High-mass matter-wave interferometry opens interesting perspectives for testing fundamental physics while posing intriguing challenges in experimental technologies. Here we explore the source and detection techniques that will be the basis for the next generation of quantum experiments with objects at least one order of magnitude more massive than the current state of the art. Magnetron sputtering provides a means to produce a continuous metal cluster beam with a large variability in material composition, structure, temperature, velocity, electric, and magnetic properties. The mass-scalable nanoparticles come with high ionization yield and a detected beam brilliance of up to ${10}^{11} {\mathrm{sr}}^{-1}$ ${\mathrm{s}}^{-1}$ clusters. We find favorably slow and narrow velocity distributions for mass-filtered clusters up to 800 kDa, which can also be readily ionized, manipulated, and detected by 266 nm UV laser light. The source-detector combination that we demonstrate here fulfills the essential requirements for a next generation of nanoparticle interferometry.

High-temperature virial expansion is a powerful tool in equilibrium statistical mechanics. In this paper we generalize this method to the study of nonequilibrium dynamics based on contour Green's functions and present the diagrammatic rules to calculate the virial expansion coefficients. As an application of our theory, we recover the dynamics of a Bose gas quenched from noninteracting to finite interaction and obtain analytically the long-time limit of the momentum distribution function of the gas, consistent with previous numerical results.

For single-photon generation based on cavity quantum electrodynamics, we investigate a practical model assuming a Gaussian wave packet. This model makes it possible to comprehensively analyze the temporal dynamics of an atom-cavity system with both adiabatic and nonadiabatic conditions using analytical expressions. These results enable us to clarify the relationship between pulse width and maximum success probability over the full range of coupling regimes. We demonstrate how to achieve a high success probability while keeping a short pulse width by optimizing the cavity transmittance parameter and the time-controlled external field. Our formulations provide a practical tool for efficient single-photon generation in a wide variety of experimental platforms.

An analysis of the homodyne tomography process that is often used to determine the Wigner functions of quantum optical states is performed to consider the effects of the spatiotemporal degrees of freedom. The homodyne tomography process removes those parts of the input state that are not associated with the mode of the local oscillator by tracing out those degrees of freedom. Using a functional approach to incorporate all the spatiotemporal degrees of freedom, we find that this reduction in the degrees of freedom introduces distortions in the observed Wigner function. The analysis also shows how the homodyne tomography process introduces a scaling caused by the efficiency and a resolution that depends on the strength of the local oscillator. As examples, we consider coherent states, Fock states, and squeezed vacuum states.

Which-path information of a quantum particle in interferometers is the key to infer the past of the quantum particle. It has led to many extensive discussions, including discussions of quantum complementarity and the path-visibility relation. The basics of these discussions are the description, detection, and control of which-path information. In this paper, we focus on the investigation of multidimensional which-path information in a nested Mach-Zehnder interferometer. A general expression of which-path information is given and can be partially extracted by different detection methods. Further analysis shows that the which-path information can be controlled by the phase differences and beam-splitting ratios between the arms of the nested Mach-Zehnder interferometer. Moreover, an alternative which-path information elimination phenomenon has been predicted and demonstrated experimentally. Our work can help us to understand the physics of quantum particles, with potential applications for quantum information processing and quantum metrology.

Understanding the demarcation line between classical and quantum is an important issue in modern physics. The development of such an understanding requires a clear picture of the various concurrent notions of “classicality” in quantum theory presently in use. Here we focus on the relationship between Kolmogorov consistency of measurement statistics—the foundational footing of classical stochastic processes in standard probability theory—and the commutativity (or absence thereof) of measurement operators—a concept at the core of quantum theory. Kolmogorov consistency implies that the statistics of sequential measurements on a (possibly quantum) system could be explained entirely by means of a classical stochastic process, thereby providing an operational notion of classicality. On the other hand, commutativity of measurement operators is a structural property that holds in classical physics and its breakdown is the origin of the uncertainty principle, a fundamentally quantum phenomenon. We formalize the connection between these two a priori independent notions of classicality, demonstrate that they are distinct in general and detail their implications for memoryless multitime quantum processes.

We report on the realization of large assembled arrays of more than 300 single ${}^{87}\mathrm{Rb}$ atoms trapped in optical tweezers in a cryogenic environment at $\sim 4$ K. For arrays with $N_{\mathrm{a}}=324$ atoms, the assembly process results in defect-free arrays in $\sim 37\%$ of the realizations. To achieve this high assembling efficiency, we equalize the loading probability of the traps within the array using a closed-loop optimization of the power of each optical tweezer, based on the analysis of the fluorescence time traces of atoms loaded in the traps.

A differential operation, which can highlight features of images, is of significant importance in fields such as machine vision. As a front-end action, optical differentiation can overcome the shortcomings of traditional computer differentiation and runs at the speed of light. Compared with the conventional first-order optical differentiation technique, a higher-order one with the correct sign of a differential field obtained can facilitate the recognition of finer features, although relevant studies are relatively scarce. Here, we propose a method to realize sign-distinguishable arbitrary-order optical differentiation by coupling the momentum and polarization of a light field, the feasibility of which is further confirmed by a proof-of-principle experiment. We hope our method will promote the development of optical analog computing, image recognition, and other relevant applications.

Adiabatic frequency conversion (AFC) is a promising alternative for shifting the frequency of optical signals. In this paper we present a comprehensive theoretical study of the energy efficiency of AFC in an all-pass resonator. Through the Cauchy-Schwarz inequality, we deduce the upper limit of the energy efficiency of the AFC process and analyze its dependence on the input pulse shape. For a fixed pulse shape, we discuss the dependence of the AFC efficiency on its relevant timescales. We show that maximum efficiency requires overcoupling of the resonator to a degree dependent on the input pulse duration. We compare and contrast the requirements for optimal AFC for pulsed and continuous-wave inputs.

We propose and experimentally realize a general scheme of differential imaging employing the idea of weak measurement. We show that the weak coupling between the system of interest and a two-level ancilla can introduce a two-beam circuit after an arbitrary preselection of the ancilla. By choosing the postselection orthogonal to the preselection measurement, an effective imaging platform based on differential operations is achieved. Experimental results on both the Sagnac interferometer and ultrathin Wollaston prism demonstrate that our imaging scheme successfully yields the boundary information of complex geometric configurations.

Dissipative solitons (DSs) widely exist in optical systems and mode-locked fiber lasers provide a versatile playground to study their dynamics. Recently, Mamyshev oscillators (MOs), which rely upon a pair of narrow filters with offset passing frequencies, have emerged as a new type of femtosecond fiber lasers. Besides creating high-energy broadband DSs, MOs have been predicted to support periodically pulsating DSs whose observation remains elusive. Here, we present a comprehensive experimental and numerical investigation of pulsating DSs in an ytterbium MO. By reducing the filter separation, we observe pulsation in both single-pulse and DS molecule states. The output pulse energy can vary as large as 40 times in our MO. Single-shot spectra measured by the dispersive Fourier transform method further enables the observation of spectral bandwidth breathing and soliton explosion in the pulsation. In addition, pulsation lasting 9 round trips and even a chaotic pulsation state are observed. Numerical simulations based on a lumped vector model qualitatively agree with our observation. Our results enrich pulsating DS dynamics and reveal the impact of filter separation on the stability of MOs.

We introduce a family of highly symmetric bipartite quantum states in arbitrary dimensions. It consists of all states that are invariant under local phase rotations and local cyclic permutations of the basis. We solve the separability problem for a subspace of these states and show that a sizable part of the family is bound entangled. We also calculate some of the Schmidt numbers for the family in $d=3$, thereby characterizing the dimensionality of entanglement. Our results allow us to estimate entanglement properties of arbitrary states, as general states can be symmetrized to the considered family by local operations.

For more than 40 years since the first ion-atom collision investigations of the two-electron process of electron transfer with excitation and its resonant and nonresonant features, a satisfactory quantum mechanical treatment has been lacking. We present now such a comprehensive transfer excitation treatment using a three-electron atomic orbital close-coupling approach within a full configuration interaction formalism, exemplified by ${\mathrm{C}}^{4+}\left(1s^2\right)+\mathrm{He}$ collisions at 0.5–18 MeV impact energies. The calculated cross sections for the production of ${\mathrm{C}}^{3+}\left(1s2p^2{}^{2}D\right)$, which shows the strongest resonance signature among the KLL Auger transitions, are found to be in excellent agreement with zero-degree Auger projectile spectroscopy measurements covering the maximum of the resonance peak and its high energy wing. At the lower energies, the theoretical results show a second maximum which is interpreted through a nonresonant one-step transfer excitation mechanism, never considered to date. The present, fully coherent, many-body treatment provides an important advance in the modeling and understanding of multielectronic processes in quantum systems under strong and ultrafast perturbations.

We present a scheme to generate photon-pair states through resonant spontaneous four-wave mixing (SFWM) in a five-level double-bowtie $(\bowtie )$ configuration subject to two pumping fields and one coupling field when a quasidark state is created between two of three ground levels. The quasidark state is used here to largely suppress linear resonant absorption (gain) of the generated photon pairs while supporting strong SFWM nonlinearities and nonclassical cross-correlation. Numerical results for cold atomic samples show that the generation efficiency may be comparable to or even larger than that obtained in the off-resonance four-level single-bowtie configuration with only one pumping field. This scheme is robust to fluctuations of atomic populations and enables one to easily control the ratio of two rates with which distinct SFWM processes generate photon-pair states.

Fluctuation theorems are one of the pillars of nonequilibrium thermodynamics. Broadly speaking, they concern the statistical distribution of quantities such as heat, work, or entropy production. Quantum experiments, however, usually can only assess these distributions indirectly, or reconstruct them a posteriori. In this Letter we report an experiment where the distribution of entropy production is obtained directly from the outcomes (clicks) of an optical experiment simulating the interaction between a two-level system and a thermal reservoir. The setup consists of entangled photon pairs, one of which is sent to an interferometer implementing a finite-temperature amplitude damping channel, and is designed so as to allow full access to the two-point measurement statistics of both system and reservoir. First, by measuring the entangled pair, we directly implement the two-point measurement scheme in the system, avoiding the destructive nature of photodetection. Second, each optical path of the interferometer is associated with a specific transition of the reservoir. Thus, by blocking all but one of the paths, we can measure the conditional entropy production, given a specific reservoir trajectory.

Many complex systems can spontaneously oscillate under nonperiodic forcing. Such self-oscillators are commonplace in biological and technological assemblies where temporal periodicity is needed, such as the beating of a human heart or the vibration of a cello string. While self-oscillation is well understood in classical nonlinear systems and their quantized counterparts, the spontaneous emergence of periodicity in quantum systems is more elusive. Here, we show that this behavior can emerge within the repeated-interaction description of open quantum systems. Specifically, we consider a many-body quantum system that undergoes dissipation due to sequential coupling with auxiliary systems at random times. We develop dynamical symmetry conditions that guarantee an oscillatory long-time state in this setting. Our rigorous results are illustrated with specific spin models, which could be implemented in trapped-ion quantum simulators.

Inspired by the recent work [Nature (London) 521, 336 (2015)], we propose a scheme to suppress the backaction-induced dephasing of the spin state in a spin-oscillator system by squeezing the quantum fluctuation of the two-mode oscillator. We show specifically that squeezing either on a single mode individually or on both modes simultaneously could be used to effectively suppress spin dephasing. Therefore, with a spin-oscillator interferometer input by the spin-dependent squeezing cat state, the sensitivity of the rotational parameter estimation can be significantly enhanced. Given that various squeezings on the two-mode vibrations of the oscillator have been experimentally demonstrated, the proposal could provide a possibility to implement the sensitive parameter estimation, particularly in the ion-trap system.

Realizing nonunitary transformations on unitary-gate-based quantum devices is critically important for simulating a variety of physical problems, including open quantum systems and subnormalized quantum states. We present a dilation-based algorithm to simulate nonunitary operations using probabilistic quantum computing with only one ancilla qubit. We utilize the singular-value decomposition (SVD) to decompose any general quantum operator into a product of two unitary operators and a diagonal nonunitary operator, which we show can be implemented by a diagonal unitary operator in a one-qubit dilated space. While dilation techniques increase the number of qubits in the calculation, and thus the gate complexity, our algorithm limits the operations required in the dilated space to a diagonal unitary operator, which has known circuit decompositions. We use this algorithm to prepare random subnormalized two-level states on a quantum device with high fidelity. Furthermore, we present the accurate nonunitary dynamics of two-level open quantum systems in a dephasing channel and an amplitude-damping channel computed on a quantum device. The algorithm presented will be most useful for implementing general nonunitary operations when the SVD can be readily computed, which is the case for most operators in the noisy intermediate-scale quantum computing era.

We report experimental observations of the rotational structure in photoassociative spectroscopy of ultralong-range Rydberg molecules (ULRRMs) in an ultracold gas of ${}^{86}\mathrm{Sr}$. ULRRM spectroscopy probes scattering wave functions at much larger internuclear separations than photoassociative spectroscopy of low-lying electronic states. At such separations photon recoil momentum can lead to the transfer of significant angular momentum to the Rydberg molecule, evidence of which is provided through the distribution of excited rotational states. The visibility of the rotational structure is enhanced because, for collisions between ground-state ${}^{86}\mathrm{Sr}$ atoms, the large, near-resonant s-wave scattering length approaches the size of the ULRRM. Similar enhancement is not seen for ${}^{84}\mathrm{Sr}$, which has a much smaller scattering length. Results are interpreted with the aid of a theory that accounts for the recoil momentum associated with photoexcitation and the large $s$-wave scattering length in the entrance channel. While the observed rotational splittings and the qualitative trends in the relative intensities of the spectral features are well described by theory, the effects of recoil momentum are more prominent in the measured data than in the theory.