An anyonic system not only has potential applications in the construction of topological quantum computers but also presents a unique property known as topological entanglement entropy (TEE) in quantum many-body systems. Recently, Bonderson et al. [Ann. Phys. (NY) 385, 399 (2017)] have formally defined the entropy of anyonic charge entanglement (ACE) in anyonic states that has been shown to be able to derive TEE. We give it operational meaning from the perspective of quantum information theory. Specifically, for an anyonic bipartite system, we define an operational measure of topological correlation based on the principle of maximum entropy, where the topological correlation is the information that cannot be accessed by local operations constrained by anyonic superselection rules (SSRs) and classical communication. For a given anyonic bipartite state with maximal rank, we prove that its topological correlation is equal to its entropy of ACE. This measure can be extended to measure nonlocal resources of other compound quantum systems in the presence of SSRs and can provide a more refined classification of correlations in a multipartite system with SSRs.

The concept of entanglement witnesses forms a useful technique to detect entanglement in realistic quantum devices. Measurement-device-independent nonlinear entanglement witnesses (MDI-NEWs) are a kind of entanglement witness that eliminates dependence on the correct alignments of measurement devices for guaranteeing the existence of entanglement and also detects more entangled states than its linear counterparts. While this method guarantees entanglement independent of measurement alignments, it is still prone to serving wrong results due to other loopholes. Here, we study the response of MDI-NEWs to two categories of faults occurring in experiments. In the first category, the detection loophole, characterized by lost and additional events of outcomes of measurements, is investigated, and bounds which guarantee entanglement are obtained in terms of the efficiency of measurement being performed. In the second category, we study noise associated with the sets of additional quantum inputs required in MDI-NEW scenarios. In this case, a type of noise is identified which still allows the MDI-NEWs to guarantee entanglement. We also show that MDI-NEWs are less or equally robust in comparison to their linear counterparts under the same noise in additional quantum inputs, although the former group detects a larger volume of entangled states in the noiseless scenario than their linear cousins.

Quantum reservoir computing is an emerging field in machine learning with quantum systems. While classical reservoir computing has proven to be a capable concept for enabling machine learning on real, complex dynamical systems with many degrees of freedom, the advantage of its quantum analog has yet to be fully explored. Here, we establish a link between quantum properties of a quantum reservoir, namely, entanglement and its occupied phase-space dimension, and its linear short-term memory performance. We find that a high degree of entanglement in the reservoir is a prerequisite for a more complex reservoir dynamics, which is key to unlocking the exponential phase space and higher short-term memory capacity. We quantify these relations and discuss the effect of dephasing in the performance of physical quantum reservoirs.

Continuous-variable quantum computing architectures based upon the Gottesman-Kitaev-Preskill (GKP) encoding have emerged as a promising candidate because one can achieve fault tolerance with a probabilistic supply of GKP states and Gaussian operations. Furthermore, by generalizing to rectangular-lattice GKP states, a bias can be introduced and exploited through concatenation with qubit codes that show improved performance under biasing. However, these codes (such as the XZZX surface code) still require weight-four stabilizer measurements and have complex decoding requirements to overcome. In this work, we study the code-capacity behavior of a rectangular-lattice GKP encoding concatenated with a repetition code under an isotropic Gaussian displacement channel. We find a numerical threshold of $\sigma =0.599$ for the noise's standard deviation, which outperforms the biased GKP planar surface code with a trade-off of increased biasing at the GKP level. This is all achieved with only weight-two stabilizer operators and simple decoding at the qubit level. Furthermore, with moderate levels of bias (aspect ratio $\le 2.4$) and nine or fewer data modes, significant reductions in logical error rates can still be achieved for $\sigma \le 0.3$, opening the possibility of using biased GKP repetition codes as a simple low-level qubit encoding for further concatenation.

The seminal multibeam Young's interference experiment established the wave-particle duality principle, connecting quantum properties (interference) and path information. To quantify this connection, we propose a framework in which quantum measurement is applied to a subsystem of an entangled state, thus producing a quantum ensemble in the other part. The applied observable is characterized by its uncertainty quality arising from entropic uncertainty relations. In the proposed approach initial entanglement appears as a resource that can be transformed into classical information or an ensemble's coherence. Our framework provides a connection between such fundamental quantum information concepts as entanglement, coherence, and uncertainty.

We propose a framework for computing the structure and dynamics for second-quantized many-nucleon Hamiltonians on quantum computers. We develop an oracle-based Hamiltonian input model that computes the many-nucleon states and nonzero Hamiltonian matrix elements of the many-nucleon system. With our Fock-state based input model, we show how to implement the sparse matrix simulation algorithms to calculate the dynamics of the second-quantized many-nucleon Hamiltonian. Based on the dynamics simulation methods, we also present the methodology for structure calculations of the many-nucleon system. In this work, we provide an explicit circuit design of our input model of the second-quantized Hamiltonian within a direct encoding scheme that maps the occupation of each available single-particle state in the many-nucleon state to the state of specific qubit in a quantum register. We analyze our method and provide the asymptotic cost in computing resources for structure and dynamics calculations of many-nucleon systems. For pedagogical purposes, we demonstrate our input model with two model problems in restricted model spaces.

Entangled states shared among distant nodes are frequently used in quantum network applications. When quantum resources are abundant, entangled states can be continuously distributed across the network, allowing nodes to consume them whenever necessary. This continuous distribution of entanglement enables quantum network applications to operate continuously while being regularly supplied with entangled states. Here, we focus on the steady-state performance analysis of protocols for continuous distribution of entanglement. We propose the virtual neighborhood size and the virtual node degree as performance metrics. We utilize the concept of Pareto optimality to formulate a multiobjective optimization problem to maximize the performance. As an example, we solve the problem for a quantum network with a tree topology. One of the main conclusions from our analysis is that the entanglement consumption rate has a greater impact on the protocol performance than the fidelity requirements. The metrics that we establish in this paper can be utilized to assess the feasibility of entanglement distribution protocols for large-scale quantum networks.

Laser-seeding attack enables an attacker to control the source output and eavesdrops on the key information, compromising the security of the continuous-variable (CV) quantum key distribution (QKD) system. In this paper, we suggest an improved optical power limiter (OPL) to defend against a laser-seeding attack on CV QKD systems. The proposed OPL utilizes a temperature monitor to detect Eve's attacks through temperature changes, providing an effective optical component for improving the security and reliability of the CV QKD system in practical environments, which is called an OPL-T scheme. In particular, we consider the security threats of a laser-seeding attack on two common CV QKD systems and prove their theoretical security degradation. Experiments are conducted to verify the effective OPL-T, and the results show that this defense scheme has an advantage in resisting the laser-seeding attack. Numerical simulations demonstrate the security analysis on CV QKD equipped with an OPL-T, which further confirms the effectiveness of our scheme. This work provides a practical solution for the security enhancements of continuous-variable quantum communication in practical applications.

Nonadiabatic holonomic quantum computation has been attracting continuous attention since it was proposed. Until now, various schemes of nonadiabatic holonomic quantum computation have been developed and many of them have been experimentally demonstrated. It is known that at the end of a computation, one usually needs to estimate the average value of an observable. However, computation errors severely disturb the final state of a computation, causing erroneous average value estimation. Thus, for nonadiabatic holonomic quantum computation, an important topic is to investigate how to better give the average value of an observable under the condition of computation errors. While the above topic is important, the previous works in the field of nonadiabatic holonomic quantum computation pay woefully inadequate attention to it. In this paper, we show that rescaling the measurement results can better give the average value of an observable in nonadiabatic holonomic quantum computation when computation errors are considered. Particularly, we show that by rescaling the measurement results, $56.25\%$ of the computation errors can be reduced when using the depolarizing noise model, a widely adopted noise model in quantum computation community, to analyze the benefit of our method.

We report an experimental and theoretical study of spin-noise correlations in a ${}^{87}\mathrm{Rb}\text{−}{}^{133}\mathrm{Cs}$ unpolarized alkali-metal vapor dominated by spin-exchange collisions. We observe strong unequal-time interspecies correlations and account for these with a first-principles theoretical model. Since the two atomic species have different spin precession frequencies, the dual-species vapor enables the use of an additional experimental handle, the applied magnetic field, for untangling various subtypes of spin correlations. In particular, the measured cross-correlation and autocorrelation spectra shed light on a number of spin-dynamic effects involving intra-atom, interatom, intraspecies, and interspecies correlations. Cross-correlation coefficients exceeding $60\%$ have been observed at low magnetic fields, where the two spin species couple strongly via spin-exchange collisions. The understanding of such spontaneously generated correlations can motivate the design of quantum-enhanced measurements with single or multispecies spin-polarized alkali-metal vapors used in quantum sensing applications.

The intrinsic polygonal symmetry of a [h]cycloacene molecule is employed to find the normal modes of the electronic and vibrational degrees of freedom using the interaction picture in a fixed molecule, i.e., without rotational degrees of freedom. Instead of considering separable solutions, the dynamics is studied implementing an electron-phonon linear coupling of first order. This coupling is described using an algebraic approach analogous to the Jaynes-Cummings model for atoms in optical cavities. Criteria for transitions are given based on the symmetry selection rules and conservation laws. A basis of electron-phonon dressed states is given and distortion phenomena usually associated with the Jahn-Teller effect in crystals are predicted.

The consequences of enforcing permutational symmetry, as required by the Pauli principle (spin-statistical theorem) on the state space of molecular ensembles interacting with the quantized radiation mode of a cavity are discussed. The Pauli-allowed collective states are obtained by means of group theory, i.e., by projecting the state space onto the appropriate irreducible representations of the permutation group of the indistinguishable molecules. It is shown that with increasing number of molecules the ratio of Pauli-allowed collective states decreases very rapidly. Bosonic states are more abundant than fermionic states and the brightness of Pauli-allowed state space (the contribution from photon-excited states) increases (decreases) with increasing fine-structure in the energy levels of the material ground- (excited) state manifold. Numerical results are shown for the realistic example of rovibrating ${\mathrm{H}}_2\mathrm{O}$ molecules interacting with an infrared cavity mode.

The radiation and absorption responses of atoms in strong laser fields for quantum probes are studied using the strong-field quantum optical method. It was shown that the emission and absorption are related, respectively, to the positive and negative frequencies of the current's autocorrelator, which can be separated into the coherent part from the expectation of the current operator and the incoherent part from its quantum fluctuations. While the current expectation determines the response to coherent-state probes, the current fluctuations have a significant impact on the response to Fock-state probes. Our studies extend the pump-probe scheme of strong-field physics into the quantum optical regime.

We present a methodology for the design of optimal Raman beam-splitter pulses suitable for cold atom inertial sensors. The methodology, based on time-dependent perturbation theory, links optimal control and the sensitivity function formalism in the Bloch sphere picture, thus providing a geometric interpretation of the optimization problem. Optimized pulse waveforms are found to be more resilient than conventional beam-splitter pulses and ensure a near-flat superposition phase for a range of detunings approaching the Rabi frequency. As a practical application, we simulated the performance of an optimized Mach-Zehnder interferometer in terms of scale-factor error and bias induced by interpulse laser intensity variations. Our findings reveal enhancements compared to conventional interferometers operating with constant-power beam-splitter pulses.

We present an efficient three-dimensional dark-focus optical trapping potential for neutral atoms and Bose-Einstein condensates. This “optical bottle” is created by a single blue-detuned light field exploiting the phenomenon of conical refraction occurring in biaxial crystals. The conversion of a Gaussian input beam to the bottle beam has an efficiency of close to 100% and the optical setup requires the addition of the biaxial crystal and a circular polarizer only. Based on the conical-refraction theory, we derive the general form of the potential, the trapping frequencies, and the potential barrier heights. We present experiments on confining a ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate in three dimensions. We determine the trap shape, the vibrational frequencies along the weak axis, as well as the lifetime of ultracold atoms in this type of potential.

We investigate the crystalline stationary states of a dipolar Bose-Einstein condensate in a planar trapping geometry. Our focus is on the ground-state phase diagram in the thermodynamic limit, where triangular, honeycomb, and stripe phases occur. We quantify the superfluid fraction by calculating the nonclassical translational inertia, which allows us to identify favorable parameter regimes for observing supersolid ground states. We develop two simplified theories to approximately describe the ground states and consider the relationship to roton softening in the uniform ground state. This also allows us to extend the phase diagram to the low density regime. While the triangular and honeycomb states have an isotropic superfluid response tensor, the stripe state exhibits anisotropic superfluidity.

In the presence of a magnetic field, multicomponent ground states appear in trapped spin-1 Bose-Einstein condensates for both ferromagnetic and antiferromagnetic types of spin-spin interaction. We aim to produce an accurate analytical description of the multicomponent states which is of fundamental importance. Despite being in the so-called regime of the Thomas-Fermi approximation (condensates with large particle number), the scenario of multicomponent states is problematic under this approximation due to the large variation in densities of the subcomponents. We generalize the variational method that we have introduced [P. K. Kanjilal and A. Bhattacharyay, Eur. Phys. J. Plus 137, 547 (2022)] to overcome the limitations of the Thomas-Fermi approximation. We demonstrate that the variational method is crucial in identifying multicomponent ground states. A comparison of the results of the variational method, which is multimodal by construction, with that of the single-mode approximation is also presented in this paper to demonstrate a marked improvement in accuracy over the single-mode approximation. We have also looked into the phase transition between phase-matched and polar states in a trapped condensate using the variational method and have identified substantial change in the phase boundary. The correspondence of the phase diagram of the trapped case with the homogeneous one identifies other limitations of the Thomas-Fermi approximation as opposed to the more accurate variational method.

We demonstrate the spontaneous formation of a Bose-Einstein condensate (BEC) of strongly bound harmonically trapped dipolar ${}^{164}\mathrm{Dy}$ atoms on the outer curved surface of an elliptical or a circular cylinder, with a distinct topology, employing the numerical solution of an improved mean-field model including a Lee-Huang-Yang-type interaction, meant to stop a collapse at high atom density, the axis of the cylindrical-shell-shaped BEC being aligned along the polarization direction of the dipolar atoms. These states are dynamically stable and a Gaussian initial state leads to the cylindrical-shell-shaped state in both imaginary-time and real-time propagation. The formation of the hollow cylindrical BEC by a real-time simulation starting from a solid cylindrical state demonstrate the possibility of the formation of such a condensate experimentally.

We study the general aspects of the dynamics of an ion trapped in an ideal multipolar radio-frequency trap while interacting with a dense cold atomic gas. In particular, we explore the dynamical stability, the energy relaxation, and the characteristic harmonic motion exhibited by a trapped ${\mathrm{Yb}}^{+}$ ion in different multipolar potentials and immersed in various cold atomic samples (Li, Na, Rb, Yb). For this purpose, we use two different molecular-dynamics simulations, one based on a time-event drive algorithm and the other based on the stochastic Langevin equation. Relevant values for experimental realizations, such as the associated ion's lifetimes and observable distributions, are presented along with some analytical expressions that relate the ion's dynamical properties with the trap parameters.

Virtual perfect absorption (VPA) is an effect simulating real absorption of light by using excitation at a complex frequency corresponding to a scattering zero. We theoretically study VPA in resonantly absorbing and amplifying media irradiated by two counterpropagating waves with exponentially growing amplitudes. We show that VPA critically depends on the medium density (i.e., level of loss or gain) deteriorating in the high-density limit. In contrast, almost ideal VPA persists in the $\mathcal{P}T$-symmetric loss-gain bilayer. For high enough gain, powerful quasilasing pulses are observed at later times symmetrically (single amplifying layer) or asymmetrically ($\mathcal{P}T$-symmetric structure) generated in both propagation directions.