Out-of-time-ordered correlators (OTOCs) have been proposed as a tool to witness quantum information scrambling in many-body system dynamics. These correlators can be understood as averages over nonclassical multitime quasiprobability distributions (QPDs). These QPDs have more information and their nonclassical features witness quantum information scrambling in a more nuanced way. However, their high dimensionality and nonclassicality make QPDs challenging to measure experimentally. We focus on the topical case of a many-qubit system and show how to obtain such a QPD in the laboratory using circuits with three and four sequential measurements. Averaging distinct values over the same measured distribution reveals either the OTOC or parameters of its QPD. Stronger measurements minimize experimental resources despite increased dynamical disturbance.

We introduce an iterative method to search for time-optimal Hamiltonians that drive a quantum system between two arbitrary, and in general mixed, quantum states. The method is based on the idea of progressively improving the efficiency of an initial, randomly chosen Hamiltonian, by reducing its components that do not actively contribute to driving the system. We show that our method converges rapidly even for large-dimensional systems and that its solutions saturate any attainable bound for the minimal time of evolution. We provide a rigorous geometric interpretation of the iterative method by exploiting an isomorphism between geometric phases acquired by the system along a path and the Hamiltonian that generates it. Our method is directly applicable as a powerful tool for state preparation and gate design problems.

We present a joint experimental and theoretical study of spin coherence properties of ${}^{39}\mathrm{K}$, ${}^{85}\mathrm{Rb}$, ${}^{87}\mathrm{Rb}$, and ${}^{133}\mathrm{Cs}$ atoms trapped in a solid parahydrogen matrix. We use optical pumping to prepare the spin states of the implanted atoms and circular dichroism to measure their spin states. Optical-pumping signals show order-of-magnitude differences depending on both matrix growth conditions and atomic species. We measure the ensemble transverse relaxation times ($T_2^{*}$) of the spin states of the alkali-metal atoms. Different alkali species exhibit dramatically different $T_2^{*}$ times, ranging from submicrosecond coherence times for high-$m_F$ states of ${}^{87}\mathrm{Rb}$ to $\sim {10}^2$ microseconds for ${}^{39}\mathrm{K}$. These are the longest ensemble $T_2^{*}$ times reported for an electron spin system at high densities ($n\gtrsim {10}^{16}{\mathrm{cm}}^{-3}$). To interpret these observations, we develop a theory of inhomogeneous broadening of hyperfine transitions of ${}^{2}S$ atoms in weakly interacting solid matrices. Our calculated ensemble transverse relaxation times agree well with experiment, and suggest ways to longer coherence times in future work.

We use a master equation to study the dynamics of two coupled macroscopic quantum systems (e.g., a Josephson junction made of two Bose-Einstein condensates or two spin states of an ensemble of trapped ions) subject to a weak continuous measurement. If the coupling between the two systems is suddenly switched on, the resulting dynamics leads to caustics (fold and cusp catastrophes) in the number-difference probability distribution and at the same time the measurement gradually induces a quantum-to-classical transition. Decoherence is often invoked to help resolve paradoxes associated with macroscopic quantum mechanics, but here, on the contrary, caustics are well behaved in the quantum (many-particle) theory and divergent in the classical (mean-field) theory. Caustics thus represent a breakdown of the classical theory towards which decoherence seems to inevitably lead. We find that measurement backaction plays a crucial role in softening the resulting singularities and calculate the modification to the Arnol'd index which governs the scaling of the caustic's amplitude with the number of atoms. The Arnol'd index acts as a critical exponent for the formation of singularities during quantum dynamics and its modification by the open nature of the system is analogous to the modification of the critical exponents of phase transitions occurring in open systems.

We theoretically investigate the excitation dynamics in a photon fluid with both local and nonlocal interactions. We show that the interplay between locality and an infinite-range nonlocality gives rise to a gapped Bogoliubov spectrum of elementary excitations which, at lower momenta, correspond to massive particles (phonons) with a relativistic energy-momentum relation. In this regime and in the presence of an inhomogeneous flow the density fluctuations are governed by the massive Klein-Gordon equation on the acoustic metric and thus propagate as massive scalar fields on a curved spacetime. We finally demonstrate that in the nonrelativistic limit the phonon modes behave as self-gravitating quantum particles with an effective Schrödinger-Newton dynamics, although with a finite-range gravitational interaction and a nonzero cosmological constant. Our photon fluid represents a viable alternative to Bose-Einstein condensate models for “emergent gravity” scenarios and offers a promising setting for analog simulations of semiclassical gravity and quantum gravity phenomenology.

The quest of demonstrating beneficial quantum error correction in near-term noisy quantum processors can benefit enormously from a low-resource optimization of fault-tolerant schemes, which are specially designed for a particular platform considering both state-of-the-art technological capabilities and main sources of noise. In this work we show that flag-qubit-based fault-tolerant techniques for active error detection and correction, as well as for encoding of logical qubits, can be leveraged in current designs of trapped-ion quantum processors to achieve this break-even point of beneficial quantum error correction. Our improved description of the relevant sources of noise, together with detailed schedules for the implementation of these flag-based protocols, provide one of the most complete microscopic characterizations of a fault-tolerant quantum processor to date. By extensive numerical simulations, we provide a comparative study of flag- and cat-based approaches to quantum error correction, and show that the superior performance of the former can become a landmark in the success of near-term quantum computing with noisy trapped-ion devices.

Reverse annealing is a relatively new variant of quantum annealing in which one starts from a classical state and increases and then decreases the amplitude of the transverse field, in the hope of finding a better classical state than the initial state for a given optimization problem. We numerically study the unitary quantum dynamics of reverse annealing for the mean-field-type $p$-spin model and show that the results are consistent with the predictions of equilibrium statistical mechanics. In particular, we corroborate the equilibrium analysis prediction that reverse annealing provides an exponential speedup over conventional quantum annealing in terms of solving the $p$-spin model. This lends support to the expectation that equilibrium analyses are effective at revealing essential aspects of the dynamics of quantum annealing. We also compare the results of quantum dynamics with the corresponding classical dynamics to reveal their similarities and differences. We distinguish between two reverse annealing protocols we call adiabatic and iterated reverse annealing. We further show that iterated reverse annealing, as has been realized in the D-Wave device, is ineffective in the case of the $p$-spin model but note that a recently introduced protocol (“$h$-gain”), which implements adiabatic reverse annealing, may lead to improved performance.

We propose a protocol for improving quantum entanglement based on a quantum scissor scheme [D. T. Pegg et al., Phys. Rev. Lett. 81, 1604 (1998)]. Compared to existing protocols for entanglement improvement, our scheme does not require biside operation on two-mode squeezed vacuum states. This greatly enhances the success probability as well as entanglement, while reducing the resources. A squared gain factor can be obtained from our scheme. In addition, our scheme is robust against the decoherence when considering more realistic cases. Finally, a comparison between the single-side quantum scissor scheme and the single-side quantum catalysis is also investigated.

Harnessing quantum processes is an efficient method to generate truly indeterministic random numbers, which are of fundamental importance for cryptographic protocols, security applications, or Monte Carlo simulations. Recently, quantum random-number generators based on continuous variables have gathered a lot of attention due to the potentially high bit rates they can deliver. Especially quadrature measurements on shot-noise-limited states have been studied in detail as they do not offer any side information to potential adversaries under ideal experimental conditions. However, they may be subject to additional classical noise beyond the quantum limit, which may become a source of side information for eavesdroppers. While such eavesdropping attacks have been investigated in theory in some detail, experimental studies are still rare. We experimentally realize a continuous-variable eavesdropping attack, based on heterodyne detection, on a trusted quantum random-number generator and discuss the limitations for secure random-number generation that arise.

The recent experimental advancement to realize ultracold gases scattering off an eightfold optical potential [K. Viebahn, M. Sbroscia, E. Carter, J.-C. Yu, and U. Schneider, Phys. Rev. Lett. 122, 110404 (2019)] heralds the beginning of a new technique to study the properties of quasicrystalline structures. Quasicrystals possess long-range order but are not periodic, and are still little studied in comparison to their periodic counterparts. Here, we consider an ultracold bosonic gas in an eightfold symmetric lattice and assume a toy model where the atoms occupy the ground states of the local minima of the potential. The ground-state phases of the system are studied, with particular interest in the local nature of the phases. The usual Mott-insulator, density wave, and supersolid phases of the standard and extended Bose-Hubbard model are observed. For nonzero long-range interactions, we find that density wave states can spontaneously break the eightfold symmetry, and may even possess no rotational symmetry. We find the local variation in the number of nearest neighbors to play a vital role in the phase transitions, local structure, and global symmetries of the ground states. This variation in the number of nearest neighbors is not a unique property of the considered eightfold lattice, and we expect our results to be generalizable to any quasicrystalline potential where there are only small position dependent variations in the site energy, tunneling, and interactions.

Signatures of quantum transport are expected to quickly vanish as dissipation is introduced in a system. This dissipation can take several forms, including that of particle loss, which has the consequence that the total probability current is not conserved. Here, we study the effect of such losses at a quantum point contact (QPC) for ultracold atoms. Experimentally, dissipation is provided by a near-resonant optical tweezer the power and detuning of which control the loss rates for the different internal atomic states as well as their effective Zeeman shifts. We theoretically model this situation by including losses in the Landauer-Büttiker formalism over a wide range of dissipative rates. We find good agreement between our measurements and our model, both featuring robust conductance plateaus. Finally, we are able to map out the atomic density by varying the position of the near-resonant tweezer inside the QPC, realizing a dissipative scanning gate microscope for cold atoms.

We study the heating time in periodically driven $D$-dimensional systems with interactions that decay with the distance $r$ as a power law $1/r^{\alpha }$. Using linear-response theory, we show that the heating time is exponentially long as a function of the drive frequency for $\alpha >D$. For systems that may not obey linear-response theory, we use a more general Magnus-like expansion to show the existence of quasiconserved observables, which imply exponentially long heating time, for $\alpha >2D$. We also generalize a number of recent state-of-the-art Lieb-Robinson bounds for power-law systems from two-body interactions to $k$-body interactions and thereby obtain a longer heating time than previously established in the literature. Additionally, we conjecture that the gap between the results from the linear-response theory and the Magnus-like expansion does not have physical implications, but is, rather, due to the lack of tight Lieb-Robinson bounds for power-law interactions. We show that the gap vanishes in the presence of a hypothetical, tight bound.

The possibility of creating crystal bilayers twisted with respect to each other has led to the discovery of a wide range of novel electron correlated phenomena the full understanding of which is still under debate. Here we propose and analyze a method to simulate twisted bilayers using cold atoms in state-dependent optical lattices. Our proposed setup can be used as an alternative platform to explore twisted bilayers which allows one to control the inter- and intralayer coupling in a more flexible way than in the solid-state realizations. We focus on square geometries but also show how it can be extended to simulate other lattices which show Dirac-like physics. This setup opens a path to observe similar physics, e.g., band narrowing, with larger twist angles, to rule out some of the mechanisms to explain the observed strongly correlated effects, as well as to study other phenomena difficult to realize with crystals. As an example of the latter we explore the quantum optical consequences of letting emitters interact with twisted-bilayer reservoirs, and predict the appearance of unconventional radiation patterns and emitter interactions following the emergent Moiré geometry.

Major obstacles against efficient long-distance quantum communication are photon losses during transmission and the probabilistic nature of Bell measurement causing exponential scaling in time and resource with distance. To overcome these difficulties, while conventional quantum repeaters require matter-based operations with long-lived quantum memories, recent proposals have employed encoded multiple photons in entanglement, providing an alternative way for scalability. In pursuing scalable quantum communications, naturally arising questions are thus whether any ultimate limit exists in all-optical scalability and whether and how it can be achieved. Motivated by these questions, we derive the fundamental limits of the efficiency and loss tolerance of the Bell measurement with multiple photons, restricted not by protocols but by the laws of physics, i.e., linear optics and no-cloning theorem. We then propose a Bell measurement scheme with linear optics, which enables one to reach both the fundamental limits: one by linear optics and the other by the no-cloning theorem. The quantum repeater based on our scheme allows one to achieve fast and efficient quantum communication over arbitrary long distances, outperforming previous all-photonic and matter-based protocols. Our work provides a fundamental building block for quantum networks within but toward the ultimate limits of all-optical scalability.

We develop an efficient variational approach to studying dynamics of a localized quantum spin coupled to a bath of mobile spinful bosons. We use parity symmetry to decouple the impurity spin from the environment via a canonical transformation and reduce the problem to a model of the interacting bosonic bath. We describe coherent time evolution of the latter using bosonic Gaussian states as a variational ansatz. We provide full analytical expressions for equations describing variational time evolution that can be applied to study in- and out-of-equilibrium phenomena in a wide class of quantum impurity problems. In the accompanying paper [Ashida et al., Phys. Rev. Lett. 123, 183001 (2019)], we present a concrete application of this general formalism to the analysis of the Rydberg central spin model, in which the spin-1/2 Rydberg impurity undergoes spin-changing collisions in a dense cloud of two-component ultracold bosons. To illustrate new features arising from orbital motion of the bath atoms, we compare our results to the Monte Carlo study of the model with spatially localized bosons in the bath, in which random positions of the atoms give rise to random couplings of the standard central spin model.

Surface-enhanced Raman scattering (SERS) allows for detection and identification of molecular vibrational fingerprints in minute sample quantities. The SERS process can also be exploited for optical manipulation of molecular vibrations. We present a quantum description of surface-enhanced resonant Raman scattering, in analogy to hybrid cavity optomechanics, and compare the resonant situation with the off-resonant SERS. Our model predicts the existence of a regime of coherent interaction between electronic and vibrational degrees of freedom of a molecule, mediated by a plasmonic nanocavity. This coherent mechanism can be achieved by parametrically tuning the frequency and intensity of the incident pumping laser and is related to the optomechanical pumping of molecular vibrations. We find that vibrational pumping is able to selectively activate a particular vibrational mode, thus providing a mechanism to control its population and drive plasmon-assisted chemistry.

We study the use of ensembles of quantum two-level systems as batteries to store and release radiative energy. Collective coupling to a tuneable cavity mode leads to enhanced charging power and a means to recover the energy on demand in a controllable manner. Individual decay and decoherence of the emitters occur on a much slower timescale, but they gradually cause population of states with lower symmetry and similar to the aging of rechargeable household batteries, they reduce the efficiency of the battery with time.

We investigate the relation between a refined version of Leggett and Garg conditions for macrorealism, namely, the no-signaling-in-time (NSIT) conditions, and the quantum mechanical notion of nondisturbance between measurements. We show that all NSIT conditions are satisfied for any state preparation if and only if certain compatibility criteria on the state-update rules relative to the measurements, i.e., quantum instruments, are met. The systematic treatment of NSIT conditions supported by structural results on nondisturbance provides a unified framework for the discussion of the the clumsiness loophole. This extends previous approaches and allows for a tightening of the loophole via a hierarchy of tests able to disprove a larger class of macrorealist theories. Finally, we discuss perspectives for a resource theory of quantum mechanical disturbance related to violations of macrorealism.

We apply a Bloch-band approach to the analysis of pulsed optical standing wave diffractive elements in optics and interferometry with ultracold atoms. We verify our method by comparison to a series of experiments with Bose-Einstein condensates. The approach provides accurate Rabi frequencies for diffraction pulses and is particularly useful for the analysis and control of diffraction phases, an important systematic effect in precision atom interferometry. Utilizing this picture, we also demonstrate a method to determine atomic band structure in an optical lattice through a measurement of phase shifts in an atomic contrast interferometer.

We theoretically analyze the problem of detecting the position of a classical dipolar scatterer in a strongly focused optical field. We suggest an optimal measurement scheme and show that it resolves the scatterer's position in three dimensions at the Heisenberg limit of the imprecision-backaction product. We apply our formalism to levitated-optomechanics experiments and show that backscattering detection provides sufficient information to feedback cool the particle's motion along the optical axis to a phonon occupancy below unity under realistic experimental conditions.