We investigate the cross influence of the pump coherence on the entanglement produced from spontaneous parametric down-conversion (SPDC) in different degrees of freedom (DOFs). We experimentally demonstrate the generation of polarization entanglement from SPDC pumped by a spatiotemporally highly incoherent (STHI) light-emitting diode. Our quantum state tomography measurements using multimode collection fibers to avoid postselection yield a two-qubit state with the concurrence of $0.531\pm 0.006$ and purity of $0.647\pm 0.005$, in excellent agreement with our theoretically predicted concurrence of 0.552 and purity of 0.652. We find that using an STHI pump reduces the entanglement and purity of the output polarization two-qubit state due to a coupling between the spatiotemporal and polarization DOFs introduced by the birefringence and dispersion of the nonlinear crystal. The viability of SPDC with STHI pumps is important for two reasons: (i) STHI sources are ubiquitous and available at a wider range of wavelengths than lasers, and (ii) the generated STHI polarization-entangled two-photon states could potentially be useful in long-distance quantum communication schemes due to their robustness to scattering.

The Trotter product formula is a key instrument in numerical simulations of quantum systems. However, computers cannot deal with continuous degrees of freedom, such as the position of particles in molecules, or the amplitude of electromagnetic fields. It is therefore necessary to discretize these variables to make them amenable to digital simulations. Here, we give sufficient conditions to conclude the validity of this approximate discretized physics. Essentially, it depends on the state-dependent Trotter error, for which we establish explicit bounds that are also of independent interest.

We study fluctuations in many-body quantum heat engines operating in the presence of collective system-bath interactions. We show that collective effects in open quantum systems can be harnessed to develop highly consistent many-body quantum engines. We consider quantum Otto engines, modeled by $n$ spins collectively coupled to thermal baths. Our results show that collective effects can significantly reduce the fluctuations in the output work, quantified by high reliability ($r$) and low thermodynamic uncertainty. In contrast to independent engines, we demonstrate a quadratic enhancement of the reliability $r$ for their collective counterparts. We extend our analysis to the case of interacting spin models commonly studied in many-body physics, such as the Lipkin-Meshkov-Glick (LMG) model, thereby broadening the regime of applicability of collective effects in quantum thermal machines significantly. This paves the way forward for realistic collective quantum thermal machines in many-body systems.

We study the change in multiparticle entanglement if one particle becomes classical, in the sense that this particle is destructed by a measurement but the gained information is encoded into a new register. We present an estimation of this change for different entanglement measures and ways of encoding. We first simplify the numerical calculation to analyze the change in entanglement under classicalization in special cases. Second, we provide general upper and lower bounds on the entanglement change. Third, we show that the entanglement change caused only by classicalization of one qubit can still be arbitrarily large. Finally, we discuss cases where no entanglement is left under classicalization for any possible measurement. Our results shed light on the storage of quantum resources and help to develop a different direction in the field of quantum resource theories.

We show that Minkowski single-particle states localized beyond the horizon modify the Unruh thermal distribution in an accelerated frame. This means that, contrary to classical predictions, accelerated observers can reveal particles emitted beyond the horizon. The method we adopt is based on deriving the explicit Wigner characteristic function for the complete description of the quantum field in the noninertial frame and can be generalized to general states.

We report on the experimental realization of a bosonic quantum degenerate mixture of highly magnetic ${}^{168}\mathrm{Er}$ and nonmagnetic ${}^{174}\mathrm{Yb}$. Quantum degeneracy is reached by forced evaporation in an all-optical trap. Formation of the two Bose-Einstein condensates is confirmed by analysis of the cloud shape and the observed inversions of the aspect ratios. The results open a path for possible new experiments on magnetic and nonmagnetic impurity physics as well as on the quantum chaotic behavior of Feshbach resonances and their dependencies on minor variations of the reduced masses.

By studying the effects of quadratic anisotropy and quartic perturbations on two-dimensional harmonic oscillators, one arrives at a simple model, termed here the Ince oscillator, whose analytic solutions are given in terms of Ince polynomials. This one model unifies diverse physical systems, including aberrated optical cavities that are shown to support Ince-Gauss beams as their modes, and the two-mode Bose-Hubbard dimer describing two coupled superfluids. The Ince oscillator model describes a topological transition which can have very different origins: in the optical case, which is fundamentally linear, it is driven by the ratio of astigmatic to spherical mirror aberrations, whereas in the superfluid case it is driven by the ratio of particle tunneling to interparticle interactions and corresponds to macroscopic quantum self-trapping.

We study the antidistinguishability problem, which is a fundamental task in quantum computing. A set of $d$ quantum states is said to be antidistinguishable if there exists a $d$-outcome positive-operator-valued measure that can perfectly identify which state was not measured. We revisit a conjecture by Havlíçek and Barrett which states that if a set of $d$ pure states has small pairwise inner products, then the set must be antidistinguishable. We develop a certificate of antidistinguishability via semidefinite programming duality and use it to provide a counterexample to this conjecture when $d=4$. Our work thus opens up again the investigation into which sets of pure states are antidistinguishable.

We study the quantized topological response of trapped one-dimensional quantum gases, which involves applying an optical pulse to a half-infinite region in an asymptotically harmonic trap and measuring the resulting density distribution. We prove that the corresponding linear response is described by a universal quantized formula in the thermodynamic limit, which is invariant under local continuous deformations of the trapping potential $V$, atom distribution $f_{\mathrm{\Lambda }}$, the spatial envelope of the optical pulse ${\mathrm{\Theta }}_p$, and the measurement region ${\mathrm{\Theta }}_m$. Our numerical analysis confirms this prediction with high accuracy, and we show that a short but finite optical pulse duration only causes a violation of quantization near the transition time. Our work presents an exciting avenue for exploring quantized topological phenomena in trapped quantum gases.

We address the problem of optimal quantum state preparation. We propose a protocol based on a stroboscopic driving of the system in its parameter space combined with repeated measurementlike interactions with an external spectator system. In the limit of infinite-rate interactions, the protocol naturally yields unit fidelity due to the quantum Zeno effect. For realistic finite-rate interactions we show that the fidelity is maximized if the discretized driving trajectory has a minimal geometric length and keeps a constant speed with respect to the Provost-Vallee metric in the parameter space. We numerically test the protocol in an interacting multiqubit system, demonstrating its possible dominance over the coherent driving. Our results can be used in various quantum information applications.

Reaching a high cavity population with a coherent pump in the strong-coupling regime of a single-atom laser is impossible due to the photon blockade effect. In this Letter, we experimentally demonstrate that in a single-atom maser based on a transmon strongly coupled to two resonators, it is possible to pump over a dozen photons into the system. The first high-quality resonator plays the role of a usual lasing cavity, and the second one presents a controlled dissipation channel, bolstering population inversion, and modifies the energy-level structure to lift the blockade. As confirmation of the lasing action, we observe conventional laser features such as a narrowing of the emission linewidth and external signal amplification. Additionally, we report unique single-atom features: self-quenching and several lasing thresholds.

We report an experimental simulation of charging process of an $XXZ$ Heisenberg Hamiltonian-driven quantum battery by using single photons and linear optics. We find entanglement is not always the most important resource to boost charging, while coherence plays a nontrivial role. We construct a two-qubit quantum battery and investigate the performance of quantum batteries and their relationship with the amount of entanglement and coherence that arise during the charging process. Our finding offers insights into the design of more efficient quantum devices and advances our understanding of quantum resources.

We report the observation of coherent oscillations in conversion efficiency of weakly bound dimers formed from a thermal gas of ultracold atoms. Finite thermal energy of the gas causes loss of coherence when a broad continuum is resonantly coupled to a discrete bound state. Restoration of the coherence can be achieved through nonadiabatic transitions of the dressed molecular energy level that are induced by a strong modulation pulse with fast envelope dynamics. Conditions to observe coherent oscillations are verified and control of their properties is demonstrated. The main experimental findings are supported by theoretical modeling and numerical calculations. The observed results may lead to a renewed interest in general studies of a discrete energy level coupled to a broadband continuum when the properties of both are fully controlled.

Motivated by recent progress in the generation of optical spatiotemporal vortex pulses (STVPs), there is a theoretical discussion about the transverse orbital angular momentum (OAM) carried by such pulses. Two recent works [K. Y. Bliokh, Phys. Rev. Lett. 126, 243601 (2021)] and [S. W. Hancock et al., Phys. Rev. Lett. 127, 193901 (2021)] claimed the OAM values which differ by a factor of 2 for circular STVPs. Here we resolve this controversy by showing that the result by Hancock et al. is correct for the total OAM, while the result by Bliokh describes the suitably defined intrinsic part of the OAM. The other, extrinsic part of the OAM originates from the fact that plane waves of the same amplitude but different frequencies in the pulse spectrum contain different densities of photons, which induces a transverse vortex-dependent shift of the photon centroid even in a STVP with symmetric energy-density distribution. We describe similar peculiarities of acoustic and quantum-relativistic (Klein-Gordon and arbitrary-spin) STVPs. In all cases, only the intrinsic OAM keeps a universal form independent of the details of the problem and similar to the OAM of monochromatic vortex beams.

We study the thermodynamic liquid-gas transition and coexistence (LGTC) for ground-state bosons under contact interactions. We find that the LGTC can be facilitated by the mismatch of spin polarization, dubbed “spin twist,” between single-particle and interaction channels of bosons with spin degrees of freedom. Such a spin twist uniquely stabilizes the gas phase by creating an effective repulsion for low-density bosons, thereby enabling LGTC in the presence of a quantum droplet at a much larger density. We have demonstrated the scheme for binary bosons subject to Rabi coupling and magnetic detuning, where the liquid-gas transition can be conveniently tuned and their coexistence can be characterized by a discontinuous density profile in a harmonic trap. The spin twist scheme for LGTC can be generalized to a wide class of quantum systems with competing single-particle and interaction orders.

We consider nonequilibrium dynamics of two initially independent reservoirs $A$ and $B$ filled with a cold Fermi gas coupled and decoupled by two quantum quenches following one another. We find that the von Neumann entropy production induced by the quench is faster than thermal transport between the reservoirs and defines the short-time dynamics of the system. We analyze the energy change in the system which adds up the heat transferred between $A$ and $B$ and the work done by the quench to uncouple the reservoirs. In the case when $A$ and $B$ interact for a short time, we notice an energy increase in both reservoirs upon decoupling. This energy gain results from the quench's work and does not depend on the initial temperature imbalance between the reservoirs. We relate the quench's work to the mutual correlations of $A$ and $B$ expressed through their von Neumann entropies. Utilizing this relation, we show that once $A$ and $B$ become coupled, their entropies grow (on a timescale of the Fermi time) faster than the heat flow within the system. This result may provide a track of quantum correlations' generation at finite temperatures which one may probe in ultracold atoms, where we expect the characteristic timescale of correlations' growth to be $\sim 0.1\mathrm{ms}$.

We investigate an ultracold mixture of Bose gases interacting via spin-changing collisions by studying the dynamics of spin fluctuations. The experimental implementation employs ${}^{23}\mathrm{Na}$ and ${}^7\mathrm{Li}$ atoms, which are prepared out of equilibrium across a wide range of initial conditions. We identify three regimes in the dynamics of the system for different initial states: a long-lived metastable regime, an instability range with strong growth of fluctuations, and a regime approaching thermal equilibrium. Theoretical modeling of the data allows us to reconstruct effective potentials which characterize the different dynamical regimes of the system.

We studied the recall efficiency as a function of optical depth of rephased amplified spontaneous emission (RASE), a protocol for generating entangled light. The experiments were performed on the ${}^3\mathrm{H}_4\to {}^1\mathrm{D}_2$ transition in the rare-earth doped crystal ${\mathrm{Pr}}^{3+}$:${\mathrm{Y}}_2{\mathrm{SiO}}_5$, using a four-level echo sequence between four hyperfine levels to rephase the emission. The efficiency of RASE was observed to increase from $3\%$ to $14\%$ as the optical depth was reduced from 2.0 to 0.8. This is a significant improvement over the previously reported nonclassical result but is well short of the predicted efficiency. We discuss the possible mechanisms limiting the protocol's performance, and suggest ways to overcome these limits.

Spectator qubits (SQs) are a tool to mitigate noise in hard-to-access data qubits. The SQ, designed to be much more sensitive to the noise, is measured frequently, and the accumulated results used rarely to correct the data qubits. For the hardware-relevant example of dephasing from random telegraph noise, we introduce a Bayesian method employing complex linear maps which leads to a plausibly optimal adaptive measurement and control protocol. The suppression of the decoherence rate is quadratic in the SQ sensitivity, establishing that the SQ paradigm works arbitrarily well in the right regime.

Breakthrough experiments have newly explored the fascinating physics of dipolar quantum droplets and supersolids. The recent realization of dipolar mixtures opens further intriguing possibilities. We show that under rather general conditions, the presence of a second component catalyzes droplet nucleation and supersolidity in an otherwise unmodulated condensate. Droplet catalyzation in miscible mixtures, which may occur even for a surprisingly small impurity doping, results from a local roton instability triggered by the doping-dependent modification of the effective dipolar strength. The catalyzation mechanism may trigger the formation of a two-fluid supersolid, characterized by a generally different superfluid fraction of each component, which opens intriguing possibilities for the future study of spin physics in dipolar supersolids.