We study the ground state of two interacting bosonic particles confined in a ring-shaped lattice potential and subjected to a synthetic magnetic flux. The system is described by the Bose-Hubbard model and solved exactly through a plane-wave Ansatz of the wave function. We obtain energies and correlation functions of the system both for repulsive and attractive interactions. In contrast with the one-dimensional continuous theory described by the Lieb-Liniger model, in the lattice case we prove that the center of mass of the two particles is coupled with its relative coordinate. Distinctive features clearly emerge in the persistent current of the system. While for repulsive bosons the persistent current displays a periodicity given by the standard flux quantum for any interaction strength, in the attractive case the flux quantum becomes fractionalized in a manner that depends on the interaction. We also study the density after a long time expansion of the system. Our results can be used to benchmark approximate schemes for the many-body problem such as the density matrix renormalization group or other variational schemes.

We have investigated the combined positive and negative orientation-selected and yield-enhanced and -suppressed molecular tunneling ionization of carbon monoxide by dual-phase control of femtosecond Fourier-synthesized laser pulses. To investigate the role of each harmonic light, two types of experiments have been performed: (1) two-color experiments using Fourier-synthesized laser fields consisting of a fundamental light and its second or third harmonics and (2) three-color experiments using Fourier-synthesized laser fields consisting of a fundamental light and its second and third harmonics obtained by changing each relative phase difference independently. Positive and negative orientation-selected ionization is induced by the $\omega +2\omega$ laser fields, and yield-enhanced and -suppressed ionization is induced mainly by $\omega +3\omega$ laser fields. The Fourier-synthesized $\omega +2\omega +3\omega$ laser fields concurrently enhance both positive and negative orientation-selected and yield-enhanced and -suppressed molecular tunneling ionization. The mechanism is discussed in connection with the sub-optical-cycle interference control of the laser waveforms.

The nitrogen-vacancy (NV) center in nanodiamonds has emerged as an excellent platform in quantum technologies due to its applications in spin manipulation and nanoscale sensing. However, their use is limited by the unavoidable charge-state conversion under optical pumping. In this study, we investigate the control of charge-state conversion in NV centers using an engineered photonic environment and thus change the available local density of optical states (LDOS). The spectral- and pump-dependent decay rate measurements are performed to study the redistribution of emission rates due to the change in LDOS and their effect on charge-state conversion. We have achieved an 8% enhancement of emission rate at the zero phonon line which is accompanied with a 10% suppression in the phonon sideband emission rate in the low-pump-power regime. In the high-pump-power regime, the charge-state conversion becomes inevitable and leads to the deterioration of LDOS-induced modification in the NV center emission lifetimes. The results are useful for efficient NV center spin readout and charge-state depletion microscopy utilizing reversible charge-state conversion.

With respect to the quantum anomalous Hall effect (QAHE), the detection of topological nontrivial large-Chern-number phases is an intriguing subject. Motivated by recent research on Floquet topological phases, this study proposes a periodic driving protocol to engineer large-Chern-number phases using the QAHE. Herein, spinless ultracold fermionic atoms are studied in a two-dimensional optical dice lattice with nearest-neighbor hopping and a $\mathrm{\Lambda }$- or V-type sublattice potential subjected to a circular driving force. Results suggest that large-Chern-number phases exist with Chern numbers $C=-3$, which is consistent with the edge-state quasienergy spectra.

We propose a reliable scheme to realize the ultrastrong Jaynes-Cummings (JC) model by simultaneously modulating the resonance frequencies of the two-level system and the bosonic mode in the ultrastrong quantum Rabi model. We find that in both the high- and low-frequency modulation regimes, the counter-rotating terms can be completely suppressed without reducing the coupling strength of the rotating-wave terms, and hence the ultrastrong JC Hamiltonian is achieved. The ultrastrong JC interaction can not only be used to implement ultrafast quantum operations such as ultrafast quantum state transfer and quantum logic gates but also will open up a new route to the demonstration of quantum phase transition associated with the JC Hamiltonian across the deep-strong-coupling point. Some discussions on the experimental implementation of this scheme with circuit-QED systems are presented.

Bose condensed light can form new phases in a dye-filled cavity due to the presence of the orientational disorder created by dye molecules which are essentially frozen on the timescale of the photonic thermalization. At longer times (few ns), molecular degrees of freedom, i.e., orientations and positions, become important. Including them on equal footing with photons can change the nature of the photonic condensation—it can proceed as a first-order phase transition which can also result in the mutual phase-separation effect—for photons and dye. The analysis is conducted within the mean-field approach in the thermodynamic limit. The role of the finite-size effects is discussed and the recommendations for the experimental detection of the transition type are formulated.

Photon interference and bunching are widely studied quantum effects that have also been proposed for high-precision measurements. Here, we construct a theoretical description of photon interferometry on rotating platforms, specifically exploring the relation between noninertial motion, relativity, and quantum mechanics. On the basis of this, we then propose an experiment where photon entanglement can be revealed or concealed solely by controlling the rotational motion of an interferometer, thus providing a route towards studies at the boundary between quantum mechanics and relativity.

We propose a scheme to generate single-cycle terahertz (THz) pulses via reflection of an optical femtosecond pulse train from a thin layer of nonlinear resonant medium. Our method is based on the coherent control of low-frequency oscillations and free induction decay in the medium. The specific single-cycle shape of generated THz pulses requires a plane wave front and detection in the near field. Our theoretical results pave the way to a new, simple, and highly efficient way to generate single-cycle waveshape-tunable THz pulses.

It is known that a spatially partially coherent light field produces better imaging contrast compared to a spatially coherent field and that the contrast increases as the spatial coherence length of the field becomes smaller. The transverse spatial coherence length of most spatially partially coherent fields increases upon propagation. As a result, the field produces progressively decreasing image quality at subsequent transverse planes. By controlling the propagation of spatial coherence, we demonstrate enhanced image quality at different transverse planes along the propagation direction through a scattering medium. Using a source with propagation-invariant spatial coherence function, we report experimental observations of imaging different transverse planes with equal contrast over a significant distance. Furthermore, we generate a spatially partially coherent source that can be tailored to have minimum-possible transverse coherence area at the plane of the object to be imaged, and using this source, we demonstrate imaging spatially separated transverse planes with maximum possible image contrast.

We present a proposal for the optical simulation of para-Fermi oscillators in arrays of coupled waveguides. We use a representation that arises as a deformation of the su(2) algebra. This provides us with a set of chiral modes and a zero-energy-like normal mode. The latter is its own chiral pair and suggest the addition of controlled losses or gains following a pattern defined by parity. In these non-Hermitian para-Fermi oscillators, the analog of the zero-energy mode presents the largest effective loss or gain and it is possible to tune the system to show sequences of exceptional points and varying effective losses or gains. These arrays can be used for mode suppression or enhancement depending on the use of loss or gain, in that order. We compare our coupled-mode-theory predictions with finite-element-method simulations to good agreement.

We present improved results of sending-or-not-sending twin-field quantum key distribution by using error rejection through two-way classical communications. Our error rejection method, especially our method of actively odd-parity pairing (AOPP) can drastically improve the performance of sending-or-not-sending twin-field protocol in both secure distance and key rate. Taking a typical experimental parameter setting, our method here improves the secure distance by 70 km to more than 100 km in comparison with the prior art results. Comparative study also shows advantageous in key rates at regime of long distance and large misalignment error rate for our method here. The numerical results show that our method here can significantly exceed the absolute limit of direct transmission key rate, and also have an advantageous key rates higher than various prior art results by 10 to 20 times.

We study the generation of a magnon blockade in a single-crystalline yttrium-iron-garnet (YIG) sphere coupled to a three-dimensional superconducting microwave resonator that interacts with a superconducting qubit. Based on the indirect coupling between the Kittel mode of YIG and the superconducting qubit mediated by the virtual photon excitation in the microwave cavity, we propose two different mechanisms for observing the magnon blockade effect in the ferromagnetic sphere. The first one is to use the strong anharmonicity of the dressed states of the coupled qubit-magnon system. However, the strong magnon antibunching can only be achieved under the strong-coupling regime. To relax this constraint, we propose another quantum interference method to produce and enhance the magnon blockade effect with the moderate-coupling strength. By optimizing the relative phase and the strength ratio of the external driving fields applied to the YIG and qubit, we show that the much stronger magnon antibunching with significantly increased steady-state mean magnon numbers can be realized by the destructive interference for the two-magnon excitation.

We report the theory for time-resolved spectator resonant interparticle Coulombic decay (ICD) processes. Following excitation by a short extreme ultraviolet pulse, the spectrum of the resonant ICD electron develops. Strong-field ionization is imagined to quench the decay at different time delays and to initiate regular ICD. In this latter process, the ICD electron signal can be measured without interference effects. The typical lifetimes of ICD processes allow for the observation of oscillations of the time- and energy-differential ionization probability. We propose to utilize this oscillation to measure lifetimes of electronic decay processes.

The interplay between antiferromagnetic interaction and hole motion is capable of inducing conducting Haldane phases with topological features described by a finite nonlocal string order parameter. Here we show that these states of matter are captured by the one-dimensional $t\text{−}{J}_z$ model, which can be experimentally realized with dressed Rydberg atoms trapped onto a one-dimensional optical lattice. In the sector with vanishing total magnetization, exact calculations associated with the bosonization technique allow us to predict that both metallic and superconducting topological Haldane states can be achieved. With the addition of an appropriate magnetic field, the system enters a domain-wall structure with finite total magnetization. In this regime, the conducting Haldane states are confined in domains separated by regions where a fully polarized Luttinger liquid occurs. A procedure to dynamically stabilize such topological phases starting from a confined Ising state is also described.

A system of bosons in a two-dimensional harmonic trap in the presence of Rashba-type spin-orbit coupling is investigated. An analytic treatment of the ground state of a single atom in the weak-coupling regime is presented and used as a basis for a perturbation theory in the interacting two-boson system. The numerical diagonalization of both the single-particle and the two-boson Hamiltonian matrices allows us to go beyond those approximations and obtain not only the ground state, but also the low-energy spectra and the different energy contributions separately. We show that the expectation value of the spin-orbit term is related to the expectation value of ${\widehat{\sigma }}_z{\widehat{L}}_z$ for the eigenstates of the system, regardless of the trapping potential. The low-energy states of the repulsively interacting two-boson system are characterized. In the presence of a sufficiently strong interaction and spin-orbit coupling strength, there is a direct energy-level crossing in the ground state of the system between states of different $J_z$, the third component of the total angular momentum, that changes its structure. This is reflected in a discontinuity in the different energy terms and it is signaled in the spatial density of the system.

In this work we extend our previous studies on the quantum transfer of a particle through a finite-bandwidth continuum under frequent detections by replacing the assumed frequent measurements with a genuine continuous monitoring with a point-contact detector. We present a quantitative comparison between the two types of measurement. We also propose possible measurements, based on state-of-the-art experiments, to test the “scaling” property between the measurement rate and the bandwidth of the reservoir, rooted in the transfer dynamics under continuous monitoring.

We observe a significant steady-state deviation from the generalized equipartition theorem, one of the pivotal results of classical statistical mechanics, in a system of confined, laser-cooled atoms. We parametrize this deviation, measure its dynamics, and show that its steady-state value quantifies the departure of nonthermal states from thermal equilibrium even for anharmonic confinement. In particular, we find that deviations from equipartition grow as the system dynamics becomes more anomalous. We present numerical simulations that validate the experimental data and reveal an inhomogeneous distribution of the kinetic energy through the system, supported by an analytical examination of the phase space.

A time-dependent periodical field can be utilized to efficiently modify the Rabi coupling of system, exhibiting nontrivial dynamics. We propose a scheme to show that this feature can be utilized for speeding up the formation of dissipative steady entanglement based on the Rydberg antiblockade mechanism. When the modulation frequency is exactly equal to the central frequency of the driving field, a sufficient residence time of the two-excitation Rydberg state is allowed for an irreversible spontaneous decay onto the target state, leading to a steady entanglement with a high fidelity $\sim 0.98$ and a shorter generation time $<400\mu \mathrm{s}$. We show that a global maximal fidelity benefits from a consistence of microwave-field coupling and spontaneous decay strengths. The scheme manifests robust insensitivities towards the imperfect initialization, the fluctuations of van der Waals interaction, and the modulation frequency. This simple approach to facilitate the generation of dissipative entangled two-qubit states may guide an experimental direction in Rydberg quantum technology and quantum information.

We consider a qubit coupled to another system (its environment) and discuss the relationship between the effects of subjecting the qubit to either a dynamical decoupling sequence of unitary operations or a sequence of projective measurements. We give a formal statement concerning the equivalence of a sequence of coherent operations on a qubit, precisely operations from a minimal set $\{{\mathbb{1}}_Q,{\widehat{\sigma }}_x\}$, and a sequence of projective measurements of observable ${\widehat{\sigma }}_x$. Using it we show that when the qubit is subjected to $n$ such successive projective measurements at certain times, the expectation value of the last measurement can be expressed as a linear combination of expectation values of ${\widehat{\sigma }}_x$ observed after subjecting the qubit to dynamical decoupling sequences of $\pi$ pulses, with $k\le n$ of them applied at subsets of these times. Performing a sequence of measurements on the qubit gives then access to the same properties of the environment and qubit-environment coupling that are affecting the coherence observed in a dynamical decoupling experiment. Analyzing the latter has been widely used to characterize the environmental dynamics (perform so-called noise spectroscopy), so our result shows how the results obtained with dynamical-decoupling-based protocols are related to those that can be obtained just by performing multiple measurements on the qubit. We also discuss in more detail the application of the general result to the case of the qubit undergoing pure dephasing and outline possible extensions to higher-dimensional systems (a qudit or multiple qubits).

Recently, intensity correlation of x-ray fluorescence, based on the principle introduced by Hanbury Brown and Twiss [R. Hanbury Brown and R. Q. Twiss, Nature (London) 177, 27 (1956)], has been proposed for high-resolution imaging of a three-dimensional arrangement of atoms. To explore the applicability of this fluorescence approach, we theoretically investigate fluorescence dynamics of nonperiodic systems subject to x-ray free electron laser (XFEL) pulses over a range of fluences from the linear to nonlinear x-ray absorption regimes as a function of system size, from a single atom to a cluster of 149 171 atoms. Fluorescence dynamics in the nonlinear x-ray regime differs from that in the weak x-ray field in that intense x-ray pulses interrupt the fluorescence dynamics by multiphoton absorption creating a dense electron environment within the sample on a femtosecond timescale. In large systems, the presence of both recombination and photoionization pathways gives rise to an enhanced $K\alpha$ and $K{\alpha }^h$ emission yield and an extended emission time beyond the lifetime of the core-excited states. Our analysis suggests that, in an intense x-ray pulse, the $K{\alpha }^h$ emission line can be a good candidate for fluorescence imaging as it has a higher yield and an emission time that is short relative to the x-ray-induced lattice distortion time.