As quantum technology rapidly advances, the need for efficient scalable methods to characterize quantum systems intensifies. Quantum state tomography and Hamiltonian learning are essential for interpreting and optimizing quantum systems, yet a unified approach remains elusive. Such an integration could enhance our understanding of the complex relationship between quantum states and Hamiltonians, contributing to the development of more efficient methodologies. In this paper, we present a method that integrates quantum state tomography and Hamiltonian learning, drawing inspiration from machine translation in the field of natural language processing (NLP). We demonstrate the effectiveness of our approach across a variety of quantum systems, successfully learning the complex relationships between quantum states and Hamiltonians. Furthermore, the scalability and few-shot learning capabilities of our method could potentially minimize the resources required for characterizing and optimizing quantum systems. Our research provides valuable insights into the relationship between quantum states and Hamiltonians, paving the way for further studies on quantum systems and advancing quantum computation and related technologies.

A sending-or-not-sending (SNS) protocol is a very promising variant in twin-field quantum key distribution (TFQKD) for its advantages at the regime of long distance and the robustness against large optical misalignment. The actively odd-parity pairing (AOPP) method can significantly enhance the key-rate performance of the original SNS TFQKD, which forms the AOPP SNS TFQKD protocol. In this paper, we introduce the advantage distillation (AD) method to further improve the communication distance for AOPP SNS TFQKD. We also prove the composable security against coherent attacks for the AOPP SNS TFQKD with AD in the finite-key regime for bit blocks with a size of 2 in the AD process. Our AD method for AOPP SNS TFQKD protocol is expected to improve the key-rate performance in practice without changing the SNS TFQKD experimental system.

We report large nonreciprocity in the transmission of shear-horizontal surface acoustic waves (SHSAWs) on ${\mathrm{Li}\mathrm{Ta}\mathrm{O}}_3$ substrate coated with a $\mathrm{Fe}\text{−}\mathrm{Co}\text{−}\mathrm{Si}\text{−}\mathrm{B}/\mathrm{Ni}\text{−}\mathrm{Fe}\text{−}\mathrm{Cu}$ magnetoelastic bilayer. The large difference in saturation magnetization of the two layers not only brings nonreciprocal spin waves (SWs), but also ensures the phonon-magnon (SAW-SW) coupling at relatively low wave numbers. It is found that the angle between the static magnetization and the spin-wave vector plays a significant role in determining the strength of magnetoelastic coupling and nonreciprocity, simultaneously. A large nonreciprocal transmission of SAWs about 30 dB (i.e., 60 dB/mm) is demonstrated at 2.33 GHz. In addition, the dispersion relation between coupled SHSAWs and nonreciprocal SWs is developed, which provide a good insight into the observed phenomena. Our results offer a convenient approach to implement nonreciprocal SAW isolators or circulators.

Efficient generation, guiding, and detection of phonons, or mechanical vibrations, are of interest in various fields, including radio-frequency communication, sensing, and quantum information. Diamond is a useful platform for phononics because of the presence of strain-sensitive spin qubits, and its high Young’s modulus, which allows for low-loss gigahertz devices. We demonstrate a diamond phononic waveguide platform for generating, guiding, and detecting gigahertz-frequency surface acoustic wave (SAW) phonons. We generate SAWs using interdigital transducers integrated on $\mathrm{Al}\mathrm{N}$/diamond and observe SAW transmission at 4–5 GHz through both ridge and suspended waveguides, with wavelength-scale cross sections (approximately 1 ${\mathrm{m}}^2$) to maximize spin-phonon interaction. This work is a crucial step for developing acoustic components for quantum phononic circuits with strain-sensitive color centers in diamond.

The geometric and dynamic phases are generally treated as independent and together give rise to the total phase of a system. Here, we present a scenario in which these two phases become strongly interdependent. We achieve this by introducing a modified Young’s double-slit configuration that supports surface plasmon polaritons (SPPs) propagating between the slits. Remarkably, by varying the slit separation distance, and hence the dynamic SPP phase, by just a single plasmon wavelength, the geometric phase can be significantly modified.

We report on the dependence of curvilinear-shaped coplanar waveguides on the near-field diffraction (NFD) patterns of spin waves propagating in perpendicularly magnetized thin films. Implementing the propagating spin-wave spectroscopy techniques on either concentrically or eccentrically shaped antennas, we show how the link budget is directly affected by the spin-wave interference, in good agreement with NFD simulations. This work demonstrates the feasibility of inductively probing a magnon interference pattern with a resolution down to $1\text{μ}{\mathrm{m}}^2$, and provides a methodology for shaping spin-wave beams from an antenna design. This methodology is successfully implemented in the case study of a spin-wave Young’s interference experiment.

Kerr cat qubits are a promising candidate for fault-tolerant quantum computers owing to the biased nature of their errors. The $ZZ$ coupling between the qubits can be utilized for a two-qubit entangling gate, but the residual coupling called $ZZ$ crosstalk is detrimental to precise computing. In order to resolve this problem, we propose a tunable $ZZ$-coupling scheme using two transmon couplers. By setting the detunings of the two couplers at opposite values, the residual $ZZ$ couplings via the two couplers cancel each other out. We also apply our scheme to the $R_{ZZ}(\mathrm{\Theta })$ gate ($ZZ$ rotation with angle $\mathrm{\Theta }$), one of the two-qubit entangling gates. We numerically show that the fidelity of the $R_{ZZ}(-\pi /2)$ gate is higher than 99.9% in a case of 16-ns gate time and without decoherence.

Parametric down-conversion is a widely exploited technique in optics to produce entangled states of photons for quantum information processing and quantum sensing. In the microwave domain, devices based on Josephson junctions, such as Josephson parametric amplifiers and voltage-biased Josephson junctions, have been successfully utilized to generate such states. However, their high susceptibility to magnetic fields has posed challenges in many applications. Here we demonstrate the generation of two-mode squeezed states via four-wave mixing in a superconducting nanowire resonator patterned from $\mathrm{Nb}\mathrm{N}$. The $\mathrm{Nb}\mathrm{N}$ nanowire exhibits a strong Kerr nonlinearity, resulting in the emission of a signal-idler pair with a cross-correlation of $g^{(2)}(0)=11.9$. Owing to the magnetic resilience and high critical temperature ($T_c$) of $\mathrm{Nb}\mathrm{N}$, our microwave parametric sources based on kinetic inductance promise an expanded range of potential applications.

In light of recent achievements in optical computing and machine learning, we consider the conditions under which all-optical computing may surpass electronic and optoelectronic computing in terms of energy efficiency and scalability. When considering the performance of a system as a whole, the cost of memory access and data acquisition is likely to be one of the main efficiency bottlenecks not only for electronic, but also for optoelectronic and all-optical devices. However, we predict that all-optical devices will be at an advantage in the case of inference in large neural network models, and the advantage will be particularly large in the case of generative models. We also consider the limitations of all-optical neural networks, including footprint, strength of nonlinearity, optical signal degradation, limited precision of computations, and quantum noise.

Unauthorized light injection has always been a vital threat to the practical security of a quantum-key-distribution (QKD) system. An optical-power limiter (OPL) based on the thermo-optical defocusing effect has been proposed and implemented, limiting the injected hacking light. As a hardware countermeasure, the performance of the OPL under various light-injection attacks will be tested to clarify the security boundary before it is widely deployed. To investigate the security boundary of the OPL in quantum cryptography, we comprehensively test and analyze the behavior of the OPL under continuous-wave (cw) light-injection attacks and pulse-illumination attacks with pulse repetition rates of $0.5$ Hz, $40$ MHz, and $1$ GHz. The test results illuminate the security boundary of the OPL, which allows one to properly employ the OPL in use cases. The methodology of testing and analysis proposed here is applicable to other power-limitation components in a QKD system.

A convenient analytical description of an ultrashort terahertz pulse and an analysis of the influence of this pulse on the Zeeman torque dynamics are extremely important tasks due to the potential applications of terahertz radiation. The theoretical expressions proposed in this paper clarify the physics of magnetic dynamics under the action of the magnetic field of a terahertz pulse and show the role of individual parameters of the material and the pulse field in this process. On the basis of the formulas obtained and the available experimental data for fcc-$\mathrm{Co}$ film, we analyze the possibilities of recovering information about the magnetic field of the pulse from the observation of magnetization dynamics, as well as the effect of fluence, Gilbert damping, and magnetic anisotropy on the magnetization dynamics induced by ultrashort terahertz radiation. The theoretical framework presented could potentially be used in the design and optimization of a new generation of terahertz detectors based on the magnetic component of the electromagnetic field.

Distributing quantum information between remote systems will necessitate the integration of emerging quantum components with existing communication infrastructure. This requires understanding the channel-induced degradations of the transmitted quantum signals, beyond the typical characterization methods for classical communication systems. Here we report on a comprehensive characterization of a Boston-Area Quantum Network (BARQNET) telecom fiber testbed, measuring the time-of-flight, polarization, and phase noise imparted on transmitted signals. We further design and demonstrate a compensation system that is both resilient to these noise sources and compatible with integration of emerging quantum memory components on the deployed link. These results have utility for future work on the BARQNET as well as other quantum network testbeds in development, enabling near-term quantum networking demonstrations and informing what areas of technology development will be most impactful in advancing future system capabilities.

We present a way to achieve fully directional, quantum-limited phase-preserving amplification in a four-port, four-mode superconducting Josephson circuit by utilizing interference between six parametric processes that couple all four modes. Full directionality, defined as the reverse isolation surpassing forward gain between the matched input and output ports of the amplifier, ensures its robustness against impedance mismatch that might be present at its output port during applications. Unlike existing directional phase-preserving amplifiers, both the minimal backaction and the quantum-limited added noise of this amplifier remains unaffected by noise incident on its output port. In addition, the matched input and output ports allow direct on-chip integration of these amplifiers with other circuit QED components, facilitating scaling up of superconducting quantum processors.