EPJ Quantum Technology最新文献

The barren plateau problem in quantum neural networks (QNNs) is a significant challenge that hinders the practical success of QNNs. In this paper, we introduce residual quantum neural networks (ResQNets) as a solution to address this problem. ResQNets are inspired by classical residual neural networks and involve splitting the conventional QNN architecture into multiple quantum nodes, each containing its own parameterized quantum circuit, and introducing residual connections between these nodes. Our study demonstrates the efficacy of ResQNets by comparing their performance with that of conventional QNNs and plain quantum neural networks through multiple training experiments and analyzing the cost function landscapes. Our results show that the incorporation of residual connections results in improved training performance. Therefore, we conclude that ResQNets offer a promising solution to overcome the barren plateau problem in QNNs and provide a potential direction for future research in the field of quantum machine learning.

Grover quantum algorithm is an unstructured search algorithm that can run on a quantum computer with the complexity of O(sqrt{N}), and is one of the typical algorithms of quantum computing. Recently, it has served as a routine for pattern-matching tasks. However, the original Grover search algorithm is probabilistic, which is not negligible for problems involving determinism. Besides that, efficient data loading is also a key challenge for the practical applications of the Grover algorithm. Here in this work, we propose a modified pattern-matching scheme with Long’s quantum search algorithm, in which the quantum circuit structure search algorithm requires fewer multi-qubit quantum gates, and can obtain the desired results deterministically. Then, the comparison of the performance of our scheme and the previous algorithms is presented through numerical simulations, indicating our algorithm is feasible with current quantum technologies which is friendly to noisy intermediate-scale quantum (NISQ) devices.

We propose a Rydberg atom-based receiver for amplitude-modulation (AM) reception utilizing a dual-tone microwave field. The pseudo-random binary sequence (PRBS) signal is encoded in the basic microwave field (B-MW) at the frequency of 14.23 GHz. The signal can be decoded by the atomic receiver itself but more obvious with the introduction of an auxiliary microwave (A-MW) field. The receiver’s amplitude variations corresponding to microwave field are simulated by solving density matrices to give this mechanism theoretical support. An appropriate AM frequency is obtained by optimizing the signal-to-noise ratio, guaranteeing both large data transfer capacity (DTC) and high fidelity of the receiver. The power of two MW fields, along with the B-MW field frequency, is studied to acquire larger DTC and wider operating bandwidth. Finally, the readout of PRBS signals is performed by both the proposed and conventional mechanisms, and the comparison proves the obvious increment of DTC with the proposed scheme. This proof-of-principle demonstration exhibits the potential of the dual-tone scheme and offers a novel pathway for Rydberg atom-based microwave communication, which is beneficial for long-distance communication and weak signal perception outside the laboratory.

Rydberg atoms have exhibited excellent potentials to become a competent platform of implementing quantum computation, which demands to execute various quantum gates fast and faithfully. We propose a dynamic mechanism of two interacting Rydberg atoms for implementing a high-fidelity SWAP gate on ground-state manifolds, where the amplitude modulation and soft quantum control of lasers driving ground-Rydberg state transitions are elaborately matched with the interaction strength between atoms so as to engineer the desired transformation of atomic states. Compared with the recent Rydberg-atom SWAP gate scheme, the present one possesses the undegraded first-order dynamics and shows an interference-induced suppression of the doubly-excited Rydberg state, so it costs shorter gate time and exhibits greater robustness against atomic decay and deviations in the interatomic separation (interaction strengths). The present mechanism of implementing a SWAP gate on interacting Rydberg atoms could facilitate high-fidelity demonstrations of atomic ground state transformation and further exploitation of peculiar dynamics.

Decoy-state quantum key distribution (QKD) is undoubtedly the most efficient solution to handle multi-photon signals emitted by laser sources, and provides the same secret key rate scaling as ideal single-photon sources. It requires, however, that the phase of each emitted pulse is uniformly random. This might be difficult to guarantee in practice, due to inevitable device imperfections and/or the use of an external phase modulator for phase randomization in an active setup, which limits the possible selected phases to a finite set. Here, we investigate the security of decoy-state QKD when the phase is actively randomized by faulty devices, and show that this technique is quite robust to deviations from the ideal uniformly random scenario. For this, we combine a novel parameter estimation technique based on semi-definite programming, with the use of basis mismatched events, to tightly estimate the parameters that determine the achievable secret key rate. In doing so, we demonstrate that our analysis can significantly outperform previous results that address more restricted scenarios.

We investigate the response bandwidth of a superheterodyne Rydberg receiver at a room-temperature vapor cell, and present an architecture of multi-channel lasers excitation to increase the response bandwidth and keep sensitivity, simultaneously. Two microwave fields, denoted as a local oscillator (LO) (E_{text{LO}}) and a signal field (E_{text{SIG}}), couple two Rydberg states transition of (|52D_{5/2}rangle to |53P_{3/2}rangle ). In the presence of the LO field, the frequency difference between two fields can be read out as an intermediate frequency (IF) signal using Rydberg electromagnetically induced transparency (EIT) spectroscopy. The bandwidth of the Rydberg receiver is obtained by measuring the output power of IF signal versus the frequency difference between two fields. The bandwidth dependence on the Rabi frequency of excitation lasers is presented, which shows the bandwidth decrease with the probe Rabi frequency, while it is quadratic dependence on the coupling Rabi frequency. Meanwhile, we investigate the effect of probe laser waist on the bandwidth, showing that the bandwidth is inversely proportional to the laser waist. We achieve a maximum response bandwidth of the receiver about 6.8 MHz. Finally, we design an architecture of multi-channel lasers excitation for increasing the response and keeping the sensitivity, simultaneously. Our work has the potential to extend the applications of Rydberg atoms in communications.

A satellite-constellation based global quantum network could allow secure quantum communication between remote users worldwide. Such a constellation could be formed of micro- or even nanosatellites, which have the advantage of being more cost-effective than larger expensive spacecrafts. At the same time, the features of quantum communication impose a number of technical requirements that are more difficult to meet when using small satellites. Full-fledged quantum communication has been demonstrated with neither a micro- nor a nanosatellite so far. The authors took up this challenge and have developed a 6U CubeSat weighting 9.5 kg. The satellite is to be launched in 2023 and has already successfully passed all the pre-flight tests. The mission is not yet intended for fully quantum communication. Nevertheless, the authors are testing such key functional elements as polarization reference-frame synchronization and acquisition, pointing and tracking system on it. Besides that, the payload accommodates a full-duplex telecommunication system operating at a bit rate of 50 Mbit/s: an up- and a downlink at wavelengths of 808 and 850 nm. After the satellite is launched, the main goal to be achieved is to demonstrate stable connection between it and an optical ground station and carry out multiple communication sessions. In quantum communication, generating secret keys from raw measurement data implies two-way exchange of significant amount of information and therefore availability of a classical communication channel with a high bandwidth is one of the crucial things. In the following mission, which envisages an overall quantum key distribution system, we plan to use the free-space optical link for such an exchange of data, whereas the RF link will only be used for telemetry and telecommand.

The quantum entanglement distribution network, serviced as the communication infrastructure which distributes quantum information among remote users, enables many applications beyond the reach of classical networks. Recently, the applications such as quantum key distribution and quantum secure direct communication, have been successfully demonstrated in the quantum entanglement distribution network. In this article, we propose a multi-user round-trip quantum clock synchronization (QCS) scheme in the quantum network, which can be implemented with one single entangled photon source located at the server. The server distributes the entangled photons to remote multiple users with the wavelength division multiplexing strategy, and each user feeds partial received photons back to the server. The clock difference between the server and each user is calculated from the one-way and round-trip propagation times, which are determined according to the time correlation of entangled photons. Afterwards, the demonstration has been conducted between the server and a user over a 75-km-long fiber link, where the measured clock difference uncertainty is 4.45 ps, and the time deviation is 426 fs with an average time of 4000 s. Furthermore, the proposed QCS scheme is linearly scalable to many users, with respect to user hardware and number of deployed fibers.

Quantum random number generator (QRNG) based on the inherent randomness of fundamental quantum processes can provide provable true random numbers which play an important role in many fields. However, the security of practical QRNGs is linked to the performance of realistic devices. In particular, devices based on the Faraday effect in a QRNG system may be affected by external magnetic fields, which will inevitably open a loophole that an eavesdropper can exploit to steal the information of generated random numbers. In this work, the effects of external magnetic fields on the security of practical QRNGs are analyzed. Taking the quantum phase fluctuation based QRNG with unbalanced Michelson interferometer as an example, we experimentally demonstrate the rotation angle of the Faraday rotation mirror (FRM) is influenced by external magnetic fields. Then, we develop a theoretical model between the rotation angle deviation of FRM and conditional min-entropy. Simulation results show that the imperfect FRM leads to a reduction in the variance of measured signal and extractable randomness. Furthermore, the impacts of practical sampling device on the extractable randomness are analyzed in the presence of imperfect FRM, which indicates suitable parameters of the sampling device can improve the security of practical QRNGs. Potential countermeasures are also proposed. Our work reveals that external magnetic fields should be carefully considered in the application of practical QRNGs.

Entity authentication is crucial for ensuring secure quantum communication as it helps confirm the identity of participants before transmitting any confidential information. We propose a practical entity authentication protocol for quantum key distribution (QKD) network systems that utilizes authentication qubits. In this protocol, authentication qubits that are encoded with pre-shared information are generated and exchanged to verify the legitimacy of each entity. By using the authentication qubit, participants can identify each other with enhanced security level through the quantum channel. The proposed protocol can be easily integrated with existing QKD systems without the need for additional hardware. In this study, we demonstrated the efficacy of the proposed scheme using a 1xN QKD network system and verified its stable operation over a deployed fiber network. Additionally, a security analysis of the proposed entity authentication protocol and architecture is provided.