Quantum Time Crystals from Hamiltonians with Long-Range Interactions

This is the final version. Available from the American Physical Society via the DOI in this recordTime crystals correspond to a phase of matter where time-translational symmetry (TTS) is broken. Up to date, they are well studied in open quantum systems, where an external drive allows us to break discrete TTS, ultimately leading to Floquet time crystals. At the same time, genuine time crystals for closed quantum systems are believed to be impossible. In this study we propose a form of a Hamiltonian for which the unitary dynamics exhibits the time crystalline behavior and breaks continuous TTS. This is based on a spin-1/2 many-body Hamiltonian which has long-range multispin interactions in the form of spin strings, thus bypassing previously known no-go theorems. We show that quantum time crystals are stable to local perturbations at zero temperature. Finally, we reveal the intrinsic connection between continuous and discrete TTS, thus linking the two realms.Government of the Russian FederationMinistry of Education and Science of Russian Federatio

Effective Hamiltonians for discrete time crystals

This is the final version. Available from AIP Publishing via the DOI in this recordWe analyze the effective Hamiltonian for the 2T discrete time crystal (2T-DTC or DTC). This effective Hamiltonian is given by spin 1/2 many-body Hamiltonian which includes all-to-all coupling terms, thus being of infinite range. We describe the possible structure of the Hamiltonian, including many-body localized version which prevents thermalization. Finally, we show how the DTC melts when symmetry breaking terms are added.Ministry of Russian Federation,President of Russian FederationRussian Science Foundatio

Machine learning of phase transitions in nonlinear polariton lattices

This is the final version. Available from Nature Research via the DOI in this record. The data that support the findings of this study are available from the corresponding author upon reasonable request.The code for the analysis is available from the corresponding author upon reasonable request.Polaritonic lattices offer a unique testbed for studying nonlinear driven-dissipative physics. They show qualitative changes of their steady state as a function of system parameters, which resemble non-equilibrium phase transitions. Unlike their equilibrium counterparts, these transitions cannot be characterised by conventional statistical physics methods. Here, we study a lattice of square-arranged polariton condensates with nearest-neighbour coupling, and simulate the polarisation (pseudospin) dynamics of the polariton lattice, observing regions with distinct steady-state polarisation patterns. We classify these patterns using machine learning methods and determine the boundaries separating different regions. First, we use unsupervised data mining techniques to sketch the boundaries of phase transitions. We then apply learning by confusion, a neural network-based method for learning labels in a dataset, and extract the polaritonic phase diagram. Our work takes a step towards AI-enabled studies of polaritonic systems.Engineering and Physical Sciences Research Council (EPSRC)Russian Foundation for Basic ResearchNATOIcelandic Research FundIcelandic Research Fun

Dynamics of spin polarization in tilted polariton rings

This is an accepted manuscript of an article published by the American Physical Society in Physical Review B on 22/04/2021, available online: https://doi.org/10.1103/PhysRevB.103.165306 The accepted version of the publication may differ from the final published version.We have observed the effect of pseudomagnetic field originating from the polaritonic analog of spin-orbit coupling [transverse electric and transverse magnetic (TE-TM) splitting] on a polariton condensate in a ring-shaped microcavity. The effect gives rise to a stable four-leaf pattern around the ring as seen from the linear polarization measurements of the condensate photoluminescence. This pattern is found to originate from the interplay of the cavity potential, energy relaxation, and TE-TM splitting in the ring. Our observations are compared to the dissipative one-dimensional spinor Gross-Pitaevskii equation with the TE-TM splitting energy, which shows good qualitative agreement.Published versio

On the possibility of a terahertz light emitting diode based on a dressed quantum well

We consider theoretically the realization of a tunable terahertz light emitting diode from a quantum well with dressed electrons placed in a highly doped p-n junction. In the considered system the strong resonant dressing field forms dynamic Stark gaps in the valence and conduction bands and the electric field inside the p-n junction makes the QW asymmetric. It is shown that the electrons transiting through the light induced Stark gaps in the conduction band emit photons with energy directly proportional to the dressing field. This scheme is tunable, compact, and shows a fair efficiency.Comment: 6 pages, 5 figure

Perivascular macrophages in health and disease

Macrophages are a heterogeneous group of cells that are capable of carrying out distinct functions in different tissues, as well as in different locations within a given tissue. Some of these tissue macrophages lie on, or close to, the outer (abluminal) surface of blood vessels and perform several crucial activities at this interface between the tissue and the blood. In steady-state tissues, these perivascular macrophages maintain tight junctions between endothelial cells and limit vessel permeability, phagocytose potential pathogens before they enter tissues from the blood and restrict inappropriate inflammation. They also have a multifaceted role in diseases such as cancer, Alzheimer disease, multiple sclerosis and type 1 diabetes. Here, we examine the important functions of perivascular macrophages in various adult tissues and describe how these functions are perturbed in a broad array of pathological conditions

Tunable optical nonlinearity for transition metal dichalcogenide polaritons dressed by a Fermi sea

This is the final version. Available from the American Physical Society via the DOI in this record. We study the system of a transition metal dichalcogenide (TMD) monolayer placed in an optical resonator, where the strong light-matter coupling between excitons and photons is achieved. We present a quantitative theory of the nonlinear optical response for exciton-polaritons for the case of a doped TMD monolayer, and analyze in detail two sources of nonlinearity. The first nonlinear response contribution stems from the Coulomb exchange interaction between excitons. The second contribution comes from the reduction of Rabi splitting that originates from phase space filling at increased exciton concentration and the composite nature of excitons. We demonstrate that both nonlinear contributions are enhanced in the presence of free electrons. As free electron concentration can be routinely controlled by an externally applied gate voltage, this opens a way of electrical tuning of the nonlinear optical response.ESPR