274 research outputs found
Defining Time Crystals via Representation Theory
Time crystals are proposed states of matter which spontaneously break time
translation symmetry. There is no settled definition of such states. We offer a
new definition which follows the traditional recipe for Wigner symmetries and
order parameters. Supplementing our definition with a few plausible assumptions
we find that a) systems with time independent Hamiltonians should not exhibit
TTSB while b) the recently studied spin glass/Floquet time crystal can be
viewed as breaking a global internal symmetry and as breaking time translation
symmetry as befits its two names
Absolute Stability and Spatiotemporal Long-Range Order in Floquet systems
Recent work has shown that a variety of novel phases of matter arise in
periodically driven Floquet systems. Among these are many-body localized phases
which spontaneously break global symmetries and exhibit novel multiplets of
Floquet eigenstates separated by quantized quasienergies. Here we show that
these properties are stable to all weak local deformations of the underlying
Floquet drives -- including those that explicitly break the defining symmetries
-- and that the models considered until now occupy sub-manifolds within these
larger "absolutely stable" phases. While these absolutely stable phases have no
explicit global symmetries, they spontaneously break Hamiltonian dependent
emergent symmetries, and thus continue to exhibit the novel multiplet
structure. The multiplet structure in turn encodes characteristic oscillations
of the emergent order parameter at multiples of the fundamental period.
Altogether these phases exhibit a form of simultaneous long-range order in
space and time which is new to quantum systems. We describe how this
spatiotemporal order can be detected in experiments involving quenches from a
broad class of initial states.Comment: Published version. Minor typos corrected, some discussions expande
Floquet Prethermalization in a Bose-Hubbard System
Periodic driving has emerged as a powerful tool in the quest to engineer new
and exotic quantum phases. While driven many-body systems are generically
expected to absorb energy indefinitely and reach an infinite-temperature state,
the rate of heating can be exponentially suppressed when the drive frequency is
large compared to the local energy scales of the system -- leading to
long-lived 'prethermal' regimes. In this work, we experimentally study a
bosonic cloud of ultracold atoms in a driven optical lattice and identify such
a prethermal regime in the Bose-Hubbard model. By measuring the energy
absorption of the cloud as the driving frequency is increased, we observe an
exponential-in-frequency reduction of the heating rate persisting over more
than 2 orders of magnitude. The tunability of the lattice potentials allows us
to explore one- and two-dimensional systems in a range of different interacting
regimes. Alongside the exponential decrease, the dependence of the heating rate
on the frequency displays features characteristic of the phase diagram of the
Bose-Hubbard model, whose understanding is additionally supported by numerical
simulations in one dimension. Our results show experimental evidence of the
phenomenon of Floquet prethermalization, and provide insight into the
characterization of heating for driven bosonic systems
Observation of discrete time-crystalline order in a disordered dipolar many-body system
Understanding quantum dynamics away from equilibrium is an outstanding
challenge in the modern physical sciences. It is well known that
out-of-equilibrium systems can display a rich array of phenomena, ranging from
self-organized synchronization to dynamical phase transitions. More recently,
advances in the controlled manipulation of isolated many-body systems have
enabled detailed studies of non-equilibrium phases in strongly interacting
quantum matter. As a particularly striking example, the interplay of periodic
driving, disorder, and strong interactions has recently been predicted to
result in exotic "time-crystalline" phases, which spontaneously break the
discrete time-translation symmetry of the underlying drive. Here, we report the
experimental observation of such discrete time-crystalline order in a driven,
disordered ensemble of dipolar spin impurities in diamond at
room-temperature. We observe long-lived temporal correlations at integer
multiples of the fundamental driving period, experimentally identify the phase
boundary and find that the temporal order is protected by strong interactions;
this order is remarkably stable against perturbations, even in the presence of
slow thermalization. Our work opens the door to exploring dynamical phases of
matter and controlling interacting, disordered many-body systems.Comment: 6 + 3 pages, 4 figure
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