is hard put to imagine
a use for a time crystal, other proposed phases of non-equilibrium matter
theoretically hold promise as nearly perfect memories and may be useful in
quantum computers.
This phase diagram shows how
changing the experimental parameters can ‘melt’ a time crystal into a normal
insulator or heat up a time crystal to a high temperature thermal state. Image
credit: N.Y. Yao et al.
Time crystals were originally
theorized by Nobel laureate Frank Wilczek in 2012.
In 2016, theoretical physicists at
Princeton University and the University of California Santa Barbara
independently proved that such a crystal could be made.
According to Dr. Yao, his team was
“the bridge between the theoretical idea and the experimental implementation.”
“From the perspective of quantum
mechanics, electrons can form crystals that do not match the underlying spatial
translation symmetry of the orderly, 3D array of atoms,” he said.
“This breaks the symmetry of the
material and leads to unique and stable properties we define as a crystal.”
The Harvard team set up its time
crystal using densely packed nitrogen vacancy centers in diamonds.
The time crystal created by the
Maryland team employs a conga line of 10 ytterbium ions whose electron spins
interact, similar to the qubit systems being tested as quantum computers.
To keep the ions out of equilibrium,
the physicists alternately hit them with one laser to create an effective
magnetic field and a second laser to partially flip the spins of the atoms,
repeating the sequence many times.
Because the spins interacted, the
atoms settled into a stable, repetitive pattern of spin flipping that defines a
crystal.
A time crystal breaks time symmetry.
In this particular case, the magnetic field and laser periodically driving the
ytterbium atoms produce a repetition in the system at twice the period of the
drivers, something that would not occur in a normal system.
“Wouldn’t it be super weird if you
jiggled the Jell-O and found that somehow it responded at a different period?”
Dr. Yao said.
“But that is the essence of the time
crystal. You have some periodic driver that has a period ‘T’, but the system somehow
synchronizes so that you observe the system oscillating with a period that is
larger than ‘T’.”
Dr. Yao worked closely with the
Maryland physicists as they made the new material, helping them focus on the
important properties to measure to confirm that the material was in fact a
stable or rigid time crystal.
He also described how the time
crystal would change phase, like an ice cube melting, under different magnetic
fields and laser pulsing.
_____
N.Y. Yao et al. 2017.
Discrete Time Crystals: Rigidity, Criticality, and Realizations. Phys.
Rev. Lett. 118 (3): 030401; doi: 10.1103/PhysRevLett.118.030401
J. Zhang et al. 2016.
Observation of a Discrete Time Crystal. arXiv: 1609.08684
Soonwon Choi et al.
2016. Observation of discrete time-crystalline order in a disordered dipolar
many-body system. arXiv: 1610.08057
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