Microwave-optical transducers and quantum memories are essential for quantum repeaters enabling a quantum internet. Despite advances in both technologies, integrating these functionalities remains challenging. Here, we theoretically propose and experimentally demonstrate an on-demand microwave-optical quantum transducer based on a Rydberg ensemble. Using cascaded electromagnetically induced transparency, we store microwave photons in a highly excited collective state and convert them into optical photons during retrieval. Leveraging an optical depth of millions for microwave photons and minimal single-photon-level dephasing, our transducer achieves around 90% area-normalized storage efficiency, 2.3 MHz bandwidth, and noise-equivalent temperature of 26 K under cavity-free conditions. Furthermore, our system is cryogenically compatible and extendable for high single-photon conversion efficiency without requiring optical cavity coupling. These findings advance practical on-demand quantum interfaces with broad applications across atomic and solid-state platforms.

There is an inherent trade-off in shortwave (SW) signal reception between achieving wide operational bandwidth and high radio sensitivity. Advances in SW radio reception hold significant potential for applications in communications, broadcasting, and over-the-horizon radar. Here, we demonstrate a proof-of-concept Rydberg SW receiver integrated with the frequency-hopping technique, achieving a highly sensitive reception compatible with large bandwidth. By shifting a carrier across the whole SW band, the atomic communication maintains a stable and high data capacity far beyond the Chu-Harrington limit. The outcomes of our work facilitate the development of Rydberg atom-based RF field sensing and communication.

The development of a microwave electrometer with inherent uncertainty approaching its ultimate limit carries both fundamental and technological significance. Recently, the Rydberg electrometer has garnered considerable attention due to its exceptional sensitivity, small-size, and broad tunability. This specific quantum sensor utilizes low-entropy laser beams to detect disturbances in atomic internal states, thereby circumventing the intrinsic thermal noise encountered by its classical counterparts. However, due to the thermal motion of atoms, the advanced Rydberg-atom microwave electrometer falls considerably short of the standard quantum limit by over three orders of magnitude. In this study, we utilize an optically thin medium with approximately 5.2e5 laser-cooled atoms to implement heterodyne detection. By mitigating a variety of noises and strategically optimizing the parameters of the Rydberg electrometer, our study achieves an electric-field sensitivity of 10.0 nV/cm/Hz^1/2 at a 100 Hz repetition rate, reaching a factor of 2.6 above the standard quantum limit and a minimum detectable field of 540 pV/cm. We also provide an in-depth analysis of noise mechanisms and determine optimal parameters to bolster the performance of Rydberg-atom sensors. Our work provides insights into the inherent capacities and limitations of Rydberg electrometers, while offering superior sensitivity for detecting weak microwave signals in numerous applications.

The braiding operations of quantum states have attracted substantial attention due to their great potential for realizing topological quantum computations. In this paper, we show that a threefold-degenerate eigensubspace can be obtained in a four-level Hamiltonian which is the minimal physical system. Braiding operations are proposed to apply to dressed states in the subspace. The topology of the braiding diagram can be characterized through physical methods once the sequential braiding pulses are adopted. We establish an equivalent relationship function between the permutation group and the output states where different output states correspond to different values of the function. The topological transition of the braiding happens when two operations overlap, which is detectable through the measurement of the function. Combined with the phase variation method, we can analyze the wringing pattern of the braiding. Therefore, the experimentally feasible system provides a platform to investigate braiding dynamics, the SU(3) physics, and the qutrit gates.

Rydberg atoms-based electric field sensing has developed rapidly over the past decade. A variety of theoretical proposals and experiment configurations are suggested and realized to improve the measurement metrics, such as intensity sensitivity, bandwidth, phase, and accuracy. The Stark effect and electromagnetically induced transparency (EIT) or electromagnetically induced absorption (EIA) are fundamental physics principles behind the stage. Furthermore, various techniques such as amplitude- or frequency-modulation, optical homodyne read-out, microwave superheterodyne and frequency conversion based on multi-wave mixing in atoms are utilized to push the metrics into higher levels. In this review, different technologies and the corresponding metrics they had achieved were presented, hoping to inspire more possibilities in the improvement of metrics of Rydberg atom-based electric field sensing and broadness of application scenarios.

Terahertz heterodyne receivers with high sensitivity and spectral resolution are crucial for various applications. Here, we present a room-temperature atomic terahertz heterodyne receiver that achieves ultrahigh sensitivity and frequency resolution. At a signal frequency of 338.7 GHz, we obtain a sensitivity of 2.88 +/- 0.09 mu V & sdot;cm-1 & sdot;Hz-1/2 for electric field measurements. The calibrated linear dynamical range spans approximately 89 dB, ranging from -110 dBV/cm to -21 dBV/cm. We demodulate a 400 symbol stream encoded in 4-state phase-shift keying, demonstrating excellent phase detection capability. By scanning the frequency of the local oscillator, we realize a terahertz spectrometer with Hz level frequency resolution. This resolution is more than two orders of magnitude higher than that of existing terahertz spectrometers. The demonstrated terahertz heterodyne receiver holds promising potential for working across the entire terahertz spectrum, significantly advancing its practical applications.

The braiding operations of quantum states have attracted substantial attention due to their great potential for realizing topological quantum computations. In this paper, we show that a three-fold degenerate eigen subspace can be obtained in a four-level Hamiltonian which is the minimal physical system. Braiding operations are proposed to apply to dressed states in the subspace. The topology of the braiding diagram can be characterized through physical methods once that the sequential braiding pulses are adopted. We establish an equivalent relationship function between the permutation group and the output states where different output states correspond to different values of the function. The topological transition of the braiding happens when two operations overlap, which is detectable through the measurement of the function. Combined with the phase variation method, we can analyze the wringing pattern of the braiding. Therefore, the experimentally-feasible system provides a platform to investigate braiding dynamics, the SU(3) physics and the qutrit gates.

A coherent microwave-to-optical conversion scheme, previously feasible only under cryogenic environments, has now been expanded to ambient conditions by using Rydberg atoms.

The hybrid quantum system composed of superconductor and cold atoms is expected to achieve fast quantum gates, long-life quantum storage and long-distance transmission through optical fibers, making it one of the most promising hybrid quantum systems to realize optical interconnection between two superconducting quantum computers. In this paper, we comprehensively review the recent research advancements in the optical interconnection of two superconducting quantum computers, based on the superconductor and cold atoms hybrid quantum system, specifically the review covers the coherent coupling between superconducting chips and cold atoms, the coherent microwave-to-optics conversion, and the long-range microwave interconnection between superconducting qubits and quantum converters. The system is expected to provide a physical and technical foundation for practical optical-fiber interconnection of two superconducting quantum computers, and have broad applications in distributed superconducting quantum computation and hybrid quantum networks.

The research of the microwave electrical field measurement which is based on the Rydberg atoms is developing fast during recent years. The prerequisite for the engineering application is the minimization and integration of the microwave electrical field measurement systems. This article introduces the basic characteristics of the Rydberg atoms, the fundamental principles of the microwave electrical field measurement, and the method of determining the resonant frequency of the transition. In addition, a transportable Rydberg atomic microwave electrometry is developed by combing the 852 nm modulation transfer frequency stabilization and 509 nm electromagnetically induced transparency frequency stabilization. Based on this instrument, we demonstrate the microwave electrical field measurement which is traced back to the standard international unit systems and the detection of the weak microwave signal.

The superconductor and cold atoms hybrid quantum system is poised to realize fast quantum gates, long-life quantum storage and long-distance transmission through optical fibers, making it one of the most promising hybrid quantum systems for achieving optical interconnection between two superconducting quantum computers. This paper provides a comprehensive review of recent research advancements in the optical interconnection of two superconducting quantum computers, based on the superconductor and cold atoms hybrid quantum system. This encompasses the coherent coupling between superconducting chips and cold atoms, the coherent microwave-to-optics conversion, and the long-range microwave interconnection between superconducting qubits and quantum converters. The system is expected to provide a physical and technical foundation for practical optical-fiber interconnection of two superconducting quantum computers, and have broad applications in distributed superconducting quantum computation and hybrid quantum networks.

Realization of practical terahertz wireless communications still faces many challenges. The receiver with high sensitivity is important for THz wireless communications. Here we demonstrate a terahertz receiver based on the cesium Rydberg atoms in a room-temperature vapor cell. The minimum detectable THz electric field is calibrated. With this receiver, the phase-sensitive conversion of amplitude-modulated or frequency-modulated terahertz waves into optical signals is performed. The results show that the atomic receiver has many advantages due to its quantum properties. Particularly, the long distance THz wireless communications is achievable using this receiver. Furthermore, the atomic receiver can be used in the THz wireless-to-optical link.

We propose a built-in fault-tolerant geometric operation to realize fast remote entanglement between superconducting qubits anchored to a 15 mK plate and Rydberg-atom qubits trapped near a 1 K plate via thermal coupled cavities. We show that this operation is robust against the detrimental effects of the thermal mode states and fluctuations in the control parameters. The operation can generate a high-fidelity entanglement between superconducting and atomic qubits under realistic experimental parameters, comparable to the results of the existing methods using auxiliary cooling systems. The scheme proposed here will promote the development of quantum network and distributed superconducting quantum computation.

Quantum transducers that can convert quantum signals from the microwave to the optical domain are a crucial optical interface for quantum information technology. Coherent microwave-to-optics conversions have been realized with various physical platforms, but all of them are limited to low efficiencies of less than 50%—the threshold of the no-cloning quantum regime. Here we report coherent microwave-to-optics transduction using Rydberg atoms and off-resonant scattering technique with an efficiency of 82 ± 2% and a bandwidth of about 1 MHz. The high conversion efficiency is maintained for microwave photons ranging from thousands to about 50, suggesting that our transduction is readily applicable to the single-photon level. Without requiring cavities or aggressive cooling for the quantum ground states, our results would push atomic transducers closer to practical applications in quantum technologies.

Atomic heterodyne dressed by a local oscillator resonant with Rydberg transitions allows high sensitivity and robust phase measurement of a microwave electric field, but it is typically limited to detection of discrete frequencies within the narrow bandwidth of Rydberg transitions. Here we demonstrate an atomic heterodyne scheme for continuous-frequency electric field measurement based on multilevel Rydberg atoms in a room-temperature vapor cell. Driven by two off-resonant microwaves acting as a tunable local oscillator field, the heterodyne receiver can retrieve the amplitude, phase, and frequency information of signal microwave in a continuous frequency band. In our experiment, the receiver achieves an electric field sensitivity of up to 1.5 mu V cm(-1) Hz(-1/2), 80-dB linear dynamic range, and over 1 GHz of continuous frequency range. We also demonstrate the reliable reception of continuously tunable phase-modulated carriers in the digital communication. This work will facilitate the application of atomic heterodyne in the areas such as radar technique, radio monitoring, and radio astronomy.

With the overall redefinition of the international system of units (SI), the classical physical standards are gradually replaced by the natural standards based on quantum physics. Due to the reproducibility, accuracy and stability of atomic system, many new technologies of quantum precision measurement have been developed based on the atomic systems. Rydberg atoms are very sensitive to external electromagnetic fields because of their large MW transition dipole moment and polarizability. With the atomic coherence effect, the ultra wideband electric field can be measured with high precision and high sensitivity. Especially for the microwave range whose frequency is higher than 1 GHz, the research of establishing microwave quantum standard is advancing steadily. In addition, all optical materials and ultra wide band atomic antenna have a wide range of applications in aerospace, military communications and other fields. In this paper, we review the researches of microwave sensing and communication based on Rydberg atoms, and introduce the progress in the quantum standard of microwave electric field strength, coherent detection, radio-over-fiber, digital communication, etc., and discuss the possible applications and research frontiers in this rapid developing field.

The traceability of microwave electric field is measured based on the Rydberg atomic quantum interference effect. The rubidium atomic vapor cell at room temperature is used as the probe, and the Autler-Townes splitting of two-photon electromagnetically-induced transparency, three-photon electromagnetically-induced transparency and three-photon electromagnetically-induced absorption effect are analyzed theoretically. The measurement limit of atomic shot noise is also discussed. This method is not only suitable for the traceability and self-calibration measurement of the microwave electric field but also for the sub-wavelength imaging and vector measurement of the microwave electric field and will provide a reference for further miniaturization and integration of the atomic microwave probe.

We report the direct characterization of energy-time entanglement of narrow-band biphotons produced from spontaneous four-wave mixing in cold atoms. The Stokes and anti-Stokes two-photon temporal correlation is measured by single-photon counters with nanosecond temporal resolution, and their joint spectrum is determined by using a narrow linewidth optical cavity. The energy-time entanglement is verified by the joint frequency-time uncertainty product of 0.063 +/- 0.0044, which does not only violate the separability criterion but also satisfies the continuous variable Einstein-Podolsky-Rosen steering inequality.

Rydberg atoms have large electric dipole moments in the microwave and terahertz frequency band. The detection of electromagnetic field intensity in this frequency band can be achieved by using quantum interference effects. Theoretically, this detection method can have a sensitivity much higher than the traditional detection methods. Therefore, electromagnetic field detection and precision measurement technology based on Rydberg atomic quantum effects has great application prospects in terahertz field strength and power measurement, terahertz communication and imaging. In this paper, we review the basic theory and experimental methods to realize the self-calibration and traceability measurement of electromagnetic field based on Rydberg atomic quantum effects. The principle and technical scheme of high-sensitivity terahertz field strength measurement, terahertz near-field high-speed imaging and terahertz digital communication based on Rydberg atom are introduced in detail. Finally, the processing terahertz detection work based on Rydberg atom by our research team is also mentioned briefly.