Discrete time crystals on quantum hardware

Short answer: A discrete time crystal is a periodically driven quantum system whose local observables spontaneously break time translation symmetry and keep a memory of the initial state. A Science Advances paper reports observing one on a chain of 57 superconducting qubits, and a recent trapped-ion paper reports a period-tripling "3T" version on a qudit processor. These are physics experiments on small processors, not computing applications.

What a discrete time crystal is

The Science Advances paper starts from the idea that a periodically driven system is naively expected to lose all memory of its initial state through thermalization, but that this can be avoided in the presence of many-body localization. It describes a discrete time crystal as a driven system whose local observables spontaneously break time translation symmetry and retain memory of the initial state indefinitely. The authors add that most earlier experiments, on platforms such as trapped ions, dipolar spin systems and superfluid quantum gases, fail to satisfy all the requirements for this order.

57 superconducting qubits

The authors report observing a discrete time crystal on a chain of 57 superconducting qubits on a quantum computer they describe as state of the art. They probe random initial states and compare vanishing and finite disorder, to tell many-body localization from prethermal dynamics. They also report results on the dynamical phase transition between the time crystal and a thermal regime, seen through critical fluctuations in the subharmonic frequency response of the system and a substantial speedup of spin depolarization.

A period-tripling crystal on qudits

The trapped-ion paper notes that time crystals have been observed on various qubit-based platforms, but that behaviour beyond period doubling has remained fairly unexplored, partly because qubit architectures natively encode two-cycle dynamics. It proposes a one-dimensional, disorder-free spin-1 Floquet model with short-range interactions that realises a 3T time crystal, and implements it on a trapped-ion qudit quantum processor. The authors observe period-tripling dynamics in local observables and spin correlations, which they say confirms a collective subharmonic response. Numerical simulations show a prethermal phase over a wide range of parameters, and they use the Quantum Fisher Information to verify multipartite entanglement, which they report is enhanced at the crossover between ergodic and localised regimes.

Limits

The papers describe the physics of driven many-body systems. They do not claim an application, and the processors used are research devices. The observed behaviour is a finite-size and finite-time experiment, even where the term "indefinitely" is used in a definition.

FAQ

What is a discrete time crystal?

A driven system whose local observables break time translation symmetry and keep memory of the initial state, according to the Science Advances paper.

How large was the superconducting experiment?

57 qubits in a chain.

What is a 3T time crystal?

One that responds at three times the drive period. A trapped-ion paper reports observing period tripling on a qudit processor.

Why use qudits?

The authors say qubit architectures natively encode two-cycle dynamics, and qudits offer a route beyond period doubling.

Is it useful for computing?

The papers do not claim so. They study the physics of non-ergodic driven systems.