It is shown that tailored breaking of the translational symmetry through weak scattering in waveguides and optical fibers can control chromatic dispersions of the individual modes at any order; thereby, it overcomes the problem of coherent classical and quantum signal transmission at long distances. The methodology is based on previously developed quantum control techniques and gives an analytic solution in ideal scattering conditions; it has been also extended to incorporate and correct nonunitary effects in the presence of weak backscattering. In practice, it requires scatterers able to couple different modes and carefully designed dispersion laws giving a null average quadratic distortion in the spectral vicinity of the operational frequency.

By etching a hole in the mirrors or by placing a scatterer in the center of a cavity, we can create a sink for light. In a Bose-Einstein condensate of photons this sink results in the creation of a so-called radial vortex, which is a two-dimensional analog of a Schwarzschild black hole. We theoretically investigate the Hawking radiation and the associated graybody factor of this Schwarzschild black hole. In particular, we determine the density-density and velocity-velocity correlation functions of the Hawking radiation, which can be measured by observing the spatial correlations in the fluctuations in the light emitted by the cavity.

Confinement of atoms inside impenetrable (hard) and penetrable (soft) cavities has been studied for nearly eight decades. However, a unified virial theorem for such systems has not yet been found. Here we provide a general virial-like equation in terms of mean square and expectation values of potential and kinetic energy operators. It appears to be applicable in both free and confined situations. Apart from that, we have derived an equation using the time-independent Schrödinger equation, which can be treated as a sufficient condition for a given stationary quantum state. A change of boundary condition does not affect these virial equations. In the hard confining condition, the perturbing (confining potential) does not affect the expression; it merely shifts the boundary from infinity to a finite region. In the soft case, on the contrary, the final expression includes contributions from the perturbing term. These are demonstrated numerically for several representative enclosed systems like harmonic oscillators (one-dimensional and three-dimensional) and hydrogen atoms. Its applicability in various other confinements (including angular) has been discussed. In essence, a virial equation has been proposed for free and confined quantum systems, from simple arguments.

We propose a resource theory of the quantum invasiveness of general quantum operations, i.e., those defined by quantum channels in Leggett-Garg scenarios. We are able to compare the resource-theoretic framework of quantum invasiveness to the resource theory of coherence. We also show that the Fisher information is a quantifier of quantum invasiveness. This result allows us to establish a direct connection between the concept of quantum invasiveness and quantum metrology, by exploring the utility of the definition of quantum invasiveness in the context of metrological protocols.

Quantum resources, such as coherence, discord, and entanglement, play a key role for demonstrating an advantage in many computation and communication tasks. In order to find the nature behind these resources, tremendous efforts have been made to explore the connections between them. In this work, we extend the single party coherence resource framework to the distributed scenario and relate it to basis-dependent discord. We show the operational meaning of basis-dependent discord in quantum key distribution and propose its resource measures. With any basis-dependent discord measure, we show how to extend it to measure coherence, discord, and entanglement by considering the relation of the resource free states. Our results lead to a unification of measures of quantum resources.

When an electron is resonantly excited from a core orbital to a Rydberg one in a molecule, complex multi-time-scale multistep processes are triggered. By using a high-resolution energy-resolved coincidence between Auger electron and ions, we identified the sequences of processes following the spectator Auger decays induced by $K$-shell excitation in ${\mathrm{O}}_2$. The first step of the spectator Auger decay can end up to a few metastable ${\mathrm{O}}_2{}^{+}$ states which autoionize into stable ${\mathrm{O}}_2{}^{2+}$. However, the majority of ${\mathrm{O}}_2{}^{+}$ states are dissociative. They evolve into several dissociation limits with complex crossings between potential-energy curves. After the molecular dissociation is finished, the generated fragments can further decay either by fluorescence emission or by autoionization. Interestingly, some electrons are always emitted after molecular dissociation.

We apply Milne's phase-amplitude representation [W. E. Milne, Phys. Rev. 35, 863 (1930)] to a scattering problem involving disjoint classically allowed regions separated by a barrier. Specifically, we develop a formalism employing different sets of amplitude and phase functions—each set of solutions optimized for a separate region—and we use these locally adapted solutions to obtain the true value of the scattering phase shift and accurate tunneling rates for ultranarrow shape resonances. We show results for an illustrative example of an attractive potential with a large centrifugal barrier.

Isothermal compressibility ${\kappa }_T$, which measures the fluidity of the liquid or the stiffness of the solids, is one of the most important physical quantities in thermodynamic physics. In ultracold atoms, this quantity has been widely explored in the measurement of the transition from normal gas to Bose-Einstein condensate, as well as the transition from superfluids to solid phases, such as supersolids and Mott insulators. This physical quantity was also examined in the spin-orbit-coupled degenerate gases in free space, showing that the combined contribution of spin-orbit coupling (SOC) and the Zeeman field may pronouncedly enhance the isothermal compressibility in the gapless Weyl phases. Motivated by the differences between the free space and the optical lattice, in this work we examine this compressibility in the spin-orbit-coupled degenerate Fermi gases in a square lattice model, in which the filling factor and particle-hole symmetry about half filling become an important influencing factor. In this case, the particle density in compressibility should be replaced by the density of a particle or hole according to their filling to ensure symmetry about half filling. With increasing particle density $n$, the compressibility decreases to a small magnitude, while $n^2{\kappa }_T$ may increase monotonically with increasing $n$ from zero to half filling. In the strong-coupling regime, when the chemical potential and pairing strength are much larger than the tunneling strength and Zeeman field, this compressibility will approach $n^2{\kappa }_T=2/U$, where $U$ is the on-site interacting strength. This limit can be explained by a one-site Hubbard model. We show that SOC and the Zeeman field can play opposite roles in the behavior of compressibilities. The combination of these two terms can give rise to pronouncedly enhanced isothermal compressibility in some proper parameter regime. The anomalous compressibilities also give rise to a anomaly in the exponent of pressure. In the lattice model, the enhanced peak can be found in both the fully gapped phase and gapless Weyl phases, which is different from that in free space, which can be realized only in the gapless phases. We derive the formalisms from the Gibbs-Duhem formalism. These observations unveil important differences between the continuous model and discrete models. Our theory may also be useful to study the physics in other superfluids, including topological superfluids with finite-momentum pairings, in which the finite-momentum pairing may also have an implicit contribution to this compressibility. In this way, we may realize superfluids with some extraordinary physical properties.

We discuss the main features of a recently introduced system capable of laser action: the complex active optical network, or lasing network (LANER) [Lepri et al., Phys. Rev. Lett. 118, 123901 (2017)]. The system is experimentally realized with optical fibers linked each other with couplers and with one or more coherently amplifying sections. The LANER displays a standard laser behavior: When the gain provided by the active sections is high enough to overcome the losses, a coherent emission is produced, with a complicated intensity spectrum. A linear theoretical description is discussed in detail, showing how the LANER can be considered as a generalization of the laser with the physical network acting as a complicated cavity. Among its main aspects, the system can be represented by directed graphs disclosing the analogies with the problem of quantum chaos on graphs. Moreover, when the links' lengths are all integer multiples of the same value, the LANER framework corresponds to a lattice problem, with the equivalence of the Brillouin zone with the cavity free spectral range. Experiments in simple configurations are also performed, reporting the evidence of lasing action and its characterization. Examples of spectra of the detected emitted intensity are obtained in different cases, in a phenomenological agreement with the numerical findings of the theory.

We investigate the collective spontaneous emission from an ensemble of two-level atoms evenly distributed inside a sphere with a low density. An initial symmetric single-excitation state is considered and polarizations for all atoms are assumed to be aligned in the same direction. We find that the superradiant decay rate of the ensemble exhibits oscillatory features dependent on the radius of the sphere and the atomic radiation wavelength, indicating that the collective emission rate can be less than the single-atom decay rate for certain parameters. Moreover, the system exhibits a hidden equivalence where the collective emission from the ensemble gives the same radiation rate as the case of a two-atom model where one atom is at the center of the sphere and the other effective large atom, composed of all other atoms, is localized at the edge of the sphere along the polarization direction. Our result hence provides a potential method towards exploring complex many-body physics by simplifying the model into a two-body problem.

Nonlinear optical frequency conversion can result in redistribution of the noise of the coherent pump light among generated optical harmonics leading to improved quality of laser emission. The property is useful for making narrow line lasers suitable for various metrology applications on a chip. We show theoretically that the resonant hyperparametric scattering allows reducing optical frequency noise in one of the generated harmonics with respect to the coherent pump light. The reduction calls for significantly dissimilar quality factors of the resonator modes and appears at the cost of enhanced frequency sensitivity to power fluctuations of the pump light. The artificial suppression of the quality factor of one of the modes participating in the scattering does not lead to significant increase of the oscillation threshold and broadens the dynamic range of the phase matching of the process. The technique is promising for creation of chip-scale parametric frequency “noise eaters.”

Platform motion poses significant challenges to high-precision optical time and frequency transfer. We give a detailed description of these challenges and their solutions in comb-based optical two-way time and frequency transfer (O-TWTFT). Specifically, we discuss the breakdown in reciprocity due to relativity and due to asynchronous sampling, the impact of optical and electrical dispersion, and velocity-dependent transceiver calibration. We present a detailed derivation of the equations governing comb-based O-TWTFT in the presence of motion. We describe the implementation of real-time signal processing algorithms based on these equations and demonstrate active synchronization of two sites over turbulent air paths to below-1-fs time deviation despite effective velocities of $\pm 25\mathrm{m}/\mathrm{s}$, which is the maximum achievable with our physical setup. With the implementation of the time transfer equation derived here, we find no velocity-dependent bias between the synchronized clocks to within a two-sigma statistical uncertainty of 330 as.

We revisit the decoherence process of a multiqubit register interacting with a thermal bosonic bath. We generalize the previous studies by considering not only the register's behavior but also a part of its environment. In particular, we are interested in information flow from the register to the environment, which we describe using recently introduced multipartite quantum state structures called spectrum broadcast structures. Working in two specific cases of (i) two-qubit register and (ii) collective decoherence, we identify the regimes where the environment acquires almost complete information about the register state. We also study in more detail the interesting causal aspects related to the finite propagation time of the field disturbances between the qubits. Finally, we describe quantum state structures which appear due to the presence of protected spaces.

We propose a quantum simulator based on driven superconducting qubits where the interactions are generated parametrically by a bichromatic magnetic flux modulation of a tunable bus element. Using a time-dependent Schrieffer-Wolff transformation, we analytically derive a multiqubit Hamiltonian which features independently tunable $XX$- and $YY$-type interactions as well as local bias fields over a large parameter range. We demonstrate the adiabatic simulation of the ground state of a hydrogen molecule using two superconducting qubits and one tunable bus element. The time required to reach chemical accuracy lies in the few microsecond range. Therefore, the proposed adiabatic protocol could be implemented using currently available superconducting circuits. Further applications of this technique may also be found in the simulation of interacting spin systems.

Sensitivity bounds for a generic interferometric phase estimation problem are the shot noise and the Heisenberg limits. The shot noise is the highest sensitivity that can be reached with separable states, while the Heisenberg limit is the ultimate bound in sensitivity which can be saturated with entangled states. The scaling of these bounds with the number of particles $N$ entering the interferometer depends on the specific Hamiltonian governing the same interferometer. In typical cases, the Hamiltonian is linear, and the shot-noise and Heisenberg limits scale with $N^{-1/2}$ and with $N^{-1}$, respectively. With interferometers described by generic, nonlinear Hamiltonian, the scalings with the number of particles can be rather different. Here we study the shot-noise and Heisenberg limits of Ising-like Hamiltonians in the presence of longitudinal and transverse fields, both in the nearest-neighbor and in the fully connected spin interaction cases. We provide the explicit forms of the states saturating the shot-noise and Heisenberg limits. These results can be relevant not only for precision measurement purposes but also to characterize quantum phase transitions and, more generally, the witnessing of multipartite entanglement.

Typically, the aim of quantum metrology is to sense target fields with high precision utilizing quantum properties. Unlike the typical aim, in this paper, we use quantum properties for adding a functionality to quantum sensors. More concretely, we propose a delegated quantum sensor (a client-server model) with security inbuilt. Suppose that a client wants to measure some target fields with high precision, but he/she does not have any high-precision sensor. This leads the client to delegate the sensing to a remote server who possesses a high-precision sensor. The client gives the server instructions about how to control the sensor. The server lets the sensor interact with the target fields in accordance with the instructions, and then sends the sensing measurement results to the client. In this case, since the server knows the control process and readout results of the sensor, the information of the target fields is available not only for the client but also for the server. We show that, by using an entanglement between the client and the server, an asymmetric information gain is possible so that only the client can obtain sufficient information on the target fields. In our scheme, the server generates the entanglement between a solid-state system (that can interact with the target fields) and a photon, and sends the photon to the client. On the other hand, the client is required to possess linear-optics elements only including wave plates, polarizing beam splitters, and single-photon detectors. Our scheme is feasible with the current technology, and our results pave the way for an application of quantum metrology.