Advanced quantum technologies最新文献

Inferring a quantum system from incomplete information is a common problem in many aspects of quantum information science and applications, where the principle of maximum entropy (MaxEnt) plays an important role. The quantum state compatibility problem asks whether there exists a density matrix $�\rho$ compatible with some given measurement results. Such a compatibility problem can be naturally formulated as a semidefinite programming (SDP), which searches directly for the existence of a $�\rho$. However, for large system dimensions, it is hard to represent $�\rho$ directly, since it requires too many parameters. In this work, MaxEnt is applied to solve various quantum state compatibility problems, including the quantum marginal problem. An immediate advantage of the MaxEnt method is that it only needs to represent $�\rho$ via a relatively small number of parameters, which is exactly the number of the operators measured. Furthermore, in case of incompatible measurement results, the method will further return a witness that is a supporting hyperplane of the compatible set. The method has a clear geometric meaning and can be computed effectively with hybrid quantum-classical algorithms.

The achievement of quantum computational advantage, also known as quantum supremacy, is a major milestone at which a quantum computer can solve a problem significantly faster than the world's most powerful classical computers. Two tasks, boson sampling and random quantum circuit sampling, have experimentally exhibited quantum advantages on photonic and superconducting platforms respectively. Classical benchmarking is essential, yet challenging, because these tasks are intractable for classical computers. This study reviews models, algorithms and large-scale simulations of these two sampling tasks. These approaches continue to hold substantial significance for research in both current noisy intermediate-scale quantum (NISQ) systems and future fault-tolerant quantum computing.

Nonlinear effect, which is induced by probe lightatom ensemble interaction, can significantly influence the performance of atomic magnetometers. The cover image shows the nonlinear interaction process between linear-polarized light and atomic ensemble operated in the spin-exchange relaxation-free regime. In article number 2400226, Jixi Lu, Bozheng Xing, and co-workers first propose that the nonlinear effect could be effectively suppressed by optimizing saturation parameters, which causes significant improvement in probe sensitivity of the atomic magnetometer.

Current research in quantum matter is strongly focused on quantum computing, communication and the quantum internet. The cover shows an artist's impression of the non-classical Fock state field in a single-mode resonator (red) operating at one frequency being directed to the input of a qubit controller (middle part) and subsequently converted to the desired non-classical (e.g. N00N state) field (green) in an output single-mode resonator operating at another frequency. The results described by Dmitry Yakovlev and co-workers in article number 2400141 open up exciting new possibilities for the quantum connection between solid-state and photonic flying qubits, enabling all-quantum data processing and data transfer.

Light–matter interaction can bridge critical phenomena, topological transitions, quantum metrology, and non-Hermitian physics, bringing novel implications. In non-Hermitian Jaynes–Cummings models with PT and anti-PT symmetries, the quantum Fisher information manifests criticality and super-universality around exceptional points (as shown in article number 2400288 by Zu-Jian Ying), providing a universally high sensitivity resource for quantum metrology. The simultaneous occurrence of conventionally incompatible critical Landau-class transition and topological transition is realized, with conceptional renovation and more potential in designing quantum sensors.

The simulation of quantum systems is one of the flagship applications of near-term NISQ (noisy intermediate-scale quantum) computing devices. Efficiently simulating the rich, non-unitary dynamics of open quantum systems remains challenging on NISQ hardware. Current simulation methods for open quantum systems employ time-stepped Trotter product formulas (“Trotterization”) which can scale poorly with respect to the simulation time and system dimension. Here, a new simulation method is proposed based on the derivation of a time-perturbative Kraus operator series representation of the system. A class of open quantum systems is identified for which this method produces circuits of time-independent depth, which may serve as a desirable alternative to Trotterization, especially on NISQ devices.

As quantum computing technology slowly matures and the number of available qubits on a QPU gradually increases, interest in assessing the capabilities of quantum computing hardware in a scalable manner is growing. One of the key properties for quantum computing is the ability to generate multipartite entangled states. In this study, aspects of benchmarking entanglement generation capabilities of noisy intermediate-scale quantum (NISQ) devices are discussed based on the preparation of graph states and the verification of entanglement in the prepared states. Thereby, entanglement witnesses that are specifically suited for a scalable experiment design are used. This choice of entanglement witnesses can detect A) bipartite entanglement and B) genuine multipartite entanglement for graph states with constant two measurement settings if the prepared graph state is based on a two-colorable graph, e.g., a square grid graph or one of its subgraphs. With this, it is experimentally verified that a bipartite entangled state comprising all qubits can be prepared on a 127-qubit IBM Quantum superconducting QPU, and genuine multipartite entanglement can be detected for states of up to 23 qubits with quantum readout error mitigation.

Carbon implantation at the nanoscale is highly desired for the engineering of defect-based qubits in a variety of materials, including silicon, diamond, silicon carbide (SiC) and hexagonal boron nitride (hBN). However, the lack of focused carbon ion beams does not allow for the full disclosure of their potential for application in quantum technologies. Here, a carbon source for focused ion beams is developed and utilized for the simultaneous creation of two types of quantum emitters in silicon, the W and G centers. Furthermore, a multi-step implantation protocol is applied for the programmable activation of the G centers with a spatial resolution better than 250 nm. This approach provides a route for significant enhancement of the creation yield of single G centers in carbon-free silicon wafers, including commercial silicon-on-insulator wafers. The experimental demonstration is an important step toward nanoscale engineering of telecom quantum emitters in silicon of high crystalline quality and isotope purity.

A null test of the two-level space of a qubit is constructed, which is both device independent and needs a small number of different experiments. Its feasibility is demonstrated on IBM Quantum, with most qubits failing the test by more than ten standard deviations. The robustness of the test against common technical imperfections, like decoherence and phase shifts, and supposedly negligible leakage, indicates that the origin of deviations is beyond known effects.