The twin-field quantum key distribution (TF-QKD) protocol has led to extensive investigation for its capacity of overcoming the rate-distance limit. One variant for TF-QKD named sending-or-not-sending (SNS) TF-QKD has also been proposed. The SNS-TF-QKD encodes the classical keys 0 and 1 in sending or not sending the phase-randomized coherent states, which helps it to tolerate large misalignment errors due to optical phase fluctuations. Nevertheless, the original SNS-TF-QKD scheme utilizes only single-photon interference in generating secret keys, which seems quite different from other TF-QKD protocols. In this paper, we prove that multiphoton interference in the SNS TF-QKD protocol can also contribute to key generation. Accordingly, an improved SNS-TF-QKD scheme with higher key rate is proposed, where events of not only single-photon but also two-photon emissions are considered as effective events.

We show that the variational quantum-classical simulation algorithm admits a finite circuit depth scaling collapse when targeting the critical point of the transverse field Ising chain. The order parameter only collapses on one side of the transition due to a slowdown of the quantum algorithm when crossing the phase transition. In order to assess performance of the quantum algorithm and compute correlations in a system of up to 752 qubits, we use techniques from integrability to derive closed-form analytical expressions for expectation values with respect to the output of the quantum circuit. We also reduce a conjecture made by Ho and Hsieh [W. W. Ho and T. H. Hsieh, SciPost Phys. 6, 029 (2019)] about the exact preparation of the transverse field Ising ground state to a system of equations.

Angular distribution of the $K{\alpha }_1$ $(1s2p{}^{1,3}P_{1,2}\to 1s^2{}^{1}S_0)$ emission line following electron-impact excitation of heliumlike spin-1/2 ${\mathrm{Tl}}^{79+}$ ions is studied within the framework of the density-matrix theory and the relativistic distorted-wave theory. In particular, we aim to explore how a (nonzero) nuclear magnetic dipole moment ${\mu }_I$ affects the $K{\alpha }_1$ angular distribution due to the hyperfine interactions. To this end, detailed calculations are performed for selected spin-1/2 ${}_{81}{}^A\mathrm{Tl}^{79+}$ ($A=187$, 205, and 207) ions with large ${\mu }_I$. It is found that the hyperfine-induced effects on the $K{\alpha }_1$ angular distribution depend strongly on the impact electron energy. When compared with the case of zero-spin ${\mathrm{Tl}}^{79+}$ ions, the $K{\alpha }_1$ angular distribution becomes much more anisotropic for low impact energies, whereas this anisotropy decreases quickly at intermediate and high impact energies. Such a behavior is in sharp contrast to the results for the fine-structure-resolved $1s2p{}^{3}P_2\to 1s^2{}^{1}S_0$ component [Phys. Rev. A 102, 042813 (2020)]. Therefore, accurate $K{\alpha }_1$ angular measurements at low impact electron energies could be employed as a sensitive tool to probe the hyperfine interaction in highly charged few-electron ions.

The separation of electronic and nuclear dynamics due to differing timescales is a useful concept for understanding ground-state molecular systems. However, coupling between these degrees of freedom is critical to understanding the evolution of most excited-state systems. We measure two-photon ionization delays of ${\mathrm{H}}_2$ and compare to calculations of the same measurement in a frozen-nuclei approximation. We find discrepancies between the vibrationally resolved measurement and bond-length-dependent theory, suggesting that nuclear motion affects ${\mathrm{H}}_2$ photoionization on attosecond timescales. We ascribe our observation to nuclear-electronic channel coupling between continuum vibrational states. Our results demonstrate that nuclear-electronic coupling cannot be neglected in the sudden ionization of molecules containing light atoms.

The phase noise of incident light fields can be converted into amplitude noise via absorption and dispersion effects in media under electromagnetically induced transparency. The conversion process is sensitive to two-photon detuning. This effect can be used to perform a different type of Rydberg spectroscopy. We present experimental measurements of the phase noise spectrum (PNS) with Rydberg atoms in a cesium vapor cell. Furthermore, to explore microwave field measurements, a modulation-enhanced PNS approach has been proposed and demonstrated.

The ground state of spin-orbit-coupled (SOC) Bose-Einstein condensates (BECs) is an intriguing subject that has not yet been fully explored. In particular, how to quickly obtain ground states for discovering their new structures is currently under active debate. In this work, we construct a theory-guided neural network (TgNN) to explore the ground states of one-dimensional BECs with and without SOC. We find that such method is markedly superior to the ordinary deep neural network due to theory guidance of the underlying problems. The former can replace the traditional method to directly give any ground state within a valid parameter plane using only imaginary-time evolution data of nine solutions as the training data, without the tedious step-by-step iterative calculation process. Furthermore, we show that the method of TgNN exhibits higher reliability and a better ability to generalize beyond regimes covered with the training data. This method provides a promising technique for discovering novel structures of ground states in parameterized spatiotemporal systems.

We revisit the question concerning the stability of nonuniform superfluid states of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type to thermal and quantum fluctuations. On general grounds, we argue that the mean-field phase diagram hosting a Lifshitz point cannot be stable to fluctuations for isotropic, continuum systems, at any temperature $T>0$ in any dimensionality $d<4$. In contrast, in layered unidirectional systems, the lower critical dimension for the onset of FFLO-type long-range order accompanied by a Lifshitz point at $T>0$ is $d=5/2$. In consequence, its occurrence is excluded in $d=2$, but not in $d=3$. We propose a relatively simple method, based on nonperturbative renormalization group, to compute the critical exponents of the thermal $m$-axial Lifshitz point continuously varying $m$, spatial dimensionality $d$, and the number of order parameter components, $N$. We point out the possibility of a robust, fine-tuning free occurrence of a quantum Lifshitz point in the phase diagram of imbalanced Fermi mixtures.

We propose a two-dimensional spectroscopic protocol for measuring the time-dependent coherences between the stationary states of a system induced by a time-dependent system-bath interaction. We also investigate the role of temporally correlated noise on coherence dephasing. This protocol enables dynamical information about the system and its coupling to the environment to be determined. Our results are based on the quantum-trajectory method, and are obtained from both approximate, analytical and exact, numerical solutions of the time-dependent Schrödinger equation. As an example, we show how this protocol can be used to investigate exciton dynamics in conjugated polymers induced by the coupling of their torsional modes with the environment.

In this paper, we describe experimental observation and characterization of emission of photon pairs through four-wave mixing (FWM) in a silica nanofiber, with high coincidence to accidental ratio (CAR, at best greater than 20 000 in the cw regime at pump power 24 mW and 40 000 in the pulsed regime at pump power ≈150 μW) and high pair emission rate (above 1 MHz in the pulsed regime with CAR around 100 at pump power ≈7 mW), despite residual presence of Raman spontaneous scattering (in the vicinity of the 12th silica Raman band) on the idler side. Moreover, these pairs of photons are observed with either a single-longitudinal-mode source or a multimode source (as well as a pulsed source) showing a factor of 2 increase of emission rate with the multimode source thanks to its random temporal fluctuation coupled to the quadratic response of FWM.

Three-dimensional (3D) nonlinear photonic crystal (NPC) has recently been realized in experiment, which provides a powerful platform for quantum-state engineering. Here, we propose a fork-grating 3D NPC scheme to manipulate orbital-angular-momentum (OAM) correlations of entangled two-photon pairs. The spiral spectra, bandwidths, and generation rates of counterpropagating and copropagating photon pairs are theoretically analyzed. Our results show that 3D NPC is of practical significance to generate high-dimensional OAM entanglement for quantum precision measurement, quantum computing, and quantum communication.

We calculate the cavity-field distribution in the Wigner representation for the two-photon resonance of the weakly driven Jaynes-Cummings (JC) oscillator in its strong-coupling limit. Using an effective four-level system, we analytically demonstrate the presence of steady-state and transient bimodality which breaks azimuthal symmetry in phase space. The two steady-state peaks are located at opposite positions and do not correspond to the two-photon amplitude of the driven transition. The developing bimodality is portrayed in parallel with the evolution of the intensity correlation function for the forward-scattered photons, before being finally contrasted to the few-photon steady-state and transient phase-space profiles for the cavity mode in the JC model driven on resonance.

We have devised an experimentally realizable model generating twin-beam states whose individual beam photon statistics are varied from thermal to Poissonian (by temporal mode averaging) keeping the nonclassical mode correlation intact. We have studied the usefulness of these states for loss measurement by considering three different estimators, comparing with the correlated thermal twin-beam states generated from spontaneous parametric down conversion or four-wave mixing. We then incorporated the photon subtraction operation into the model and demonstrated their performance in loss estimations with respect to unsubtracted states at both fixed squeezing and per photon exposure of the absorbing sample. For instance, at fixed squeezing, for two photon subtraction, up to three times advantage is found. An unexpected result in the latter case is that in some operating regimes the photon subtraction scheme can also give up to $20\%$ advantage over the correlated thermal beam result and no advantages are obtained when the statistics of each beam turns to Poissonian. We have also made a comparative study of these estimators for finding the best measurement for loss estimations. We present results for all the values of the model parameters changing the statistics of twin-beam states from thermal to Poissonian.

We study theoretically two vibrating quantum emitters trapped near a one-dimensional waveguide and interacting with propagating photons. We demonstrate that in the regime of strong optomechanical interaction the coupling of light to multiple emitter vibrations can lead to the formation of spatially localized multiphonon modes, exhibiting parity-time $\left(\mathcal{PT}\right)$ symmetry breaking. These localized multiphonon states have been interpreted by analyzing the energy spectrum in the quasiclassical approximation.

We report on a surprisingly efficient acceleration of ambient gas ions in the presence of a laser-irradiated nanoparticle. Near-infrared laser pulses with an intensity of $2\times {10}^{15}\mathrm{W}{\mathrm{cm}}^{-2}$ are used to irradiate a single CsCl nanoparticle surrounded by a helium gas with a density of around ${10}^{15}{\mathrm{cm}}^{-3}$. In this interaction ${\mathrm{He}}^{+}$ ions gain higher maximum kinetic energy than ${\mathrm{Cs}}^{+}$ and ${\mathrm{Cl}}^{+}$ ions. We suggest that ${\mathrm{He}}^{+}$ ions are accelerated by a positive charge at the nanoparticle surface whereas the movement of ${\mathrm{Cs}}^{+}$ and ${\mathrm{Cl}}^{+}$ ions is interlinked to the expansion of the nanoplasma electrons. Our results reveal an essential contribution of the environment to the interaction of intense light with dense matter.

We introduce jump-time unraveling as a distinct description of open quantum systems. As our starting point, we consider quantum jump trajectories, which emerge, physically, from continuous quantum measurements, or, formally, from the unraveling of Markovian quantum master equations. If the stochastically evolving quantum trajectories are ensemble-averaged at specific times, the resulting quantum states are solutions to the associated quantum master equation. We demonstrate that quantum trajectories can also be ensemble-averaged at specific jump counts. The resulting jump-time-averaged quantum states are then solutions to a discrete, deterministic evolution equation, with time replaced by the jump count. This jump-time evolution represents a trace-preserving quantum dynamical map if and only if the associated open system does not exhibit dark states. In the presence of dark states, on the other hand, jump-time-averaged states may decay into the dark states and the jump-time evolution may eventually terminate. Jump-time-averaged quantum states and the associated jump-time evolution are operationally accessible in continuous measurement schemes, when quantum jumps are detected and used to trigger the readout measurements. We illustrate the jump-time evolution with the examples of a two-level system undergoing relaxation or dephasing, a damped harmonic oscillator, and a free particle exposed to collisional decoherence.

If a wave function is written in polar form it becomes possible to write the Schrödinger equation of nonrelativistic quantum mechanics in a form analogous to the classical Hamilton-Jacobi equation with an extra term known as the quantum potential. Time-dependent supersymmetry is a procedure for finding new solutions of the Schrödinger equation if one solution is known. In this paper a time-dependent supersymmetry transformation is applied to a wave function in this polar form and it is shown that the classical potential plus the quantum potential is a conserved quantity under this transformation under certain circumstances. This leads to a modification of our view of the role of the quantum potential and also to a deeper appreciation of the function of a supersymmetry transformation.

Non-Gaussian quantum states of light are critical resources for optical quantum information processing, but methods to generate them efficiently remain challenging to implement. Here we introduce a generic approach for non-Gaussian state production from input states populating discrete frequency bins. Based on controllable unitary operations with a quantum frequency processor, followed by photon-number-resolved detection of ancilla modes, our method combines recent developments in both frequency-based quantum information and non-Gaussian state preparation. Leveraging and refining the $K$-function representation of quantum states in the coherent basis, we develop a theoretical model amenable to numerical optimization and, as specific examples, design quantum frequency processor circuits for the production of Schrödinger cat states, exploring the performance tradeoffs for several combinations of ancilla modes and circuit depth. Our scheme provides a valuable framework for producing complex quantum states in frequency bins, paving the way for single-spatial-mode, fiber-optic-compatible non-Gaussian resources.

High-harmonic generation (HHG) from an atomic gas target provides a valuable tool to generate high-energy radiation on the tabletop. The polarization properties of this radiation can be manipulated if the HHG process is driven by a bi-elliptical orthogonal two-color laser beam (BEOTC beam). Here we theoretically consider this setup within the so-called strong-field approximation and a quantum-orbit approach. We demonstrate that this approach allows the complete description of the geometry of the outgoing elliptically polarized harmonic radiation. In particular, we identify and classify the relevant quantum orbits that contribute to the process and analyze the harmonic spectrum, ellipticities, and offset angles in detail for $\omega \text{−}2\omega$ BEOTC beams with a fundamental wavelength of $\lambda =800\mathrm{nm}$. For these specific beam parameters, we find that the ellipticity as well as the offset angle of the polarization ellipse with respect to the BEOTC driving beam changes rapidly for consecutive harmonic orders. We show that this oscillatory behavior of the harmonic ellipticity can be explained via the superposition of the dipole radiation emitted from the target atom during different time intervals within one optical cycle of the BEOTC beam. This understanding of the HHG process driven by BEOTC beams will facilitate the generation of highly energetic radiation with complex polarization properties on the tabletop.

Inspired by the advantage of unconventional photon blockade (UCPB), we propose a simple cavity-driven quantum electrodynamics model composed of a single two-level atom embedded in an asymmetrical Fabry-Perot cavity. We derive the necessary matching conditions of frequency and intensity required for unconventional photon blockade. In the asymmetric cavity, the optimal atom-cavity coupling strength to get UCPB is related to the driving direction, which causes the equal-time second-order correlation function $g^{\left(2\right)}\left(0\right)$ of the cavity field depending on the driving direction, too. Due to the direction dependent effect, the output photons can be bunched in one direction incident while they are antibunched in the opposite direction incident, which is the so-called nonreciprocal UCPB (NUCPB). A giant NUCPB can be obtained under the appropriate parameter regime, where the output photons are fully antibunching in the opposite driving case. Our findings show a prospective path to producing giant quantum nonreciprocity.

We propose a method for spatially rerouting single photons or light in a coherent state with a small average photon number by purely electronic means, i.e., without using mechanical devices such as micromirror arrays. The method is based on mapping the quantum state of the incoming light onto a spin-wave in an atomic ensemble, as is done in quantum memories of light. Then the wave vector of the spin-wave is modified in a controlled way by an applied magnetic field gradient. Finally, by reapplying the same control beam as for storing, the signal pulse is released in a new direction that depends on the deflected wave vector of the spin-wave. We show by numerical simulation that efficiencies can be achieved for arbitrary deflection angles in the plane that are comparable with simple photon storage and reemission in the forward direction, and we propose a method for eliminating the stored momentum as a source of decoherence in the quantum memory. In a reasonable parameter regime, the rerouting should be achievable on a timescale on the order of a few to $\sim 100$ microseconds, depending on the deflection angle. The shifts in the wave vector that can be achieved using the Zeeman effect, with otherwise minimal changes to the spin-wave, can also be used to complement existing ac-Stark spin-wave manipulation methods.