The observation of single-photon optomechanical effects is a desired task in cavity optomechanics. However, the realization of ultrastrong optomechanical interaction remains a big challenge. Here we present an all-optical scheme to simulate ultrastrong optomechanical coupling based on a Fredkin-type interaction, which consists of two exchange-coupled modes with the coupling strength depending on the photon number in another controller mode. This coupling enhancement is assisted by the displacement amplification according to the physical idea of the Bogoliubov approximation, which is realized by utilizing a strong driving to pump one of the two exchanging modes. Our numerical simulations demonstrate that the enhanced optomechanical coupling can enter the single-photon strong-coupling and even ultrastrong-coupling regimes. We also show the creation of macroscopic quantum superposed states and the implementation of a weak-to-strong transition for quantum measurement in this system. This work will pave the way to quantum simulation of single-photon optomechanical effects with current experimental platforms.

We propose a cavity quantum electrodynamic system consisting of a five-level atom coupled to a single mode of the cavity electromagnetic field. The study is focused on the regime of strong coupling between the cavity and atom. Pump laser fields and cavity fields connect the split energy levels of the atom. Instead of the well-known two-level Dicke model obtained by adiabatic elimination of the high-energy levels, we consider the pump lasers' detunings to the atomic transitions to be very small such that we can examine the influence of the higher-energy states. We have studied the effect of an external coherent drive and incoherent pumping on these higher-energy levels and observed the enhancement of intracavity photon numbers due to quantum coherence effects. The amplification of intracavity photons is achieved even without a population inversion. However, the effect of the coherent and incoherent drive is negligible for very large detunings when the higher-energy states are adiabatically eliminated. At zero and small detunings, the system reaches the steady state at an earlier instant of time for higher incoherent pumping. We find an almost agreeable steady-state behavior of the system's exact full quantum dynamics model and its semiclassical approximation. Our model tries to accurately simulate the open system by considering the cavity decay, spontaneous decay, and dephasing of the system.

Cavity-free lasing generation from gas constituents has been studied in the past decade since it promises great potentials in remote sensing and optical diagnostics techniques. Here we report on experimental investigations of temporal characteristics of H-atom lasing emission at 656 nm by examining the dependences of its durations and delays on the pump-laser-pulse energies. An indirect measurement was also performed to test the delays of the lasing pulse for varying H-atom concentrations. The results show that the lasing pulse exhibits considerable superfluorescence signatures. Analysis based on experimental parameters by using deductive expressions of superfluorescence theory shows good agreement to this conjecture. Our investigations on fundamentals could pave the way to a better understanding of the lasing generation and further applications of lasing-based optical diagnostics.

We study a system of 2D trapped bosons in a quasiperiodic potential via ab initio path integral Monte Carlo simulations, focusing on its finite-temperature properties, which have not yet been explored. Alongside the superfluid, normal fluid, and insulating phases, we demonstrate the existence of a Bose glass phase, which is found to be robust to thermal fluctuations, up to about half of the critical temperature of the noninteracting system. Local quantities in the trap are characterized by employing zonal estimators, allowing us to trace a phase diagram; we do so for a set of parameters within reach of current experiments with quasi-2D optical confinement.

To capture a three-dimensional (3D) image, conventional methods often require multiple two-dimensional images of the scene from different perspectives. Here we show the reconstruction of a 3D scene with a fixed camera using quantum correlated photon pairs from a single source. Correlated photon pairs are effectively mirror images of each other so, by measuring the emission position of one photon, and the emission angle of the other, the entire propagation trajectory of the pair, and, subsequently, the 3D scene, can be reconstructed. The high photon correlation and low photon flux from a quantum source also makes it well suited for 3D imaging of light sensitive samples.

Quantum memory effects can be induced even when the (time-dependent) dynamical degrees of freedom associated to the environment are not affected at all by the open system during their joint coupled evolution. In this paper, based on a completely positive bipartite representation of the system-environment dynamics, we found the more general interactions that lead to this class of quantum non-Markovian “casual bystander” environments. General properties of the resulting dynamics are studied with a focus on the system-environment correlations, a collisional measurement-based representation, and the quantum regression hypothesis. Memory effects are also characterized through an operational approach, which in turn allows one to detect when the studied properties apply. Single and multipartite qubits dynamics support and exemplify the developed results.

We combine experimental and theoretical approaches to explore excited rotational states of molecules embedded in helium nanodroplets using ${\mathrm{CS}}_2$ and ${\mathrm{I}}_2$ as examples. Laser-induced nonadiabatic molecular alignment is employed to measure spectral lines for rotational states extending beyond those initially populated at the 0.37 K droplet temperature. We construct a simple quantum-mechanical model, based on a linear rotor coupled to a single-mode bosonic bath, to determine the rotational energy structure in its entirety. The calculated and measured spectral lines are in good agreement. We show that the effect of the surrounding superfluid on molecular rotation can be rationalized by a single quantity, the angular momentum, transferred from the molecule to the droplet.

Optimization of unitary transformations in variational quantum algorithms benefits highly from efficient evaluation of cost function gradients with respect to amplitudes of unitary generators. We propose several extensions of the parameter-shift rule to formulating these gradients as linear combinations of expectation values for generators with general eigenspectra (i.e., with more than two eigenvalues). Our approaches are exact and do not use any auxiliary qubits; instead they rely on a generator eigenspectrum analysis. Two main directions in the parameter-shift-rule extensions are (1) polynomial expansion of the exponential unitary operator based on a limited number of different eigenvalues in the generator and (2) decomposition of the generator as a linear combination of low-eigenvalue operators (e.g., operators with only two or three eigenvalues). These techniques have a range of scalings for the number of needed expectation values with the number of generator eigenvalues from quadratic (for polynomial expansion) to linear and even ${log}_2$ (for generator decompositions). This allowed us to propose efficient differentiation schemes for commonly used two-qubit transformations (e.g., match gates, transmon gates, and fSim gates) and ${\widehat{S}}^2$-conserving fermionic operators for the variational quantum eigensolver.

Recently, higher-order topological insulators have drawn much attention from researchers. For chiral-symmetric one-dimensional systems, the mean chiral displacement, a bulk observable related to the topological invariant, can distinguish topological states from trivial phases. Naturally, there exists an analogous quality for the higher-order one. In a previous work [T. Mizoguchi, Y. Kuno, and Y. Hatsugai, Phys. Rev. Lett. 126, 016802 (2021)], the mean chiral quadrupole moment is introduced. Inspired by this work, we investigate the quadrupolar phase of a higher-order topological insulator on a spin quantum simulator. With focus on a two-particle noninteracting Benalcazar-Bernevig-Hughes model, by changing the amplitude of hopping interactions, we first observe quantum phase transitions from topological phase to trivial phase using a spin simulator experimentally. Since the mean chiral quadrupole moment has robustness against environmental noise, we also test its dependence on perturbations. As an exploration of the bulk observable of a higher-order topological insulator, it may provide one path to investigate arbitrary dimensional systems.

As an important resource, the multiqubit entangled state is widely concerned in quantum computing and quantum communication. Based on the multiqubit partially entangled state, we propose a quantum multicast scheme. In the proposed scheme, arbitrary $N$-qubit Greenberger-Horne-Zeilinger state could be sent to $N$ receivers via a $2N$-qubit partially entangled state. Specially, the proposed scheme is accomplished with controlled-not gates, which ensure the scheme is more efficient and easier to be implemented. In addition, we construct the quantum circuit simulations and the partially entangled channel on IBMQ to prove the feasibility of the scheme. Finally, the performance of the proposed scheme is analyzed to show that the scheme owns the higher efficiency.

We propose a general procedure for the reconstruction of time-dependent dipole moment via transient absorption spectrum, within the framework of linear response theory. The reconstruction method is demonstrated with two examples: the self-reconstruction of extreme-ultraviolet-light-atom interaction (analytically) and the infrared-dressed helium system (numerically). A correlation coefficient is introduced to quantify the fidelity of the reconstruction, which reaches almost 100$\%$ over a wide range of laser parameters and thus indicates the robustness of our reconstruction method. Our theory provides a solid basis for understanding the recent experiment [Phys. Rev. Lett. 121, 173005 (2018)], explaining when and why the dipole moment can be well reconstructed solely from the absorption spectrum even without exact knowledge of the attosecond probe pulse.

We demonstrate a method of qubit state preparation and measurement (SPAM) based on a weakly open quantum channel that achieves a higher fidelity than previous demonstrations, even with low detection efficiency. The trapped-ion qubit SPAM infidelity ($2.1_{-0.4}^{+0.5}\times {10}^{-4}$) is limited by setup-specific errors from ion loss and imperfect population transfer between qubit eigenstates, and we show that full transfer would yield an inaccuracy less than $8\times {10}^{-5}$. The high precision of this method revealed errors caused by a rare ($\approx {10}^{-4}$ likelihood) magnetic-dipole decay that we measure and correct by driving an additional transition. Since this scheme allows fluorescence collection for effectively unlimited periods, high fidelity is achievable even with limited optical access and low quantum efficiency photon detectors.

Complications in preparing and preserving quantum correlations stimulate recycling of a single quantum resource in information processing and communication tasks multiple times. Here, we consider a scenario involving multiple independent pairs of observers acting with unbiased inputs on a single pair of spatially separated qubits sequentially. In this scenario, we address whether more than one pair of observers can demonstrate quantum advantage in some specific $2\to 1$ and $3\to 1$ random access codes. Interestingly, we not only address these in the affirmative but also illustrate that unbounded pairs can exhibit quantum advantage. Furthermore, these results remain valid even when all observers perform suitable projective measurements and an appropriate separable state is initially shared.

The monogamy relations of entanglement are highly significant. However, they involve only amounts of entanglement shared by different subsystems. Results on monogamy relations between entanglement and other kinds of correlations, and particularly classical correlations, are very scarce. Here we experimentally observe a tradeoff relation between internal quantum nonseparability and external total correlations in a photonic system and found that even purely classical external correlations have a detrimental effect on internal nonseparability. The nonseparability we consider, measured by the concurrence, is between different degrees of freedom within the same photon, and the external classical correlations, measured by the standard quantum mutual information, are generated between the photons of a photon pair using the time-bin method. Our observations show that to preserve the internal entanglement in a system, it is necessary to maintain low external correlations, including classical ones, between the system and its environment.

Based on a sequential communication game, semi-device-independent certification of an unsharp instrument has recently been demonstrated [K. Mohan, A. Tavakoli, and N. Brunner, New J. Phys. 21, 083034 (2019)]. In this paper, we provide semi-device-independent self-testing protocols in the prepare-measure scenario to certify multiple unsharpness parameters along with the states and the measurement settings. This is achieved through the sequential quantum advantage shared by multiple independent observers in a suitable communication game known as the parity-oblivious random-access code. We demonstrate that in the three-bit parity-oblivious random-access code at most three independent observers can sequentially share quantum advantage. The optimal pair (triple) of quantum advantages enables us to uniquely certify the qubit states, the measurement settings, and the unsharpness parameter(s). The practical implementation of a given protocol involves inevitable losses. In a suboptimal scenario, we derive a certified interval within which a specific unsharpness parameter has to be confined. We extend our treatment to the four-bit case and show that at most two observers can share quantum advantage for the qubit system. Further, we provide an outline to argue that four sequential observers can share the quantum advantage for the two-qubit system, thereby enabling the certification of three unsharpness parameters.

We study a model consisting of a central $\mathcal{PT}$-symmetric trimer with non-Hermitian strength parameter $\gamma$ coupled to two semi-infinite Su-Schrieffer-Heeger (SSH) leads. We show the existence of two zero-energy modes, one of which is localized while the other is antilocalized. For the remaining eigenvalues, we demonstrate two qualitatively distinct types of $\mathcal{PT}$-symmetry breaking. Within a subset of the parameter space corresponding to the topologically nontrivial phase of the SSH chains, a gap opens within the broken-$\mathcal{PT}$-symmetry regime of the discrete eigenvalue spectrum. For smaller values of $\gamma$, the eigenvalues are embedded in the two SSH bands and hence become destabilized primarily due to the resonance interaction with the continuum. We refer to this as reservoir-assisted $\mathcal{PT}$-symmetry breaking. As the value of $\gamma$ is increased, the eigenvalues exit the SSH bands and the discrete eigenstates become more strongly localized in the central trimer region. This approximate decoupling results in the discrete spectrum behaving more like the independent trimer, including both a region in which the $\mathcal{PT}$ symmetry is restored (the gap) and a second region in which it is broken again. At the exceptional point (EP) marking the boundary between the gap and the second $\mathcal{PT}$-symmetry-broken region, two of the eigenstates coalesce with the localized zero-energy mode, resulting in a third-order exceptional point. At the other boundaries of the parameter space at which the gap vanishes, similar higher-order EPs can form as pairs of the discrete eigenstates coalesce with either of the two zero-energy states. The EPs of order $N$ formed of the localized zero-energy state give rise to characteristic dynamics $\sim t^{2N-2}$ in the evolution of an initial state, which we propose to measure in a photonic lattice experiment.

Efficient verification of quantum states and gates is crucial to the development of quantum technologies. Although the sample complexities of quantum state verification and quantum gate verification have been studied by many researchers, the number of experimental settings has received little attention and is poorly understood. In this work we study systematically quantum state verification and quantum gate verification with a focus on the number of experimental settings. We show that any bipartite pure state can be verified by only two measurement settings based on local projective measurements. Any bipartite unitary in dimension $d$ can be verified by $2d$ experimental settings based on local operations. In addition, we introduce the concept of entanglement-free verification and clarify its connection with minimal-setting verification. Finally, we show that any two-qubit unitary can be verified with at most five experimental settings; moreover, a generic two-qubit unitary (except for a set of measure zero) can be verified by an entanglement-free protocol based on four settings. In the course of study we clarify the properties of Schmidt coefficients of two-qubit unitaries, which are of independent interest.

Midcircuit measurement and reset are crucial primitives in quantum computation, but such operations require strong interactions with selected qubits while maintaining isolation of neighboring qubits, which is a significant challenge in many systems. For trapped ion systems, measurement is performed with laser-induced fluorescence. Stray light from the detection beam and fluorescence from the measured ions can be significant sources of decoherence for unmeasured qubits. We present a technique using ion micromotion to reduce these sources of decoherence by over an order of magnitude. We benchmark the performance with a different method, based on randomized benchmarking, to estimate the magnitude of crosstalk errors on nearby qubits. Using the Honeywell System Model H0, we demonstrate measurement and reset on select qubits with low crosstalk errors on neighboring qubits.