Continuous-variable quantum key distribution (CV-QKD) provides an effective way to obtain the high secret key rate, but is limited by practical techniques. To overcome this limitation, we investigate the performance of CV-QKD protocols when non-Gaussian operations are applied to both sides of the channels, including photon subtraction and addition, and photon catalysis. A comparison is also made between Gaussian modulation and non-Gaussian modulation. We show that the optimal transmission distance cannot be improved by non-Gaussian modulation at the sender's side, but it can be improved by single-photon catalysis at the receiver's side. For a given modulation variance that exceeds a certain threshold, the performance of the zero-photon catalysis at the sender's side is the best when compared with other non-Gaussian modulations (except single-photon catalysis at the receiver's side), and it can always reach the optimal distance. Our paper will provide a useful insight for the use of non-Gaussian modulation to other CV-QKD protocols.

In this paper, we propose a robust and high-fidelity scheme to construct a nondestructive Rydberg parity meter by optimized inverse engineering, after which we can know the parity information of the bipartite state without destroying it, implying that the output bipartite state can be further used in the rest quantum information processing tasks. Specifically speaking, we build a model with two systematic Rydberg atoms and one auxiliary Rydberg atom and control them by utilizing optimized inverse engineering against systematic error. Also, since the accumulated time for the Rydberg atoms simultaneously being in Rydberg states is minimized, the mechanical effect and the further possible ionization are almost avoided. Moreover, numerical simulation shows that the present scheme is robust against the systematic error, the dephasing, and the thermal noise, demonstrating feasibility in the experiment.

Recent experimental progress in cooling, trapping, and quantum logic spectroscopy of highly charged ions (HCIs) made HCIs accessible for high-resolution spectroscopy and precision fundamental studies. Based on these achievements, we explore a possibility to develop optical clocks using transitions between the ground and a low-lying excited state in ${\mathrm{Cf}}^{15+}$ and ${\mathrm{Cf}}^{17+}$ ions. Using a high-accuracy relativistic method of calculation, we predicted the wavelengths of clock transitions, calculated relevant atomic properties, and analyzed a number of systematic effects (such as the electric quadrupole, micromotion, and quadratic Zeeman shifts of the clock transitions) that affect the accuracy and stability of the optical clocks. We also calculated magnetic dipole hyperfine-structure constants of the clock states and the blackbody radiation shifts of the clock transitions.

A physical interpretation is provided for the formation of the multiphoton-regime plateau feature in the spectra of nonlinear and correlated process of high-order harmonic generation (HHG) of two-active-electron atoms interacting with an intense linearly polarized laser field [Phys. Rev. A 88, 053412 (2013) for beryllium]. While in the strong-field tunneling regime the plateau feature is well known to be due to rescattering effects of the freed electron, its counterpart in the multiphoton regime is due to atomic resonance effects involving both singly excited states and doubly excited states. Here, we propose a strategy to uncover which kind of these states are responsible for this phenomenon. By studying the sensitivity of HHG to the initial state in helium, we show that this multiphoton-regime plateau structure mostly originates from singly excited states.

We unravel the correlation effects of the second-order quantum phase transitions emerging on the ground state of a harmonically trapped spin-1 Bose gas, upon varying the involved Zeeman terms, as well as its breathing dynamics triggered by quenching the trapping frequency. It is found that the boundaries of the associated magnetic phases are altered in the presence of interparticle correlations for both ferromagnetic and antiferromagnetic spin-spin interactions, an effect which becomes more prominent in the few-body scenario. Most importantly, we unveil a correlation-induced shrinking of the antiferromagnetic and broken-axisymmetry phases implying that ground states with bosons polarized in a single spin component are favored. Turning to the dynamical response of the spinor gas it is shown that its breathing frequency is independent of the system parameters while correlations lead to the formation of filamentary patterns in the one-body density of the participating components. The number of filaments is larger for increasing spin-independent interaction strengths or for smaller particle numbers. Each filament maintains its coherence and exhibits an anticorrelated behavior while distinct filaments show significant losses of coherence and are two-body correlated. Interestingly, we demonstrate that for an initial broken-axisymmetry phase an enhanced spin-flip dynamics takes place which can be tuned either via the linear Zeeman term or the quench amplitude.

When a quantum particle is launched with a finite velocity in a disordered potential, it may surprisingly come back to its initial position at long times and remain there forever. This phenomenon, dubbed “quantum boomerang effect,” was introduced in Prat et al. [Prat, Delande, and Cherroret, Quantum boomeranglike effect of wave packets in random media, Phys. Rev. A 99, 023629 (2019)]. Interactions between particles, treated within the mean-field approximation, are shown to partially destroy the boomerang effect: the center of mass of the wave packet makes a U turn, but does not completely come back to its initial position. We show that this phenomenon can be quantitatively interpreted using a single parameter, the average interaction energy.

We study the transport of a spin-orbit-coupled atomic matter wave using a moving Dirac $\delta$-potential well. In a spin-orbit-coupled system, bound states can be formed in both ground and excited energy levels with a Dirac $\delta$ potential. Because Galilean invariance is broken in a spin-orbit-coupled system, moving of the potential will induce a velocity-dependent effective detuning. This induced detuning breaks the spin symmetry and makes the ground-state transporting channel be spin-$\uparrow$ ($\downarrow$) favored while makes the excited-state transporting channel be spin-$\downarrow$ ($\uparrow$) favored for a positive-direction (negative-direction) transporting. When the $\delta$-potential well moves at a small velocity, both the ground-state and the excited-state channels contribute to the transportation, and thus both the spin components can be efficiently transported. However, when the moving velocity of the $\delta$-potential well exceeds a critical value, the induced detuning is large enough to eliminate the excited bound state, and makes the ground bound state the only transporting channel, in which only the spin-$\uparrow$ ($\downarrow$) component can be efficiently transported in a positive (negative) direction. This work demonstrates a prototype of unidirectional spin transport.

We investigate the low-lying spin excitation of a trapped dipolar spin-1 condensate with antiferromagnetic spin-exchange interactions by numerically solving the Bogoliubov–de Gennes equations. We explore the behavior of the excited modes with different dipolar interaction strength and in different trap geometries. We also discuss the origin of the Nambu-Goldstone (NG) zero modes in different phases by analyzing the broken continuous symmetries of the ground-state wave function. In particular, we find a quasi-NG mode in the antiferromagnetic phase. These studies lead to the discovery of an unusual quantum phase and the exact critical trap aspect ratio for one of the phase transitions.

We report an experimental demonstration of period doubling of dissipative soliton resonance (DSR) pulses in a fiber laser. DSR occurs in the close vicinity consisting of a set of parameters of a dissipative system, where the energy of the generated soliton increases without limit while the pulse peak power is clamped. Consequently, DSR pulses are immune to the appearance of period doubling, which is a threshold effect. However, period doubling of DSR pulses is experimentally demonstrated and numerically duplicated. The typical DSR feature, i.e., clamped pulse peak power and linear variation in pulse width with respect to the pump power change, is lost during the transition from period one to period doubling. However, the DSR performance reappears once the period doubling is fully developed. The breach and recurrence of DSR operation during pulse evolution results from the simultaneous change of multiple parameters because of the increasing pump power. In addition, DSR pulse narrowing with pump power increasing under the period-doubling state was experimentally observed.

We study the extent of spontaneous emission inhibition in self-assembled photonic crystals where the light path is routinely affected by unavoidable imperfections of the crystal and its finite-size. We discuss the role of finite-size effects that inadvertently modify the local density of optical states (LDOS) using time-resolved decay rate measurements from the single domains of real synthesized photonic crystals. We have obtained 34% contrast in the measured emission lifetimes at the stop gap wavelength in comparison to a wavelength outside the stop gap. We have shown a remarkable variation in the emission lifetimes at the stop gap for several domains across the sample which is nullified within a single domain. The results manifest wavelength-dependent linear scaling of lifetimes with the finite-size of the crystal domain. This is a signature of direct dependence of LDOS suppression on the crystal's finite-size which is also found to be in accordance with a recent theoretical model. The precise single-domain measurements result in a robust modification of lifetime in an otherwise weakly classified self-assembled photonic crystal.

Radiation from magnetic and electric dipole moments is a key subject in the theory of electrodynamics. Although people treat the problem thoroughly in the context of the frequency domain, the problem is still not well understood in the context of the time domain especially if dipole moments arbitrarily vary in time under the action of external forces. Here, we scrutinize the instantaneous power radiated by magnetic and electric dipole moments and report findings that are different from the conventional understanding of their instantaneous radiation found in textbooks. In contrast to the traditional far-field approach based on the Poynting vector, our analysis employs a near-field method based on the induced electromotive force, leading to corrective terms that are found to be consistent with time-domain numerical simulations, unlike previously reported expressions. Beyond its theoretical value, this paper may also have significant impact in the field of time-varying metamaterials especially in the study of radiation from subwavelength meta-atoms, scatterers, and emitters that are temporally modulated.

This work theoretically investigates high-order harmonic generation in rare-gas atoms driven by two temporally delayed ultrashort laser pulses. Apart from their temporal delay, the two pulses are identical. Using a single-atom model of the laser-matter interaction it is shown that the photon energy of the generated harmonics is controllable within the range of one eV—a bandwidth comparable to the photon energy of the fundamental field—by varying the time delay between the generating laser pulses. It is also demonstrated that high-order harmonics generated by double pulses have advantageous characteristics, which mimick certain properties of an extreme ultraviolet monochromator. With the proposed method, a simpler setup at a much lower cost and comparatively higher spectral yield can be implemented in contrast to other approaches.

Optically levitated rotors are prime candidates for torque sensors whose precision is limited by the fluctuations of the rotation frequency. In this work we investigate an optically levitated rotor at its fundamental thermal limit of frequency stability, where rotation-frequency fluctuations arise solely due to coupling to the thermal bath.

We propose a generalized system of nonequilibrium cavity QED with interacting dipoles coupled to a single-mode of an electromagnetic field in strong-, ultrastrong-, and deep-strong-coupling regimes. To illustrate the applicability of the system, an extended Dicke model is developed for atoms undergoing Raman transitions between the ground states in the presence of laser fields and considering dipole-dipole interactions; the latter has been neglected in many previous works. We have studied the effect of a ferroelectric and an antiferroelectric arrangement on the phase transition for both a finite and an infinite number of dipoles. An additional superradiant phase is observed in the deep-strong-coupling regime due to influence of the dipole-dipole interaction term. A high degree of dipole-dipole entanglement occurs for the antiferroelectric arrangement in the deep-strong regime, whereas it gets disentangled quite rapidly for the ferroelectric arrangement. A sharp transition of system parameters is observed in the ultrastrong-coupling regime and beyond. The dipole-dipole interaction also influences the spectra of the system, inducing a significant shift in the peaks, and modifies the average number of photons emitted.

We present a study of spontaneously arisen spatially entangled photon pairs via intermodal four-wave mixing in a graded-index multimode optical fiber. Unique dispersive features of the fiber allow spectral indistinguishability of two different phase-matched processes, producing entangled pairs of spatial qubits. The bases are realized as superpositions of orthogonal transverse fiber modes having opposite parities. In particular, we take into consideration the spectral properties of the processes by examining their joint spectral amplitudes. It is shown that illuminating graded-index optical fiber with different pump wavelengths has an impact upon efficiency parameters accordingly the degree of spatial entanglement and gives rise to photon pairs with various spectral purities. Photons with higher spectral purity enable desired single-photon based interactions to take place, whereas photons with lower spectral purity exhibit hybrid entanglement in frequency and transverse mode. We also discuss Wigner function formalism and parity-displacement-based realization to characterize spatial properties of the states, as well as to verify quantum entanglement through a violation of Clauser-Horne-Shimony-Holt inequality.

Cavity-mediated two-qubit gates, for example between solid-state spins, are attractive for quantum network applications. We propose three schemes to implement a controlled phase-flip gate mediated by a cavity. The main advantage of all these schemes is the possibility to perform them using a cavity with high cooperativity, but not in the strong-coupling regime. We calculate the fidelity of each scheme in detail, taking into account the most important realistic imperfections, and compare them to highlight the optimal conditions for each scheme. Using these results, we discuss which quantum system characteristics might favor one scheme over another.

In this paper we reconsider the single-parameter quantum Fisher information (QFI) and compare it with the two-parameter one. We find simple relations connecting the single-parameter QFI (both in the asymmetric and symmetric phase shift cases) to the two-parameter Fisher matrix coefficients. Following some clarifications about the role of an external phase [Phys. Rev. A 85, 011801(R) (2012)], the single-parameter QFI and its overoptimistic predictions have been disregarded in the literature. We show in this paper that both the single- and two-parameter QFI have physical meaning, and their predicted quantum Cramér-Rao bounds are often attainable with the appropriate experimental setup. Moreover, we give practical situations of interest in quantum metrology, where the phase sensitivities of a number of input states approach the quantum Cramér-Rao bound induced by the single-parameter QFI, outperforming the two-parameter QFI.

The so-called quantum vampire effect, or, namely, collapse-free action at a distance by photon annihilation operator, is introduced in Optica 2, 112 (2015). Here, we investigate the quantum vampire effect in scenarios involving multiple photon annihilation on two different optical beams. We show that all $m$-time photon annihilations in some specific state will generate the same output, meaning that $m$ vampires can hide both their locations and their numbers in each optical mode. This demonstrates another interesting and strange phenomenon of quantum optics that originates from the property of the annihilation operator: $\widehat{a}|0\rangle =0$. However, the annihilation operator is a nonphysical and nonunitary operation, and realistic implementation of photon annihilation is performed with the beam splitter and photon detector. We find that realistic photon annihilation will “leave” some shadow that makes the quantum vampire easier to track in real scenarios. We design a simple quantum measurement to catch realistic quantum vampires.

In an atomic ensemble, quantum information is typically carried as single collective excitations. It is very advantageous if the creation of single excitations is efficient and robust. A Rydberg blockade enables deterministic creation of single excitations via a collective Rabi oscillation by precisely controlling the pulse area, being sensitive to many experimental parameters. In this paper, we implement the adiabatic rapid passage technique to the Rydberg excitation process in a mesoscopic atomic ensemble. We make use of a two-photon excitation scheme with an intermediate state off-resonant and sweep the laser frequency of one excitation laser. We find the chirped scheme preserves internal phases of the collective Rydberg excitation and is more robust against variance of laser intensity and frequency detuning.