We propose a method to calculate the operator entanglement and entangling power of a noisy nonunitary operation in terms of linear entropy. By decomposing the Kraus operators of noisy evolution as the sum of products of Pauli matrices, we derive the analytical expression of the operator entanglement for a general nonunitary operation. The definition of entangling power is extended from the ideal unitary operation case to the nonunitary case via a Kraus operator representation and the analytical expression of the entangling power for a general nonunitary operation is derived. To demonstrate the effectiveness of the above method, we investigate the properties of operator entanglement and entangling power of nonunitary operations caused by phase damping noise. Our findings imply that the pure phase damping noise has its own operator entanglement and entangling power, which increase exponentially with time and asymptotically approach their respective upper bounds. In addition, when the phase damping noise is added to an ideal operation, such as an iswap operation or a controlled-$Z$ operation, it can make the operation's entangling power grow exponentially with the strength of noise, but leave its operator entanglement invariant. In this sense, we can conclude that, for a general operation, operator entanglement is a more intrinsic property than entangling power.

Dephasing and decay are the intrinsic dissipative processes prevalent in any open quantum system and the dominant mechanisms for the loss of coherence and entanglement. This inadvertent effect not only can be overcome but can even be capitalized on in a dissipative quantum simulation by means of tailored couplings between the quantum system and the environment. In this context it has been demonstrated that universal quantum computation can be performed using purely dissipative elements, and furthermore, the efficient preparation of highly entangled states is possible. In this article, we are interested in nonequilibrium phase transitions appearing in purely dissipative systems and the exploration of quantum phases in terms of a dissipative quantum simulation. To elucidate these concepts, we scrutinize exemplarily two paradigmatic models: the transverse-field Ising model and the considerably more complex ${\mathbb{Z}}_2$ lattice gauge theory. We show that the nonequilibrium phase diagrams parallel the quantum phase diagrams of the Hamiltonian “blueprint” theories.

We consider the expected violations of Bell inequalities from random pure states. More precisely, we focus on a slightly generalized version of the Collins-Gisin-Linden-Massar-Popescu inequality, which concerns Bell experiments of two parties, two measurement options, and $N$ outcomes, and analyze their expected quantum violations from random pure states for varying $N$, assuming the conjectured optimal measurement operators. It is seen that for small $N$ the Bell inequality is not violated on average, while for larger $N$ it is. Both ensembles of unstructured as well as structured random pure states are considered. Using techniques from random matrix theory this is obtained analytically for small and large $N$ and numerically for intermediate $N$. The results show a beautiful interplay of different aspects of random matrix theory, ranging from the Marchenko-Pastur distribution and fixed-trace ensembles to the $O\left(n\right)$ model.

In this paper, we mainly study the local indistinguishability of mutually orthogonal product basis quantum states in $d\otimes d$. In $3\otimes 3$, Bennett et al. [ Phys. Rev. A 59, 1070 (1999)] presented nine orthogonal product basis quantum states which cannot be distinguished by local operations and classical communication (LOCC). In the work by Zhang et al. [Z.-C. Zhang et al., Phys. Rev. A 90, 022313 (2014)], this result was generalized in $d\otimes d$, where $d$ is odd. In this paper, we aim to construct locally indistinguishable orthogonal product basis quantum states in $d\otimes d$. For the general $d\otimes d(d>2)$ quantum system, we first construct $4d-4$ orthogonal product states, and prove these states are locally indistinguishable using a very simple but quite effective method. Then, based on these states, we construct some classes of locally indistinguishable orthogonal product basis quantum states (OPBS) in $d\otimes d(d>2)$. Finally, we construct some LOCC indistinguishable OPBS in multipartite quantum systems. All of the above results demonstrate the phenomenon of nonlocality without entanglement.

Measurement-device-independent quantum key distribution (MDI-QKD) can provide enhanced security compared to traditional QKD, and it constitutes an important framework for a quantum network with an untrusted network server. Still, a key assumption in MDI-QKD is that the sources are trusted. We propose here a MDI quantum network with a single untrusted source. We have derived a complete proof of the unconditional security of MDI-QKD with an untrusted source. Using simulations, we have considered various real-life imperfections in its implementation, and the simulation results show that MDI-QKD with an untrusted source provides a key generation rate that is close to the rate of initial MDI-QKD in the asymptotic setting. Our work proves the feasibility of the realization of a quantum network. The network users need only low-cost modulation devices, and they can share both an expensive detector and a complicated laser provided by an untrusted network server.

We present an extension of the resolvent operator method to extract fully differential ionization probabilities resulting from the interaction of ultrashort laser pulses with ${\text{H}}_2{}^{+}$ by including all electronic and vibrational (dissociative) degrees of freedom. The wave function from which ionization probabilities are extracted is obtained by solving the time-dependent Schrödinger equation in a grid for the case of ${\text{H}}_2{}^{+}$ oriented parallel to the polarization direction of the field. The performance of the method is illustrated by using pulses in the xuv domain. Correlated kinetic-energy (CKE) and correlated angular and nuclear kinetic-energy $\left(\text{CAK}{}_{\text{N}}\right)$ spectra have been evaluated and used to analyze the underlying mechanisms of the photoionization process. In particular, for pulses with a central frequency $\omega =0.8$ a.u., which is smaller than the vertical ionization potential of ${\text{H}}_2{}^{+}$, we show the opening of the one-photon ionization channel by decreasing the pulse duration down to less than 1 fs. An analysis of the CKE and $\text{CAK}{}_{\text{N}}$ spectra allows us to visualize individual contributions from one- and two-photon ionization processes, as well as to study the variation of these contributions with pulse duration. The latter information is difficult to extract when only the kinetic energy release (KER) spectrum is measured. This points out the importance of performing multiple-coincidence measurements for better elucidation of competing ionization mechanisms, such as those arising when ultrashort pulses are used.

We address the use of entangled qubits as quantum probes to characterize the noise induced by complex environments. In particular, we show that a joint measurement on entangled probes can improve estimation of the correlation time for a broad class of environmental noises compared to sequential strategies involving single-qubit preparation. The enhancement appears when the noise is faster than a threshold value, a regime which may always be achieved by tuning the coupling between the quantum probe and the environment inducing the noise. Our scheme exploits time-dependent sensitivity of quantum systems to decoherence and does not require dynamical control on the probes. We derive the optimal interaction time and the optimal probe preparation, showing that it corresponds to multiqubit Greenberger-Horne-Zeilinger states when entanglement is useful. We also show the robustness of the scheme against depolarization or dephasing of the probe, and discuss simple measurements approaching optimal precision.

Here we propose a route to the high-$Q$ perfect absorption of light by introducing the concept of a Fano antilaser. Based on the drastic spectral variation of the optical phase in a Fano-resonant system, a spectral singularity for scatter-free perfect absorption can be achieved with an order of magnitude smaller material loss. By applying temporal coupled mode theory to a Fano-resonant waveguide platform, we reveal that the required material loss and following absorption $Q$ factor are ultimately determined by the degree of Fano spectral asymmetry. The feasibility of the Fano antilaser is confirmed using a photonic crystal platform, to demonstrate spatiospectrally selective heating. Our results utilizing the phase-dependent control of device bandwidths derive a counterintuitive realization of high-$Q$ perfect conversion of light into internal energy, and thus pave the way for a new regime of absorption-based devices, including switches, sensors, thermal imaging, and optothermal emitters.

We present a scheme to generate an all-range long repulsive Casimir-Polder potential between a perfect negative reflecting surface and a ground-state atom. The repulsive potential is stable and does not decay with time. The Casimir-Polder potential is proportional to $z^{-2}$ at short atom-surface distances and to $z^{-4}$ at long atom-surface distances. Because of these advantages, this potential can help in building quantum reflectors, quantum levitating devices, and waveguides for matter waves.

In a gas of ultracold atoms whose scattering length is controlled by a magnetic Feshbach resonance, atoms can be associated into universal dimers by an oscillating longitudinal magnetic field. In addition to the harmonic resonance with frequency near that determined by the dimer binding energy, there is a subharmonic resonance with half that frequency. If the thermal gas contains dimers, they can be dissociated into unbound atoms by the oscillating magnetic field. We show that the transition rates for association and dissociation can be calculated by treating the oscillating magnetic field as a sinusoidal time-dependent perturbation proportional to the contact operator. Many-body effects are taken into account through transition matrix elements of the contact operator. We calculate both the harmonic and subharmonic transition rates analytically for association in a thermal gas of atoms and for dissociation in a thermal gas of dimers.

We report a time-resolved study of the thermalization dynamics and the lasing to photon Bose-Einstein condensation crossover by in situ monitoring the photon kinetics in a dye microcavity. When the equilibration of the light to the dye temperature by absorption and reemission is faster than photon loss in the cavity, the optical spectrum becomes Bose-Einstein distributed and photons accumulate at low-energy states, forming a Bose-Einstein condensate. The thermalization of the photon gas and its evolution from nonequilibrium initial distributions to condensation is monitored in real time. In contrast, if photons leave the cavity before they thermalize, the system operates as a laser.

Postselected weak measurement has aroused broad interest for its distinctive ability to amplify small physical quantities. However, the low postselection efficiency to obtain a large weak value has been a big obstacle to its application in practice since it may waste resources, and reduce the measurement precision. An improved protocol was proposed in Pang et al., Phys. Rev. Lett. 113, 030401 (2014) to make the postselected weak measurement dramatically more efficient by using entanglement. Such a protocol can increase the Fisher information of the measurement to approximately saturate the well-known Heisenberg limit. In this paper, we review the entanglement-assisted protocol of postselected weak measurement in detail, and study its robustness against technical noises. We focus on readout errors. Readout errors can greatly degrade the performance of postselected weak measurement, especially when the readout error probability is comparable to the postselection probability. We show that entanglement can significantly reduce the two main detrimental effects of readout errors: inaccuracy in the measurement result and the loss of Fisher information. We extend the protocol by introducing a majority vote scheme to postselection to further compensate for readout errors. With a proper threshold, almost no Fisher information will be lost. These results demonstrate the effectiveness of entanglement in protecting postselected weak measurement against readout errors.

We consider a simple quantum model of atom-molecule conversion where bosonic atoms can combine into diatomic molecules and vice versa. The many-particle system can be expressed in terms of the generators of a deformed $\text{su}\left(2\right)$ algebra, and the mean-field dynamics takes place on a deformed version of the Bloch sphere, a teardrop shaped surface with a cusp singularity. We analyze the mean-field and many-particle correspondence, which shows typical features of quantum-classical correspondence. We demonstrate that semiclassical methods can be employed to recover full many-particle features from the mean-field description in cold atom systems with atom-molecule conversion, and we derive an analytic expression for the many-particle density of states in the limit of large particle numbers.

In a recent paper [Phys. Rev. A 90, 043830 (2014)] it was shown that the so-called aperture response tensor is the central concept in the microscopic quantum theory of light diffraction from a small hole in a flat screen. It was further shown that the quantum mechanical theory of diffraction only requires a preknowledge of the incident field plus the electronic properties of identical screens with and without a hole. Starting from the quantum mechanical expression for the linear conductivity tensor, we study the related causal conductivity tensor paying particular attention to diamagnetic electron dynamics. Using a nonlocal-potential separation assumption, we present a calculation of the diamagnetic causal surface conductivity for a jellium quantum-well screen using a two-dimensional Hartree-Fock model. In the diamagnetic case the difference between the light-unperturbed electron densities for screens with $\left(n^0\right)$ and without $\left(n_{\infty }^0\right)$ holes are the primary quantities for the diffraction theory. In a central part (Sec. IV) of this article we determine $n^0$ via a quantum-mechanical two-dimensional extinction-theorem approach related to elastic electron scattering from a hole with an electronic selvedge. For heuristic purposes we illustrate aspects of the extinction-theorem theory by applying the approach for an infinitely high potential barrier to the vacuum hole. Finally, we calculate and discuss the aperture response tensor in the small hole limit and in the zeroth-order Born approximation. Our final result for the aperture response tensor establishes the bridge to the anisotropic electric dipole polarizability tensor of the hole. It turns out that the effective optical aperture (hole) size relates closely to the extension of the relevant electronic wave functions scattered from the hole.

The proton radius puzzle questions the self-consistency of theory and experiment in light muonic and electronic bound systems. Here we summarize the current status of virtual particle models as well as Lorentz-violating models that have been proposed in order to explain the discrepancy. Highly charged one-electron ions and muonic bound systems have been used as probes of the strongest electromagnetic fields achievable in the laboratory. The average electric field seen by a muon orbiting a proton is comparable to hydrogenlike uranium and, notably, larger than the electric field in the most advanced strong-laser facilities. Effective interactions due to virtual annihilation inside the proton (lepton pairs) and process-dependent corrections (nonresonant effects) are discussed as possible explanations of the proton size puzzle. The need for more experimental data on related transitions is emphasized.

An ever broader range of physical platforms provides the possibility to study and engineer quantum dynamics under continuous measurements. In many experimental arrangements the system of interest is monitored by means of an ancillary device, whose sole purpose is to transduce the signal from the system to the measurement apparatus. Here we present a method of adiabatic elimination when the transducer consists of an arbitrary number of bosonic modes with Gaussian dynamics while the measured object can be any quantum system. Crucially, our approach can cope with the highly relevant case of finite temperature of the transducer, which is not easily achieved with other methods. We show that this approach provides a significant improvement in the readout of superconducting qubits in circuit QED already for a few thermal excitations and makes it possible to adiabatically eliminate optomechanical transducers relevant for frequency conversion between microwave and optical fields.