The parameter estimation of the Hamiltonian of an unknown quantum system is essential in quantum information processing. However, the general methods based on quantum state tomography and process tomography are resource demanding and challenging in large quantum systems. On the other hand, if the interaction form of a system is known, one can adopt the quantum quench protocol to reconstruct the generic many-body Hamiltonian with local interactions, requiring significantly lower complexity and measurement cost compared to the general tomography technique. Here we report an experimental demonstration of the quench method to determine the scalar coupling constants in a three-spin chain. In the experiments, we demonstrate that how the quality of the estimation is affected by the duration of the quench and the number of input states. Our results show that the quantum quench protocol is a promising strategy to implement Hamiltonian estimation for many-body systems, especially when the cost of measurement is crucial in practical cases.

We study the excitation and ionization of cesium Rydberg states in an electric field ($\approx 2200\mathrm{V}/\mathrm{cm}$) near the classical field-ionization threshold ($\approx -280{\mathrm{cm}}^{-1}$ binding energy) by three-dimensional (3D) ion-electron coincidence spectroscopy. Cesium atoms are produced by an effusive oven and excited with lasers to Stark-shifted Rydberg states or directly ionized. Using a double time-of-flight setup we record 3D ($X, Y$, time of flight) coincidence imaging of electrons and ions. Above-threshold photoionization creates broad images with poor electron-ion spatial correlation. Fast ionizing states produce very good correlations and the images reveal the electric-field map of the ionization region. Slow ionizing states show that the relatively high atomic velocity is detrimental to the correlations. Experimental data are accurately reproduced by detailed Monte Carlo excitation and ionization simulations based on Stark maps obtained with local-frame transformation (LFT) theory. Agreement between our spectroscopic experiment and LFT theory is very good, with better that hundreds of megahertz accuracy. But, on rare particular states, several gigahertz discrepancy is found. This study can be used to select appropriate states for the creation of ion and electron beams with high brightness, good correlation, and low energy dispersion.

This study develops an experimental method of deducing the profile of interaction induced between impurities in a trapped gas of ultracold Fermi or Bose atoms, which are often referred to as Fermi or Bose polarons. In this method, we consider a two-body Fermi or Bose polaron collision experiment in which impurities and atoms interact only weakly. Numerical simulations of the quantum dynamics reveal the possibility to obtain information regarding the nonlocal-induced interaction between two polarons from a measured profile of the polaron wave packet at several snapshots. This is because the potential of the induced interaction is well balanced by the quantum potential whenever the WKB approximation for the relevant Schrödinger equation is applicable.

We demonstrate detection of a vector light shift (VLS) using the quantum lock-in method. The method offers precise VLS measurement with little affect from real magnetic field fluctuation. We detect a VLS on a Bose-Einstein condensate (BEC) of ${}^{87}\mathrm{Rb}$ atoms caused by an optical trap beam with a resolution less than 1 Hz. We demonstrate the elimination of a VLS by controlling the beam polarization to realize a long coherence time of an $F=2$ BEC in the stretched state. Quantum lock-in VLS detection should find wide application, including the study of spinor BECs, electric dipole moment searches, and precise magnetometry.

Studying the collective pairing phenomena in a two-component Fermi gas, we predict the appearance near the transition temperature $T_c$ of a well-resolved collective mode of quadratic dispersion. The mode is visible both above and below $T_c$ in the system's response to a driving pairing field. When approaching $T_c$ from below, the phononic and pair-breaking branches, characteristic of the zero temperature behavior, reduce to a relatively low energy-momentum region. Elsewhere they are replaced by the quadratically dispersed pairing resonance, which thus acts as a precursor of the phase transition. In the strong-coupling and Bose-Einstein condensate regime, this mode is a weakly damped propagating mode associated with a Lorentzian resonance. Conversely, in the BCS limit it is a relaxation mode of pure imaginary eigenenergy. At large momenta, the resonance disappears when it is reabsorbed by the lower edge of the pairing continuum. At intermediate temperatures between 0 and $T_c$ we unify the newly found collective phenomena near $T_c$ with the phononic and pair-breaking branches predicted from previous studies, and we exhaustively classify the roots of the analytically continued dispersion equation, and show that they provided a very good summary of the pair spectral functions.

The recoil of atoms in arrays due to the emission or absorption of photons is studied for subwavelength interatomic spacing. The atoms in the array interact with each other through collective dipole-dipole interactions and with the incident laser field in the low-intensity limit. Shining uniform light on the array gives rise to patterns of excitation and recoil in the array. They arise due to the interference of different eigenmodes of excitation. The relation between the recoil and the decay dynamics is studied when the array is in its excitation eigenstates. The recoil experienced by a subradiant collective decay is substantially larger than that from independent atom decay. A method to calculate the rate of recoil when steady state has been achieved with a constant influx of photons is also described.

A theory of pulse transmission is presented in which the medium through which the pulse propagates is characterized by an inhomogeneous distribution that is either an atomic frequency comb (AFC) or a medium of randomly spaced frequency spikes (RSM). For an AFC, we obtain analytic expressions for the transmitted field amplitude, which is composed of the (partially) transmitted incident pulse, plus a train of equally spaced echoes. For RSM, we derive expressions for the average transmitted field amplitude and field intensity. In the limit that the spike width is much less than the spike separation, normally encountered with AFC, the overall atom-field dynamics is dominated by dispersion—absorption plays a negligible role. The importance of the average group time delay of the pulse is discussed.

We present an all-fiber emitter-cavity quantum electrodynamics (QED) scheme which makes it possible to realize the strong photon blockade phenomenon in the strong coupling regime. To begin with, we study the emitter-cavity system where the two-level emitter and cavity are driven by a laser field under different driving strengths, respectively. Then, we theoretically derive that through driving the single-mode nanofiber cavity and a single two-level emitter simultaneously with two classical fields, the photon antibunching effect and the intracavity mean photon number will be tremendously enhanced. Compared with the systems driving an emitter or the cavity, respectively, the second-order correlation function at zero time decay $g^{\left(2\right)}\left(0\right)$ in our system can reach the minimum value as 0.0003, which is much smaller than a single-emitter-cavity QED system of emitter drive or cavity drive. In particular, under the condition of quantum interference model, the strong phenomenon of photon antibunching can be flexibly reached by controlling of the intensity relationship between the dual-pumping fields in the strong coupling regime. Our proposal can be achieved in the near future and has great potential applications in high-quality single-photon sources and quantum communication processing.

In this Letter, we analyze the quantum dynamics of the perceptron model: a particle is constrained on an $N$-dimensional sphere, with $N\to \infty$, and subjected to a set of randomly placed hard-wall potentials. This model has several applications, ranging from learning protocols to the effective description of the dynamics of an ensemble of infinite-dimensional hard spheres in Euclidean space. We find that the jamming transition with quantum dynamics shows critical exponents different from the classical case. We also find that the quantum jamming transition, unlike the typical quantum critical points, is not confined to the zero-temperature axis, and the classical results are recovered only at $T=\infty$. Our findings have implications for the theory of glasses at ultralow temperatures and for the study of quantum machine-learning algorithms.

We study the average quantum coherence over the pure state decompositions of a mixed quantum state. An upper bound of the average quantum coherence is provided, and sufficient conditions for the saturation of the upper bound are shown. These sufficient conditions always hold for two- and three-dimensional systems. This provides a tool to estimate the average coherence experimentally by measuring only the diagonal elements, which remarkably requires less measurements compared with state tomography. We then describe the pure state decompositions of the qubit state in the Bloch sphere geometrically. For any given qubit state, the optimal pure state decomposition achieving the maximal average quantum coherence as well as three other pure state decompositions are shown in the Bloch sphere. The order relations among their average quantum coherence are invariant for any coherence measure. The results presented in this paper are universal and suitable for all coherence measures.

Entanglement is a powerful tool for quantum sensing, and entangled states can greatly boost the discriminative power of protocols for quantum illumination, quantum metrology, or quantum reading. However, entangled state protocols generally require the retention of an idler state, to which the probes are entangled. Storing a quantum state is difficult and so technological limitations can make protocols requiring quantum memories impracticable. One alternative is idler-free protocols that utilize nonclassical sources but do not require any idler states to be stored. Here we apply such a protocol to the task of channel position finding. This involves finding a target channel in a sequence of background channels, and has many applications, including quantum sensing, quantum spectroscopy, and quantum reading.

Quantum control can be employed in quantum metrology to improve the precision limit for the estimation of unknown parameters. The optimal control, however, typically depends on the actual values of the parameters and thus needs to be designed adaptively with the updated estimations of those parameters. Traditional methods, such as gradient ascent pulse engineering (GRAPE), need to be rerun for each new set of parameters encountered, making the optimization costly, especially when many parameters are involved. Here we study the generalizability of optimal control, namely, optimal controls that can be systematically updated across a range of parameters with minimal cost. In cases where control channels can completely reverse the shift in the Hamiltonian due to a change in parameters, we provide an analytical method which efficiently generates optimal controls for any parameter starting from an initial optimal control found by either GRAPE or reinforcement learning. When the control channels are restricted, the analytical scheme is invalid, but reinforcement learning still retains a level of generalizability, albeit in a narrower range. In cases where the shift in the Hamiltonian is impossible to decompose to available control channels, no generalizability is found for either the reinforcement learning or the analytical scheme. We argue that the generalization of reinforcement learning is through a mechanism similar to the analytical scheme. Our results provide insights into when and how the optimal control in multiparameter quantum metrology can be generalized, thereby facilitating efficient implementation of optimal quantum estimation of multiple parameters, particularly for an ensemble of systems with ranges of parameters.

We uncover a highly nontrivial dependence of the spin-noise (SN) resonance broadening induced by the intense probe beam. The measurements were performed by probing the cell with cesium vapor at the wavelengths of the transition $6{}^{2}S_{1/2}\leftrightarrow 6{}^{2}P_{3/2}$ ($D_2$ line) with the unresolved hyperfine structure of the excited state. The light-induced broadening of the SN resonance was found to differ strongly at different slopes of the $D_2$ line and, generally, varied nonmonotonically with light power. We discuss the effect in terms of the phenomenological Bloch equations for the spin fluctuations and demonstrate that the SN broadening behavior strongly depends on the relation between the pumping and excited-level decay rates, the spin precession, and decoherence rates. To reconcile the puzzling experimental results, we propose that the degree of optical perturbation of the spin system is controlled by the route of the excited-state relaxation of the atom or, in other words, that the act of optical excitation of the atom does not necessarily break down completely its ground-state coherence and continuity of the spin precession. Spectral asymmetry of the effect, in this case, is provided by the position of the “closed” transition $F=4\leftrightarrow F^{\prime }=5$ at the short-wavelength side of the line. This hypothesis, however, remains to be proven by microscopic calculations.

We study carrier-envelope phase (CEP) -dependent photoelectron momentum distributions (PEMDs) generated by strong-field ionization of Xe atoms with linearly polarized sub-two-cycle near-infrared laser pulses. The PEMDs in the polarization direction end with a cutoff structure whose position and shape are analyzed using the adiabatic theory of rescattering [T. Morishita and O. I. Tolstikhin, Phys. Rev. A 96, 053416 (2017)]. This theory predicts that the cutoff, wherein the PEMD begins to exponentially decay, is located at a quantum caustic that is shifted towards higher energies with respect to the corresponding classical caustic at which long and short backward rescattering trajectories coalesce. The observed position of the cutoff in the PEMDs agrees well with the adiabatic theory, confirming the quantum shift of the caustic. This result paves the way to a more accurate procedure for extracting electron-ion elastic scattering differential cross sections from CEP-dependent cutoffs in strong-field PEMDs.

Rabi flopping is a paradigm of two-level quantum systems in the presence of an oscillatory driving field. Different from atoms, the Rabi flopping in a molecule depends on its bond length and orientation with respect to the light polarization direction. Here we explore the Rabi flopping in the strong-field dissociation of ${\mathrm{H}}_2{}^{+}$ in which the two lowest electronic states are photon coupled. ${\mathrm{H}}_2{}^{+}$ aligned along different angles experiences different laser intensities, building the Rabi flopping with angle-dependent frequencies. During the laser-${\mathrm{H}}_2{}^{+}$ interaction, the electron completes different fractions of Rabi flopping for ${\mathrm{H}}_2{}^{+}$ aligned in different directions, leading to the angular nodes and maxima in the dissociative proton momentum distribution. Our study reveals that one-photon dissociation undergoes the alternation of absorbing and emitting one photon, instead of the one-off photon absorption. This study also provides a universal explanation for the angular distribution of the dissociative fragments in strong laser fields.

Motivated by recent experiments with ultracold magnetic atoms trapped in optical lattices where the orientation of atomic dipoles can be fully controlled by external fields, we study, by means of quantum Monte Carlo, the ground-state properties of dipolar bosons trapped in a two-dimensional lattice with density-induced hopping and where the dipoles are tilted along the $xz$ plane. We present ground-state phase diagrams of the above system at different tilt angles. We find that as the dipolar interaction increases, the superfluid phase at half-filling factor is destroyed in favor of either a checkerboard or stripe solid phase for tilt angle $\theta \lesssim {30}^{\circ }$ or $\theta \gtrsim {30}^{\circ }$, respectively. More interesting physics happens at tilt angles $\theta \gtrsim {58}^{\circ }$, where we find that as the dipolar interaction strength increases, solid phases first appear at filling factor lower than 0.5. Moreover, unlike what is observed at lower tilt angles, we find that at half filling, a stripe supersolid intervenes between the superfluid and stripe solid phase.

A compact scheme for the preparation of macroscopic multipartite entanglement is proposed and analyzed. In this scheme, the vibrational modes of a mechanical resonator constitute continuous-variable (CV) subsystems that entangle with each other as a result of their interaction with a two-level system (TLS). By properly driving the TLS, we show that a selected set of modes can be activated and prepared in a multipartite entangled state. We first study the entanglement properties of a three-mode system by evaluating the genuine multipartite entanglement, and we investigate its usefulness as a quantum resource by computing the quantum Fisher information. Moreover, the robustness of the state against the qubit and thermal noises is studied, proving a long-lived entanglement. To examine the scalability and structural properties of the scheme, we derive an effective model for the multimode system through elimination of the TLS dynamics. This work provides a step toward a compact and versatile device for creating a multipartite noise-resilient entangled state in vibrational modes as a resource for CV quantum metrology, quantum communication, and quantum computation.

We propose theoretically an optomechanical system with double cavities to explore the thermophonon transport from the thermal bath of the mechanical oscillator to the coupled system. We find that the direction and magnitude of thermophonon flux in the system can be controlled flexibly by coupling an active cavity with gain to the driven cavity. In particular, the injected squeezing vacuum can reverse the nonequilibrium characteristics of the system and change the thermophonon flux from positive to negative. We also investigate in detail the influence of the driving power and the photon tunneling strength on the flux, which can widen the energy transfer channel of the system. The results obtained here have a potential application in the thermal noise energy harvesting and rectification by the optomechanical setup.