Superfluid flow past a potential barrier is a well studied problem in ultracold Bose gases, however, fewer studies have considered the case of flow through a disordered potential. Here we consider the case of a superfluid flowing through a channel containing multiple point-like barriers, randomly placed to form a disordered potential. We begin by identifying the relationship between the relative position of two point-like barriers and the critical velocity of such an arrangement. We then show that there is a mapping between the critical velocity of a system with two obstacles, and a system with a large number of obstacles. By establishing an initial superflow through a point-like disordered potential, moving faster than the critical velocity, we study how the superflow is arrested through the nucleation of vortices and the breakdown of superfluidity, a problem with interesting connections to quantum turbulence and coarsening. We calculate the vortex decay rate as the width of the barriers is increased, and show that vortex pinning becomes a more important effect for these larger barriers.

We theoretically investigate the merging behavior of two identical supersolids through dipolar Bose-Einstein condensates confined within a double-well potential. By adiabatically tuning the barrier height and the spacing between the two wells for specific trap aspect ratios, the two supersolids move toward each other and lead to the emergence of a variety of ground-state phases, including a supersolid state, a macrodroplet state, a ring state, and a labyrinth state. We construct a phase diagram that characterizes various states seen during the merging transition. Further, we calculate the force required to pull the two portions of the gas apart, finding that the merged supersolids act like a deformable plastic material. Our work paves the way for future studies of layer structure in dipolar supersolids and the interaction between them in experiments.

Interactions between a source of light and atoms are ubiquitous in nature. The study of them is interesting on the fundamental level as well as for applications. They are at the core of quantum information processing tasks and quantum thermodynamics protocols. However, even for a two-level atom interacting with a field in rotating wave approximation there exists no exact solution. This touches upon a basic problem in quantum field theory, where we can only calculate the transitions in the time asymptotic limits (i.e., minus and plus infinity), while we are not able to trace the evolution. In this paper we want to get more insight into the time evolution of a total system of a two-level atom and a continuous-mode quantum field. We propose an approximation, which we are able to apply systematically to each order of Dyson expansion, resulting in a greatly simplified formula for the evolution of the combined system at any time. Our tools include a proposed non-Hermitian renormalization method. As a sanity check, by applying our framework, we derive the known optical Bloch equations.

Localization is a critical phenomenon in quantum walks that has attracted much attention in quantum simulations. In this paper, we present a unified framework for investigating the localization due to two causes, single-position defects and topology. Furthermore, we show that a specific phase defect can act as a topological switch, which can turn on (off) topological boundary states between two regions with the same (different) topological numbers. Remarkably, the localized state turned on by the switch is protected by topological features. Thus, the switch is a topologically nontrivial defect that significantly differs from ordinary defects. Our results provide new intuitive insight into the topological features of quantum walks and shed new light on manipulating quantum walks.

We study the conditions to produce Markovian semigroups under the convex combinations of various generalized Weyl dephasing channels. We show that the convex combinations of generalized Weyl dephasing channels corresponding to two classes of subgroups of ${\mathbb{Z}}_d\times {\mathbb{Z}}_d$ could yield Markovian semigroups. We analyze the (non)invertibility and (non-)Markovianity of generalized Weyl dephasing channels. Surprisingly, we find two results that are quite different from the known conclusions. One is that the noninvertibility and non-Markovianity of generalized Weyl dephasing channels are dependent on the parity of the dimension of the system. The other is that the invertibility and Markovianity of generalized Weyl dephasing channels are equivalent.

In continuous-variable (CV) quantum key distribution (QKD), the reconciliation efficiency and frame error rate of the information reconciliation have a significant impact on the secret key rate. For a fixed-rate multiedge-type low-density parity-check (LDPC) code, the reconciliation efficiency and frame error rate will inevitably vary due to the fluctuations of the signal-to-noise ratio (SNR) of the CV QKD system, which degrades the performance of the system. We propose a high-efficiency rate-adaptive information reconciliation scheme by combining raptorlike (RL) LDPC codes with the addition of trusted noises. We establish the model of adding trusted noises and combine it with the RL LDPC codes to optimize the secret key rate under the time-varying channel. The simulation results show that our scheme can maintain a high reconciliation efficiency of more than 94.4% and a low frame error rate within 15% fluctuation range of the SNR. Furthermore, we implement the hardware acceleration of the proposed rate-adaptive scheme on a graphics processing unit and achieve a decoding throughput of 65.5 Mbits/s by optimizing the storage of the parity-check matrix. Our results are useful for the practical CV QKD under the realistic time-varying channel.

Raman qubits, represented by two ground or metastable quantum states coupled via an intermediate state, hold some advantages over directly coupled qubits, most notably much longer radiative lifetimes, shorter gate duration, and lower radiation intensity due to using electric-dipole allowed optical transitions. They are also relatively simple to implement and control, making them an attractive option for building quantum gates for quantum computers. In this work, we present a simple and fast tomographic method to measure the errors of Raman qubit gates possessing the Morris-Shore dynamic symmetry. The latter occurs when the qubit states are on two-photon resonance and the driving fields have the same time dependence. The method is based on repeating the same gate multiple times, which amplifies the small coherent errors to sufficiently large values, which can be measured with high accuracy and precision. Then the (small) gate errors can be determined from the amplified errors by using the analytical connections between them.

Universal blind quantum computation allows a client who has limited quantum abilities to delegate his or her private computation to an untrusted quantum server. The first universal blind quantum computation protocol was proposed by Broadbent, Fitzsimons, and Kashefi. In their work the computation resource is the so-called brickwork state, which can be constructed by a number of specific single qubits. The number of single qubits is in theory linearly related to the size of the quantum circuit. However, due to the fixed structure of the brickwork states, the actual number of qubits is usually far more than the linear size. In this work we mainly construct three improved brickwork states whose structures are no longer fixed, thus they can efficiently reduce the client's qubit consumption. Using those improved brickwork states we propose a class of efficient universal blind quantum computation protocols. In our basic protocol, to implement a single-qubit basic gate the client needs to prepare only two ancillary qubits, while in the original protocol it will consume four ancillary qubits. Making use of our improved brickwork states the qubit consumption of implementing a nonadjacent two-qubit gate such as a control-$X$ gate can be sharply reduced.

A promising alternative for atomic gravimetry has been proposed using a magnetic-field gradient, which has a $T^3$ scaling on the phase and does not require counterpropagating Raman beams. We analyze the limits in the precision of this method coming from the thermal expansion of the atomic cloud. We determine the requirements of microwave power and magnetic-field gradient to keep a resonant excitation. We find that there is a minimum microwave power required for the measurement. At high microwave powers, the relative precision in gravimetry (${\sigma }_g/g$) approaches ${10}^{-7}$ in a single measurement, which is good enough for many field applications.

Temporal cavity solitons are optical pulses that can persist in passive resonators, such as microresonators and fiber-based resonators. They can preserve their shape and energy during the propagation. In addition, other typical nonlinear states including Turing rolls, chaotic state, and breathers also emerge in the Kerr resonator. Normally, the occurrence of these states is not simultaneous due to their distinct parameter regimes. However, their coexistence has been observed in scenarios where neighboring resonances corresponding to both the same and orthogonal polarizations partially overlap. The coexisting nonlinear states in the strongly birefringent resonator have not been thoroughly explored due to the challenge of superimposing multiple resonances. In this paper, we investigate the feasibility of inducing coexistence states within a strongly birefringent fiber ring resonator exhibiting anomalous dispersion. The dynamic effects are discussed in the context of zero and nonzero net birefringence. By flexibly controlling the overlap of the nonlinear resonances corresponding to two orthogonal polarizations, we can attain the coexistence of both homogeneous (two polarizations exhibit the same states) and inhomogeneous states (two polarizations exhibit different states). These results could have significant implications for high-compacity optical communication, LiDAR, dual-comb spectroscopy, and so on.

The widely used experimental technique of continuous-wave detection assumes counting pulses of photocurrent from a click-type detector inside a given measurement time window. With such a procedure we miss out the photons detected after each photocurrent pulse during the detector dead time. Additionally, each pulse may initialize a so-called afterpulse, which is not associated with the real photons. We derive the corresponding quantum photocounting formula and experimentally verify its validity. Statistics of photocurrent pulses appears to be nonlinear with respect to the quantum state, which is explained by the memory effect of the previous measurement time windows. Expressions—in general, nonlinear—connecting statistics of photons and pulses are derived for different measurement scenarios. We also consider an application of the obtained results to quantum state reconstruction with unbalanced homodyne detection.

Detection of entanglement through partial knowledge of the quantum state is a challenge to implement efficiently. Here we propose a separability criterion for detecting bipartite entanglement in arbitrary-dimensional quantum states using partial moments of the realigned density matrix. Our approach enables the detection of both distillable and bound entangled states through a common framework. We illustrate the significance of our method through examples of states belonging to both the above categories, which are not detectable using comparable other schemes relying on partial state information. The formalism of employing partial realigned moments proposed here is further shown to be effective for two-qubit systems too, with a slight modification of our separability criterion.

The $N$-qubit Dicke states $|D_k^N\rangle$, of Hamming weight $k$, are a class of entangled states which play an important role in quantum algorithm optimization. We present a general calculation of entanglement entropy in Dicke states, which we use to describe the $|D_k^N\rangle$ entropy cone. We demonstrate that all $|D_k^N\rangle$ entropy vectors emerge symmetrized and use this to define a min-cut protocol on star graphs which realizes $|D_k^N\rangle$ entropy vectors. We identify the stabilizer group for all $|D_k^N\rangle$, under the action of the $N$-qubit Pauli group and two-qubit Clifford group, which we use to construct $|D_k^N\rangle$ reachability graphs. We use these reachability graphs to analyze and bound evolution of $|D_k^N\rangle$ entropy vectors in Clifford circuits.

Causal inference revealing causal dependencies between variables from empirical data has found applications in multiple subfields of scientific research. A quantum perspective of correlations holds the promise of overcoming the limitation of Reichenbach's principle and enabling causal inference with only observational data. However, it is still not clear how quantum causal inference can provide operational advantages in general cases. Here, we have devised a photonic setup and experimentally realized an algorithm capable of identifying any two-qubit statistical correlations generated by the two basic causal structures under an observational scenario, thus revealing a universal quantum advantage in causal inference over its classical counterpart. We further demonstrate the explainability and stability of our causal discovery method, which is widely sought in data processing algorithms. Employing a fully observational approach, our result paves the way for studying quantum causality in general settings.

In this work, we employ the squared Hilbert-Schmidt norm of the Gram matrix for a quantum channel as a reversibility quantifier of this channel, which is shown to be complementary to the entropy of this channel, and derive a complementary relation between the reversibility of a quantum channel and its complementary channel. For a natural unitality measure of a quantum channel, we show that it is equivalent to the entropy of the corresponding complementary channel. By quantifying the disturbance of a quantum channel as the decrease of correlations in a maximally entangled state locally passing through this channel, we eventually establish a reversibility-unitality-disturbance triality relation. To illustrate and compare these quantities, we further evaluate them for some prototypical channels associated with some special quantum information processing tasks and computations, such as the quantum teleportation channel, DQC1 channel, Mach-Zehnder interferometry channel, dephrasure channel and so on, including both unital and nonunital cases.

Optimal probe states and measurements are always required to achieve the best estimation precision in quantum metrology. In the actual physical environment, ubiquitous noise hinders estimation precision and amplifies the challenges of optimizing the probe states and measurements. In this study we extend a theoretical proposal, presented in Phys. Rev. A 103, 032615 (2021), to encompass noisy general Pauli channels for SU(2) phase estimation. By utilizing a superposition of different causal orders of two channels, we establish theoretically a probe-state-independent criterion for the estimation precision. We show that the probe-state-independent property results from the noncommutativity of the Kraus operators of the parameter-encoding channels. Based on this criterion, one can find some parameter-encoding channels without the requirement of precisely preparing the probe state. Moreover, our scheme requires only deterministic projection measurement on the control qubit. In this way, one can simultaneously avoid optimizing both the probe states and measurements. In addition, we also show that the estimation precision and probe-state-independent property of the indefinite causal order scheme are independent of representations of the Kraus operators. This alleviates the experimental challenges in realizing quantum channels. We demonstrate experimentally the advantages of the indefinite causal order for phase estimation of an SU(2) unitary transformation in the three kinds of quantum noise channels. Our result shows that the dynamic evolution of indefinite causal order can outperform the conventional cascaded estimation scenario at high noise levels, exhibiting high robustness and feasibility in practical estimation tasks.

Obtaining information from a quantum system through a measurement typically disturbs its state. The postmeasurement states for a given measurement, however, are not unique and highly rely on the chosen measurement model, complicating the puzzle of information disturbance. Two distinct questions are then in order. First, what is the minimum disturbance a measurement may induce? Second, when a fixed disturbance occurs, how informative is the possible measurement in the best-case scenario? Here we propose various approaches to tackle these questions and provide explicit solutions for the set of unbiased binary qubit measurements and postmeasurement state spaces that are equivalent to the image of a unital qubit channel. In particular, we show there are different tradeoff relations between the sharpness of this measurement and the average fidelity of the premeasurement and postmeasurement state spaces as well as the sharpness and quantum resources preserved in the postmeasurement states in terms of coherence and discordlike correlation once the measurement is applied locally.

Mode-pairing quantum key distribution (MP-QKD) can surpass the repeaterless rate-transmittance bound (Pirandola-Laurenza-Ottaviani-Banchi bound) without requiring global phase locking, exhibiting remarkable flexibility. However, MP-QKD necessitates equal communication distances in two channels, which is a challenging requirement in practical applications. To address this limitation, we extend the original MP-QKD to asymmetric cases. Our decoy-state estimation confirms that asymmetric channel transmittances and asymmetric intensities do not compromise the security of the protocol. We focus on the pulse-intensity relationship, a key factor for optimizing the performance of asymmetric MP-QKD. Unlike previous asymmetric protocols, the intensities of different bases in asymmetric MP-QKD cannot be decoupled. We introduce an optimal-pulse-intensity method, adaptable to various scenarios, to enhance key rates by calculating ideal pulse intensities. Simulation results in various representative scenarios indicate that our method effectively reduces the impact of asymmetric channel distances on MP-QKD performance, enhancing its practical applicability.