Like a perpetual motion machine, a time crystal forever cycles between states without consuming any energy. Physicists claim to have built this new phase of matter inside a quantum computer.
Quantum many-body systems display rich phase structure in their low-temperature equilibrium states. However, much of nature is not in thermal equilibrium at all. Remarkably, it was recently predicted that out-of-equilibrium systems can exhibit novel dynamical phases that may otherwise be forbidden by equilibrium thermodynamics, a paradigmatic example being the discrete time crystal (DTC). Concretely, dynamical phases can be defined in periodically driven many-body localized systems via the concept of eigenstate order. In eigenstate-ordered phases, the entire many-body spectrum exhibits quantum correlations and long-range order, with characteristic signatures in late-time dynamics from all initial states. It is, however, challenging to experimentally distinguish such stable phases from transient phenomena, wherein few select states can mask typical behavior.
Now, a team at Google and collaborators implemented a continuous family of tunable CPHASE gates on an array of superconducting qubits to experimentally observe an eigenstate-ordered DTC. They were able to demonstrate the characteristic spatiotemporal response of a DTC for generic initial states. This work employs a time-reversal protocol that discriminates external decoherence from intrinsic thermalization, and leverages quantum typicality to circumvent the exponential cost of densely sampling the eigenspectrum. In addition, they were able to locate the phase transition out of the DTC with an experimental finite-size analysis.
Taken together, these results establish a scalable approach to study non-equilibrium phases of matter on current quantum processors.
For more detailed information, read the original Google paper here.
Read the full article at: www.quantamagazine.org