Quantum Information Processing最新文献

In many quantum algorithms, including Hamiltonian simulation, efficient quantum circuit implementation of diagonal unitary matrices is an important issue. For small unitary diagonal matrices, a method based on Walsh operators is known and allows an exact implementation. Whereas, as the matrix size increases, the required resources increase linearly regarding the matrix size, so an efficient approximate implementation is indispensable. In this study, we specify the approximation using piecewise polynomials when the diagonal unitary matrix is generated by a known underlying function. It accelerates the implementation by an exponential factor compared to the exact one. In more detail, we modify a previous method, which we call PPP (phase gate for piecewise-defined polynomial), and propose a novel one called LIU (linearly interpolated unitary diagonal matrix). By introducing a coarse-graining parameter, calculated from the underlying function and the desired error bound, we evaluate the explicit gate counts for different methods as functions of some norms of the given function, the grid parameter, and the allowable error. It helps us to figure out the efficient quantum circuits in practical settings of different grid parameters and error bounds, in addition to an asymptotic speedup when the grid parameter goes to infinity. As an application, we apply our method to the first-quantized Hamiltonian simulation and estimate the quantum resources (gate count and ancillary qubits). It reveals that the error coming from the approximation of the potential function is not negligible compared to the error from the Trotter-Suzuki formula.

The establishment of the global quantum communication network relies on the effective integration of free-space links and optical fiber networks. Mode-pairing quantum key distribution (MP-QKD) represents an advancement in measurement device-independent quantum key distribution (MDI-QKD). By eliminating the need for a complex light-field setup, it breaks the rate–distance limit and can be theoretically applied to the light-field setup of free-space MDI-QKD. In this work, we propose a mode-pairing quantum key distribution model that integrates satellite-based links and fiber links, and its performance is analyzed by simulating the probability distribution of atmospheric transmittance (PDT) between satellites and ground stations. Additionally, the effects of pairing interval and misalignment error are discussed. This work may provide some important references for the future widespread application of satellite-to-ground multipath quantum key distribution.

In this paper, we propose a lightweight quantum key distribution (QKD) protocol for two participants within a unidirectional quantum channel environment that inherently prevents Trojan horse attacks. Our protocol features a novel chain method for encoding and decoding single-photon sequences, thus addressing the common limitations of the traditional QKD protocol, which treats photons independently. A notable advantage of our approach is the simplification achieved by requiring only the disclosure of the first photon basis. Furthermore, our method significantly improves the detection rate of measure-resend attacks. When a single photon of a sequence of photons is attacked by an eavesdropper, the detection rate can reach nearly 16.67% if half of them are decoy photons, offering a 3% enhancement compared to protocols without the chain method. In cases where the entire sequence is attacked, checking just twelves photons can achieve a detection rate of 99%, which is five photons fewer than that required by traditional protocols without the chain method. In addition, a privacy amplification method is introduced for the QKD protocol by sharing a hash function, to maintain high efficiency while enhancing security, as a practical solution for quantum communication.

Twists are defects that are used to encode and process quantum information in topological codes like surface and color codes. Color codes can host three basic types of twists, namely charge-permuting, color-permuting, and domino twists. In this paper, we study domino twists from the viewpoint of computation. Specifically, we give a systematic construction for domino twists in qubit color codes. We also present protocols for measurement of logical qubits. Finally, we show that all Clifford gates can be implemented by braiding twists.

This review delves into quantum sensing from its foundational principles to transformative applications across various fields, encompassing a range of qubit systems such as spin chains, superconducting qubits, and NV centre ensembles. The complex interplay between quantum mechanics and macroscopic systems is explored, highlighting phenomena like quantum entanglement and Bell states. Key challenges, including decoherence and measurement noise, are addressed, providing insights into enhancing quantum sensing capabilities. A comparison of critical attributes for evaluating qubit performance across various quantum technologies and discussion on quantum noise mitigation techniques are provided.

Absolute separable (AS) quantum states are those states from which it is impossible to create entanglement, even under global unitary operations. It is known from the resource theory of non-absolute separability that the set of absolute separable states forms a convex and compact set, and global unitaries are free operations. We show that the action of a quantum switch controlled by an ancilla qubit over the global unitaries can break this robustness of AS states and produce ordinary separable states. First, we consider bipartite qubit systems and find the effect of quantum switch starting from the states sitting on the boundary of the set of absolute separable states . As particular examples, we illustrate what happens to modified Werner states and Bell diagonal (BD) states. For the Bell diagonal states, we provide the structure for the set of AS BD states and show how the structure changes under the influence of a switch. Further, we consider numerical generalization of the global unitary operations and show that it is always possible to take AS states out of the convex set under switching operations. We also generalized our results in higher dimensions.

A popular machine-learning model for regression tasks, including stock-market prediction, weather forecasting and real-estate pricing, is the classical support vector regression (SVR). However, a practically realisable quantum SVR remains to be formulated. We devise annealing-based algorithms, namely simulated and quantum-classical hybrid, for training two SVR models and compare their empirical performances against the SVR implementation of Python’s scikit-learn package for facial-landmark detection (FLD), a particular use case for SVR. Our method is to derive a quadratic-unconstrained-binary formulation for the optimisation problem used for training a SVR model and solve this problem using annealing. Using D-Wave’s hybrid solver, we construct a quantum-assisted SVR model, thereby demonstrating a slight advantage over classical models regarding FLD accuracy. Furthermore, we observe that annealing-based SVR models predict landmarks with lower variances compared to the SVR models trained by gradient-based methods. Our work is a proof-of-concept example for applying quantum-assisted SVR to a supervised-learning task with a small training dataset.

Quantum discord is an important measure of quantum correlations that goes beyond the paradigm of quantum entanglement. However, calculating quantum discord involves optimization over measurements, which is computationally challenging and often infeasible. This raises the intriguing question of Gaussian extremality—whether the quantum discord of a reference Gaussian state can provide a meaningful bound to the quantum discord of the original state. In this paper, we investigate this question by comparing the Gaussian discord of a reference Gaussian state with the quantum discord.

The fully entangled fraction (FEF) measures the proximity of a quantum state to maximally entangled states. FEF (>frac{1}{d}), in (d otimes d) systems, is a significant benchmark for various quantum information processing protocols including teleportation. Quantum conditional entropy (QCE) on the other hand is a measure of correlation in quantum systems. Conditional entropies for quantum systems can be negative, marking a departure from conventional classical systems. The negativity of quantum conditional entropies plays a decisive role in tasks like state merging and dense coding. In the present work, we investigate the relation of these two important yardsticks. Our probe is mainly done in the ambit of states with maximally mixed marginals, with a few illustrations from other classes of quantum states. We start our study in two-qubit systems, where for the Werner states, we obtain lower bounds to its FEF when the conditional Rényi (alpha -)entropy is negative. We then obtain relations between FEF and QCE for two-qubit Weyl states. Moving on to two qudit states, we find a necessary and sufficient condition based on FEF, for the isotropic state to have negative conditional entropy. In two qudit systems, the relation between FEF and QCE is probed for the rank-deficient and generalized Bell diagonal states. FEF is intricately linked with k-copy nonlocality and k- copy steerability. The relations between FEF and QCE facilitates to find conditions for k- copy nonlocality and k- copy steerability based on QCE. We obtain such conditions for certain classes of states in two qubits and two qudits. Applications of the relations obtained are provided in the context of work extraction, faithful entanglement and entropic uncertainty relations.

This article investigates thermal entanglement and quantum teleportation in a bipartite system composed of two spin-(frac{1}{2}) qubits, exposed to an external magnetic field along the Z-axis, within the framework of the squeezed spin model. We employ concurrence to quantify both the thermal entanglement in our system and the entanglement of the replicated output state in a quantum teleportation protocol through this system. Thus, we adopt fidelity to evaluate the quality of teleportation. It is shown that at the system’s ground state, a pure state favors maximal entanglement, while a mixed state leads to an absence of entanglement regardless of the magnetic field. At very low temperatures, increasing the magnetic field induces transitions from the entangled state to a separable state, but this transition is modulated by the intensity of interactions in the XY-plane. The intensities of interactions along the X- and Y-axes are studied to understand their effect on the system’s entanglement. Two spin squeezing mechanisms, one-axis twisting and two-axis counter twisting, are compared, revealing that two-axis counter twisting offers better entanglement. Finally, we explore quantum teleportation through squeezed spin states, demonstrating its feasibility with high fidelity at high temperatures and without a magnetic field, provided that the intensities of interactions in the XY-plane are negligible. By increasing the intensities (mu ) and (chi ), fidelity improves. Intriguingly, our analysis suggests that quantum teleportation, with increased fidelity, is achievable only with the one-axis twisting spin squeezing mechanism, remaining out of reach for two-axis counter twisting.