Multimode photon subtraction provides an experimentally feasible option to construct large non-Gaussian quantum states in continuous variable quantum optics. The non-Gaussian features of the state can lead towards the more exotic aspects of quantum theory, such as negativity of the Wigner function. However, the payoff for states with such delicate quantum properties is their sensitivity to decoherence. In this paper, we present a general model that treats the most important source of decoherence in a purely optical setting: losses. We use the framework of open quantum systems and master equations to describe losses in $n$-photon-subtracted multimode states, where each photon can be subtracted in an arbitrary mode. As a main result, we find that mode-dependent losses and photon subtraction generally do not commute. In particular, the losses do not only reduce the purity of the state, they also change the modal structure of its non-Gaussian features. We then conduct a detailed study of single-photon subtraction from a multimode Gaussian state, which is a setting that lies within the reach of present-day experiments.

We propose a straightforward experimental protocol to test whether qubit-environment entanglement is generated during pure dephasing of a qubit. The protocol is implemented using only measurements and operations on the qubit—it does not involve the measurement of the system-environment state of interest, but the preparation and measurement of the qubit in two simple variations. A difference in the time dependencies of qubit coherence between the two cases testifies to the presence of entanglement in the state of interest. Furthermore, it signifies that the environment-induced noise experienced by the qubit cannot be modeled as a classical stochastic process independent of the qubit state. We demonstrate the operation of this protocol on a realistically modeled nitrogen-vacancy center spin qubit in diamond interacting with a nuclear spin environment, and show that the generation of entanglement should be easily observable in this case.

Semiquantum key distribution protocols are designed to allow two parties to establish a shared secret key, secure against an all-powerful adversary, even when one of the users is restricted to measuring and preparing quantum states in one single basis. While interesting from a theoretical standpoint, these protocols have the disadvantage that a two-way quantum communication channel is necessary which generally limits their theoretical efficiency and noise tolerance. In this paper, we construct a semiquantum key distribution (SQKD) protocol which actually takes advantage of this necessary two-way channel, and, after performing an information theoretic security analysis against collective attacks, we show it is able to tolerate a channel noise level higher than any prior SQKD protocol to date. We also compare the noise tolerance of our protocol to other two-way fully quantum protocols, along with BB84 with classical advantage distillation. We also comment on some practical issues involving semiquantum key distribution (in particular, concerning the potential complexity in physical implementation of our protocol as compared with other standard QKD protocols). Finally, we develop techniques that can be applied to the security analysis of other (S)QKD protocols reliant on a two-way quantum communication channel.

The states of three-qubit systems split into two inequivalent types of genuine tripartite entanglement, namely, the Greenberger-Horne-Zeilinger (GHZ) type and the $W$ type. A state belonging to one of these classes can be stochastically transformed only into a state within the same class by local operations and classical communications. We provide local quantum operations, consisting of the most general two-outcome measurement operators, for the deterministic transformations of three-qubit pure states in which the initial and the target states are in the same class. We explore these transformations, originally having standard GHZ and standard $W$ states, under the local measurement operations carried out by a single party and $p$ ($p=2,3$) parties (successively). We find a notable result that the standard GHZ state cannot be deterministically transformed to a GHZ-type state in which all its bipartite entanglements are nonzero, i.e., a transformation can be achieved with unit probability when the target state has at least one vanishing bipartite concurrence.

Quantum correlations between parts of a composite system most clearly reveal themselves through entanglement. Designing, maintaining, and controlling entangled systems is very demanding, which raises the stakes for understanding the efficacy of entanglement-free, yet quantum correlations, exemplified by quantum discord. Discord is defined via conditional mutual entropies of parts of a composite system and its direct measurement is hardly possible even via full tomographic characterization of the system state. Here we design a simple protocol to detect and quantify quantum discord in an unentangled bipartite system. Our protocol relies on a characteristic of discord that can be extracted from repeated direct measurements of certain correlations between subsystems of the bipartite system. The proposed protocol opens a way of extending experimental studies of discord to electronic systems, but can also be implemented in quantum-optical systems.

Three-fragment dissociation dynamics of ${\mathrm{C}}_2{\mathrm{H}}_2{}^{2+}$ dications and ${\mathrm{C}}_2{\mathrm{H}}_2{}^{3+}$ trications induced by electron capture of slow (3-keV/u) ${\mathrm{Ar}}^{8+}$ ions are investigated. Using cold target recoil ion momentum spectroscopy, the complete kinematic information and thus kinetic energy releases are determined for the three-fragment channels, ${\mathrm{C}}_2{\mathrm{H}}_2{}^{2+}\to {\mathrm{H}}^{+}+{\mathrm{C}}_2{}^{+}+\mathrm{H}$ and ${\mathrm{C}}_2{\mathrm{H}}_2{}^{3+}\to {\mathrm{H}}^{+}+{\mathrm{C}}_2{}^{+}+{\mathrm{H}}^{+}$. Then by analyzing the complete kinematics with a Dalitz plot and Newton diagram, different fragmentations, i.e., concerted or sequential pathway, are identified. For dications, the sequential fragmentation ${\mathrm{C}}_2{\mathrm{H}}_2{}^{2+}\to {\mathrm{H}}^{+}+{\mathrm{C}}_2{\mathrm{H}}^{+}\to {\mathrm{H}}^{+}+{\mathrm{C}}_2{}^{+}+\mathrm{H}$ is dominant. However, the trications mainly dissociate via the synchronous concerted fragmentation. The sequential pathway ${\mathrm{C}}_2{\mathrm{H}}_2{}^{3+}\to {\mathrm{H}}^{+}+{\mathrm{C}}_2{\mathrm{H}}^{2+}\to {\mathrm{H}}^{+}+{\mathrm{C}}_2{}^{+}+{\mathrm{H}}^{+}$, which was found to be significant at higher collision velocities, is not observed here. This distinction reveals the important role of projectile velocity on the fragmentation dynamics for some specific channels.

An ion in a Penning trap induces image charges on the surfaces of the trap electrodes. These induced image charges are used to detect the ion's motional frequencies, but they also create an additional electric field, which shifts the free-space cyclotron frequency typically at a relative level of several ${10}^{-11}$. In various high-precision Penning-trap experiments, systematics and their uncertainties are dominated by this so-called image charge shift (ICS). The ICS is investigated in this work by a finite-element simulation and by a dedicated measurement technique. Theoretical and experimental results are in excellent agreement. The measurement is using singly stored ions alternately measured in the same Penning trap. For the determination of the ion's magnetron frequency with relative precision of better than 10 parts per billion, a Ramsey-like technique has been developed. In addition, numerical calculations are carried out for other Penning traps and agree with older ICS measurements.

Dissipations in the waveguide-cavity systems destroy quantum interference between the incident signal photons and the cavity photons linearly and, thus, usually limit the manipulation of the photon transport. Here, we adopt a four-mode microring cavity including both second-order (${\chi }^{\left(2\right)}$) and third-order (${\chi }^{\left(3\right)}$) nonlinearities side coupled to two different waveguides to manipulate photon transport against dissipative effects. The results show that the incident signal photons can be amplified due to the optical parametric amplification (OPA) originated from ${\chi }^{\left(3\right)}$ nonlinearities. Owing to the coherent interference between cavity photons with different frequencies, the transmission spectra of the signal field are split into two amplified peaks. The transmission intensities are not always linearly reduced by increasing the dissipations but determined by the competition between the OPA and the dissipations. It means that we can utilize the OPA process to overcome the dissipative effects to manipulate photon transport and, thus, control the transmission intensities of the incident signal photon from $T_c=0$ to $T_c>100\%$. This competition can also be used to convert the signal field to the idler field with amplified efficiencies.

Quantum networks provide a platform for astronomical interferometers capable of imaging faint stellar objects. In a recent work [E. T. Khabiboulline et al., Phys. Rev. Lett. 123, 070504 (2019)], we presented a protocol that circumvents transmission losses with efficient use of quantum resources and modest quantum memories. Here we analyze a number of extensions to that scheme. We show that it can be operated as a truly broadband interferometer and generalized to multiple sites in the array. We also analyze how imaging based on the quantum Fourier transform provides improved signal-to-noise ratio compared to classical processing. Finally, we discuss physical realizations including photon-detection-based quantum state transfer.

Multipartite pure states are equivalent under stochastic local operations and classical communication (SLOCC) whenever they can be mapped into one another by invertible local operations. It is shown that this is equivalent to the interconvertibility through finite sequences of elementary local operations. A multipartite version of the Gauss-Jordan elimination strategy is then obtained, enabling the analytical SLOCC classification of previously unknown examples. It is argued that the problem of SLOCC classification is equivalent to the problem of classifying the multipartite fully reduced forms that the coefficient matrix of a state can assume after being subjected to this Gauss-Jordan elimination procedure. The method is applied to examples of so-called locally maximally entangleable and hypergraph states, showing different SLOCC equivalences. Moreover, possible physical implications for states with a Dicke-like structure are sketched.

We investigate magnetically tunable Feshbach resonances in ultracold collisions between ground-state Yb and Cs atoms, using coupled-channel calculations based on an interaction potential recently determined from photoassociation spectroscopy. We predict resonance positions and widths for all stable isotopes of Yb, together with resonance decay parameters where appropriate. The resonance patterns are richer and more complicated for fermionic Yb than for spin-zero isotopes, because there are additional level splittings and couplings due to scalar and tensorial Yb hyperfine interactions. We examine collisions involving Cs atoms in a variety of hyperfine states and identify resonances that appear most promising for experimental observation and for magnetoassociation to form ultracold CsYb molecules.

We unveil the dynamical formation of multiple localized structures in the form of dark-bright and dark-antidark solitary waves that emerge upon quenching a one-dimensional particle-imbalanced Bose-Bose mixture. Interspecies interaction quenches drive the system out of equilibrium while the so-called miscible-immiscible threshold is crossed in a two-directional manner. Dark-bright entities are spontaneously generated for quenches towards the phase separated regime and dark-antidark states are formed in the reverse process. The distinct mechanisms of creation of the aforementioned states are discussed in detail and their controlled generation is showcased. In both processes, it is found that the number of solitary waves generated is larger for larger particle imbalances, a result that is enhanced for stronger postquench interspecies interactions. Additionally the confining geometry highly affects the production of both types of states with a decaying solitary wave formation occurring for tighter traps. Furthermore, in both of the aforementioned transitions, the breathing frequencies measured for the species differ significantly for highly imbalanced mixtures. Finally, the robustness of the dynamical formation of dark-bright and dark-antidark solitons is also demonstrated in quasi-one-dimensional setups.

We analyze the behavior of a mobile quantum impurity, restricted to a one-dimensional motion, in a bath formed by a two-leg bosonic ladder through a combination of field theory [Tomonaga-Luttinger liquid (TLL)] and numerical (density-matrix renormalization group) techniques. We compute the Green's function of the impurity as a function of time at different momenta. We find a power-law decay at zero momentum, which signals the breakdown of a quasiparticle description of the impurity motion. We compute the exponent both for the limits of weak and strong impurity-bath interactions. At small impurity-bath interaction, the impurity experiences the ladder as a single channel one-dimensional bath, but with an effective coupling reduced by a factor of $\sqrt{2}$. We compare the numerical results for the exponent at zero momentum with a semianalytical expression, initially established for the chain, and find excellent agreement without adjustable parameters. We find an increase of the exponent with increasing transverse tunneling in the bath. At small tunneling, the exponent is compatible with the TLL prediction, while larger tunneling shows strong deviations. Finally, we show that, as a function of the momentum of the impurity, two different regimes of decay of the Green's function exist, similar to the single chain case. The power-law regime occurs for small momentum, while at large momentum the Green's function shows a faster decay, corresponding to the one expected in a polaronic regime. In this last regime, we compute the lifetime of the polaron numerically. We compute the critical momentum marking the transition between these two regimes. We compare with analytical predictions based on the structure factor of the bath and find good agreement with the numerical results. Finally, we discuss the consequences of our results for cold atomic experiments.

We propose here a fully backaction-evading scheme for the measurement of the entanglement between two nanomechanical resonators. The system, which consists of two mechanical oscillators, coupled to a single mode of an electromagnetic resonant cavity through a radiation-pressure interaction term, is driven by two pump tones and four detection tones. As previously discussed in the literature, the former induce entanglement between the two mechanical oscillators, while we show here that a specific choice of phase and amplitude of the detection tones allows for direct pairwise reconstruction of the collective quadrature fluctuations of the mechanical oscillators belonging to quantum-mechanics-free subspaces, thereby providing direct evidence of the entanglement properties of the two mechanical resonators.

We demonstrate how to manipulate quantum chaos with a single photon in a hybrid quantum device combining cavity QED and optomechanics. Specifically, we show that this system changes between integrable and chaotic relying on the photon state of the injected field. This onset of chaos originates from the photon-dependent chaotic threshold of the qubit-field coupling induced by the optomechanical interaction. By deriving the Loschmidt Echo, we observe clear differences in the sensitivity to perturbations in the regular versus chaotic regimes. We also present the classical analog of this chaotic behavior and find good correspondence between chaotic quantum dynamics and classical physics. Our work opens up a route to achieve quantum manipulations, which are crucial elements in engineering new types of on-chip quantum devices and quantum information science.

We investigate the influence of an excited-state quantum phase transition on the quantum-speed-limit time of an open quantum system. The system consists of a central qubit coupled to a spin environment modeled by a Lipkin-Meshkov-Glick Hamiltonian. We show that when the coupling between qubit and environment is such that the latter is located at the critical energy of the excited-state quantum phase transition, the qubit evolution shows a remarkable slowdown, marked by an abrupt peak in the quantum-speed-limit time. Interestingly, this slowdown at the excited-state quantum phase transition critical point is induced by the singular behavior of the qubit decoherence rate, being independent of the Markovian nature of the environment.

Spontaneous synchronization between coupled periodic systems occur in a wealth of classical physical setups. Here, we show theoretically that the phase of two distinct quantum harmonic oscillators spontaneously synchronize when they are strongly coupled to a common bosonic quantum dissipative environment at zero temperature, in the absence of any driving mechanism. To do so, we compute the dynamics of the oscillators with an exact master equation obtained from a path integral formalism. Above some value of the system-environment coupling strength, we observe numerically a strongly reduced damping and a frequency locking in the dynamics of the oscillators. Beyond the synchronization mechanism, we also describe the rich phenomenology of the dynamics in the model and notably identify the additional emergence of a regime with long-lived oscillations.

The expectation values of radial geometric quantities $\langle r_{<}\rangle , \langle r_{>}\rangle , \langle r\rangle$, and $\langle r_{12}\rangle$ and angular ones $\langle {\theta }_{12}\rangle$ and $\langle cos{\theta }_{12}\rangle$ are investigated in detail for two-electron systems. Although these quantities can be calculated straightforwardly in the framework of Hartree-Fock or configuration-interaction methods, their computations based on explicitly correlated type wave functions are nontrivial tasks. In this work we employ the Hylleraas configuration-interaction basis functions to produce accurate system energies and wave functions for the He atom and He-like ions. Computational methods are developed to accurately and efficiently calculate the geometric quantities, especially for inner and outer electron radii, i.e., $\langle r_{<}\rangle$ and $\langle r_{>}\rangle$, and the average interelectronic angle $\langle {\theta }_{12}\rangle$. Compared to previous Hartree-Fock and multiconfiguration Hartree-Fock predictions and the configuration-interaction calculations based on Slater-type orbitals, our present work improve significantly the accuracy of all geometric quantities for He atom in the ground and singly excited states. The application of the present method to He-like ions with different nuclear charge reveals asymptotic power laws for both radial and angular quantities as they approach to the high-$Z$ limit. Further extensions of the present work to other systems are discussed.

We explore the recently introduced concept of a generalized Hanbury Brown–Twiss (HBT) effect as applied to the focal plane of a lens that is used to focus a random electromagnetic beam. We find that the strength of the HBT correlation can be increased by the lens. Furthermore, the associated Stokes scintillations of the focused field display a surprisingly complicated spatial behavior. We illustrate, using the sum rules for scintillations and correlations, how focused fields can be tuned for different applications.

Orbital angular momentum (OAM) entanglement states have great potential in enhancing the information capacity in quantum communications. To make full use of the OAM degree of freedom, the measurement and distinction of OAM Bell states become a key step that needs to be accomplished. However, there is still no solution to achieve complete distinction of OAM Bell states at the single-photon level. Here we demonstrate the complete OAM Bell-state measurement without auxiliary photons by using linear optics. Furthermore, we propose a quantum communication network via OAM Bell-state multiplexing, perform a proof-of-principle experiment in the first- and second-order OAM subspaces for two users, and achieve average success probabilities of obtaining information of 91.6% for User-I and 89.1% for User-II. In principle, our scheme allows more users to be added by using high-order OAM Bell states. Our research extends the manipulation method for high-dimensional quantum states and paves the way for the application of OAM multiplexing in quantum communications.