The addressability of NV centers in diamond as single qubits has enabled various possibilities of using them as quantum sensors individually or as building blocks for quantum information processes. They have high sensitivity to fluctuations of magnetic fields in the environment, which can be generated by spin or pseudospin dynamics in a magnetically active material in proximity. In this talk, I would like to share our recent study of magnetic noise emitted by a magnetic Weyl semimetal, considering the transport of charge, spin, and valley degrees of freedom. We show these three flavors of noise have distinct spectral characteristics, which can be resolved in a single-qubit relaxometry measurement. This provides a noninvasive approach to study the intrinsic valley transport in magnetic Weyl semimetals. Along the other line, I would like to mention our work on dissipative couplings between a pair of spin qubits induced by a magnetic medium, which could help to establish entangled states even in the absence of coherent couplings.
Quantum many-body systems display rich phase structure in their low-temperature equilibrium states. However, much of nature is not in thermal equilibrium. 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. Here we implement a continuous family of tunable CPHASE gates on an array of superconducting qubits to experimentally observe an eigenstate-ordered DTC. We demonstrate the characteristic spatiotemporal response of a DTC for generic initial states. Our 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, we locate the phase transition out of the DTC with an experimental finite-size analysis. These results establish a scalable approach to study non-equilibrium phases of matter on current quantum processors.
Fermions are often considered as somewhat strange quantum objects due to their anti-commuting properties. Cellular automata describe deterministic changes for bit-configurations, as for classical computing. At first sight these two issues do not seem to be much related. We show that probabilistic cellular automata, for which initial conditions are given by a probability distribution, can describe certain quantum many body systems of interacting fermions. The notions of wave functions, non-commuting operators for observables and quantum rules for expectation values arise in a natural way from the classical statistics for the probabilistic cellular automata. This constitutes an example how quantum mechanics can emerge from a classical statistical system. The famous particle-wave duality combines the discreteness of the bit observables or fermionic occupation numbers with the continuity of the probabilistic information and its evolution in time.