We experimentally address emergent spectrospatial correlations in a one-dimensional amplifying system that sustains Anderson-localized lasing modes. The structure is based on a periodic-on-average random template, and hence follows the frequency behavior determined by passbands and band gaps of a photonic crystal. A spectrospatial correlation map constructed for weak disorder reveals strong correlations and anticorrelations that extend over multiple band gaps. The origin of the correlation is traced to two parameters, namely the probability of generation of modes, and the congruence in the mode profiles of corresponding modes. Specific locations on the correlation map are identified that reflect the relative contribution of the two parameters. The experimental setup with a fine control on disorder strength allowed us to capture the systematic evolution of the correlation map with disorder, which uncovers an interesting variation in spectrospatial correlations in such Anderson-localized lasing systems.

Over the last few years it was pointed out that certain observables of time-evolving quantum systems may have singularities at certain moments in time, mimicking the singularities physical systems have when undergoing phase transitions. These were given the name of dynamical phase transitions. They were shown to exist in certain integrable (exactly solvable) quantum systems, and were seen numerically and experimentally in some models which were not integrable. The “universality classes” of such singularities were not yet convincingly established, however. We argue that random field Ising models feature singularities in time which may potentially be present in a wider variety of quantum systems, in particular in those which are many-body localized, and describe these singularities in detail analytically.

We experimentally investigate the non-Markovianity problems in a quantum channel implemented by an all-optical setup. The quantum channel simulates the dynamics of a two-level (qubit) system surrounded by bosonic modes, with the system-environment interaction treated without applying the rotating-wave approximation (RWA). Therefore, it allows us to demonstrate the important role of counterrotating-wave terms in influencing the memory effects of the environment. The experimental and theoretical results are in good agreement and show quite different behaviors of the non-Markovianity measure between RWA and non-RWA channels. One can find the nonmonotonic dependence of the non-Markovianity measure on the coupling and detuning parameters in the non-RWA case, which is attributed to the competing mechanisms caused by the counterrotating-wave terms. We emphasize that the non-RWA channel provides a more clear description of the open-system dynamics and the counterrotating-wave terms are indispensable in the study of non-Markovianity.

A non-Hermitian topological insulator with a real spectrum is interesting in the theory of non-Hermitian extension of topological systems. We find an experimentally realizable example of a two-dimensional non-Hermitian topological insulator with a real spectrum. We consider the two-dimensional Su-Schrieffer-Heeger (SSH) model with gain and loss. We introduce a non-Hermitian polarization vector to explore topological phase and show that topological edge states in the band gap exist in the system.

Polarization of photons is the key physical quantity in measurements of quantum spin correlations. According to a variant of quantum and classical electrodynamics formulated by us on the grounds of a hypothesis of existence of a preferred frame of reference (PF) a modification of the Malus law is expected if photonic states are frame dependent. A search for a quantum PF of the photon has been conducted by way of dedicated measurements of optical rotation using a high-precision commercial polarimeter. The data yield an upper limit of the PF velocity with respect to the Earth of $3.0\mathrm{km}/\mathrm{s}$ at the time of the measurements, allowing us to reject the hypothesis that the cosmic microwave background radiation frame might be a PF for the photon.

We generalize the approach by Braunstein and Caves, [Phys. Rev. Lett. 72, 3439 (1994)]. to quantum multiparameter estimation with general states. We derive a matrix bound of the classical Fisher information matrix due to each measurement operator. The saturation of all these bounds results in the saturation of the matrix Helstrom Cramér-Rao bound. Remarkably, the saturation of the matrix bound is equivalent to the saturation of the scalar bound with respect to any given positive definite weight matrix. Necessary and sufficient conditions are obtained for the optimal measurements that give rise to the Helstrom Cramér-Rao bound associated with a general quantum state. To saturate the Helstrom bound with separable measurements or collective measurement entangling only a small number of identical states, we find it is necessary for the symmetric logarithmic derivatives to commute on the support of the state. As an important application of our results, we construct several local optimal measurements for the problem of estimating the three-dimensional separation of two incoherent optical point sources.

In this article we study lossless compression of strings of pure quantum states of indeterminate-length quantum codes which were introduced by Schumacher and Westmoreland. Past work has assumed that the strings of quantum data are prepared to be encoded in an independent and identically distributed way. We discuss the notion of quantum stochastic ensembles, allowing us to consider strings of quantum states prepared in a general way. For any quantum stochastic ensemble we define an associated quantum dynamical system and prove that the optimal average codeword length via lossless coding is equal to the quantum dynamical entropy of the associated quantum dynamical system.

We propose a theoretical scheme to investigate the dynamics of quantum correlation between two nitrogen-vacancy-center ensembles (NVEs) coupled to a common superconducting coplanar waveguide resonator, driven by a broadband microwave squeezed field working as a squeezed-vacuum reservoir. Based on the reduced master equation for NVEs by the adiabatical elimination method in the bad-resonator limit, our results reveal quantum correlation preservation due to the interplay between the nonclassical feature of the squeezing reservoirs and the original dissipation mechanisms. The required operations are very close to the capabilities of current superconducting circuit-QED techniques. Our work may open interesting perspectives for devising active decoherence-immune quantum information devices.

The advantages of quantum information processing are in many cases obtained as consequences of quantum interactions, especially for computational tasks where two-qubit interactions are essential. In this work, we establish the framework of analyzing and quantifying loss or gain of information on a quantum system when the system interacts with its environment. We show that the information flow, the theoretical method of characterizing (non-)Markovianity of quantum dynamics, corresponds to the rate of the minimum uncertainty about the system given quantum side information. Thereafter, we analyze the information exchange among subsystems that are under the performance of quantum algorithms, in particular, the amplitude amplification algorithms where the computational process relies fully on quantum evolution. Different realizations of the algorithm are considered, such as (i) quantum circuits, (ii) analog computation, and (iii) adiabatic computation. It is shown that, in all the cases, our formalism provides insight into the process of amplifying the amplitude from the information flow or leakage on the subsystems.

Randomized benchmarking (RB) is an efficient and robust method to characterize gate errors in quantum circuits. Averaging over random sequences of gates leads to estimates of gate errors in terms of the average fidelity. These estimates are isolated from the state preparation and measurement errors that plague other methods such as channel tomography and direct fidelity estimation. A decisive factor in the feasibility of randomized benchmarking is the number of sampled sequences required to obtain rigorous confidence intervals. Previous bounds were either prohibitively loose or required the number of sampled sequences to scale exponentially with the number of qubits in order to obtain a fixed confidence interval at a fixed error rate. Here, we show that, with a small adaptation to the randomized benchmarking procedure, the number of sampled sequences required for a fixed confidence interval is dramatically smaller than could previously be justified. In particular, we show that the number of sampled sequences required is essentially independent of the number of qubits and scales favorably with the average error rate of the system under investigation. We also investigate the fitting procedure inherent to randomized benchmarking in light of our results and find that standard methods such as ordinary least squares optimization can give misleading results. We therefore recommend moving to more sophisticated fitting methods such as iteratively reweighted least squares optimization. Our results bring rigorous randomized benchmarking on systems with many qubits into the realm of experimental feasibility.

It is commonly thought that thermal photons with transverse electric polarization cannot exist in a planar metallic cavity whose size $a$ is smaller than the thermal wavelength ${\lambda }_T$, due to the absence of modes with $\lambda <2a$. Computations based on a realistic model of the mirrors contradict this expectation and show that a micron-sized metallic cavity is filled with nonresonant radiation having transverse electric polarization, following a non-Planckian spectrum, whose average density at room temperature is orders of magnitudes larger than that of a blackbody. We show that the spectrum of this radiation can be measured by observing the transition rates between hyperfine ground-state sublevels $1S_{1/2}(F,m_F)\to 1S_{1/2}(F^{\prime },m_F^{\prime })$ of D atoms passing in the gap between the mirrors of a Au cavity. Such a measurement would also shed light on a puzzle in the field of dispersion forces, regarding the sign and magnitude of the thermal Casimir force. Recent experiments with Au surfaces led to contradictory results, whose interpretation is highly controversial.

The $4f^6{}^7{F}_0\to 4f^6{}^5{D}_0$ intraconfiguration transition in $\mathrm{Sm}:{\mathrm{SrF}}_2$ is forbidden for ${\mathrm{Sm}}^{2+}$ ions in the octahedrally symmetric substitution sites in ${\mathrm{SrF}}_2$. We report the direct observation of this transition using laser-induced fluorescence at cryogenic temperatures, and measurements of the excited-state lifetime and the excitation cross section. To the best of our knowledge, this optical transition has the longest-lived excited state ever observed in a solid.

Measurement of the branching ratios for $6P_{1/2}$ decays to $6S_{1/2}$ and $5D_{3/2}$ in ${}^{138}\mathrm{Ba}^{+}$ are reported with the decay probability from $6P_{1/2}$ to $5D_{3/2}$ measured to be $p=0.268177\pm {\left(37\right)}_{\mathrm{stat}}–{\left(20\right)}_{\mathrm{sys}}$. This result differs from a recent report by $12\sigma$. A detailed account of systematics is given, and the likely source of the discrepancy is identified. The new value of the branching reported here is combined with previous experimental results to give a new estimate of $\tau =7.855\left(10\right)\mathrm{ns}$ for the $6P_{1/2}$ lifetime. In addition, ratios of matrix elements calculated from theory are combined with experimental results to provide improved theoretical estimates of the $6P_{3/2}$ lifetime and the associated matrix elements.

The confined variational method is applied to investigate the low-energy elastic scattering of orthopositronium from ${\text{H}}_2$ by first-principles quantum mechanics. Describing the correlations with explicitly correlated Gaussians, we obtain accurate $s$-wave phase shifts and pickoff annihilation parameters for different incident momenta. By a least-squares fit of the data to the effective-range theory, we determine the $s$-wave scattering length $A_S=2.02a_0$ and the zero-energy value of the pickoff annihilation parameter, ${}^1{Z}_{\text{eff}}=0.1839$. The obtained ${}^1{Z}_{\text{eff}}$ agrees well with the precise experimental value of 0.186(1) [G. L. Wright et al., J. Phys. B 16, 4065 (1983)] and the obtained $A_S$ agrees well with the value of $2.1\left(2\right)a_0$ estimated from the average experimental momentum-transfer cross section for positronium energy below 0.3 eV [F. Saito et al., J. Phys. B 36, 4191 (2003)].

Electron-impact ionization of an atom or ion in the presence of a neighboring atom is studied. The latter is first collisionally excited by the incident electron, whose energy is assumed to be high but nonrelativistic. Afterwards, the excitation energy is transferred radiationlessly via a two-center Auger process to the other atom or ion, leading to its ionization. We show that the participation of the neighboring atom manifests in a very pronounced resonance peak in the energy-differential cross section and can substantially enhance the total cross section of electron-impact ionization. We also discuss the influence of the neighboring atom on the angular distribution of the ejected electron.

An ultrafast electro-optic effect of the zinc sulfide crystal is predicted employing a numerical pump-probe simulation. The numerical results indicate that the time dependence of the off-diagonal part of the dielectric function of ZnS exhibits a phase shift with respect to the pump laser field. The phase shift coincides with the time-resolved dynamical Franz-Keldysh effect, which is the modulation of the isotropic part of the dielectric function. While the probe frequency dependence around the band gap is not intense, it becomes intense at higher photon energies of approximately 42 eV.

We investigate the low-energy dynamics of two coupled anisotropic Bose-Einstein condensates forming a long Josephson junction. The theoretical study is performed in the framework of the two-dimensional Gross-Pitaevskii equation and the Bogoliubov–de Gennes formalism. We analyze the excitation spectrum of the coupled Bose condensates and show how low-energy excitations of the condensates lead to multiple-frequency oscillations of the atomic populations in the two wells. This analysis generalizes the standard bosonic Josephson equation approach. We also develop a one-dimensional hydrodynamic model of the coupled condensates, that is capable of reproducing the excitation spectrum and population dynamics of the system.

We propose a scheme for controlling the movement of dimers, trimers, and other multimers in optical lattices by modulating the lattice potential. In deep optical lattices the propagation of deeply bound atomic clusters is slowed by the high energy cost of virtual intermediate states. Adapting the well-known method of lattice modulation spectroscopy, the movement of the clusters can be made resonant by utilizing sequences of bound-bound transitions. Using the scheme, the mobility of each specific cluster can be selectively controlled by tuning the modulation frequency. We formulate a simple and intuitive model and confirm the validity of the model by numerical simulations of dimers and trimers in a one-dimensional optical lattice.

We consider the phase-ordering dynamics of an isolated quasi-two-dimensional spin-1 Bose gas quenched into an easy-plane ferromagnetic phase. Preparing the initial system in an unmagnetized antiferromagnetic state the subsequent ordering involves both polar core and Mermin-Ho spin vortices, with the ratio between the different vortices controllable by the quench parameter. Ferromagnetic domain growth occurs as these vortices annihilate. The distinct dynamics of the two types of vortices means that the domain growth law is determined by two macroscopic length scales, violating the standard dynamic scaling hypothesis. Nevertheless we find that universality of the ordering process manifests in the decay laws for the spin vortices. To provide a fuller picture of the phase ordering dynamics we also present results for the scaling of the order parameter correlation function and the hydrodynamic decomposition of the system kinetic energy.

Quantum memories are essential for large-scale quantum information networks. Along with high efficiency, storage lifetime, and optical bandwidth, it is critical that the memory adds negligible noise to the recalled signal. A common source of noise in optical quantum memories is spontaneous four-wave mixing. We develop and implement a technically simple scheme to suppress this noise mechanism by means of quantum interference. Using this scheme with a Raman memory in warm atomic vapor, we demonstrate over an order of magnitude improvement in noise performance. Furthermore we demonstrate a method to quantify the remaining noise contributions and present a route to enable further noise suppression. Our scheme opens the way to quantum demonstrations using a broadband memory, significantly advancing the search for scalable quantum photonic networks.