This topical review addresses how Rydberg atoms can serve as building blocks for emerging quantum technologies. Whereas the fabrication of large numbers of artificial quantum systems with the uniformity required for the most attractive applications is difficult if not impossible, atoms provide stable quantum systems which, for the same species and isotope, are all identical. Whilst atomic ground-states provide scalable quantum objects, their applications are limited by the range over which their properties can be varied. In contrast, Rydberg atoms offer strong and controllable atomic interactions that can be tuned by selecting states with different principal quantum number or orbital angular momentum. In addition Rydberg atoms are comparatively long-lived, and the large number of available energy levels and their separations allow coupling to electromagnetic fields spanning over 6 orders of magnitude in frequency. These features make Rydberg atoms highly desirable for developing new quantum technologies. After giving a brief introduction to how the properties of Rydberg atoms can be tuned, we give several examples of current areas where the unique advantages of Rydberg atom systems are being exploited to enable new applications in quantum computing, electromagnetic field sensing, and quantum optics.

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Rydberg atom quantum technologies

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In this work, we demonstrate the use of a Rydberg atom-based sensor for determining the angle of arrival of an incident radio frequency (RF) wave or signal. The technique uses electromagnetically induced transparency in Rydberg atomic vapor in conjunction with a heterodyne Rydberg atom-based mixer. The Rydberg atom mixer measures the phase of the incident RF wave at two different locations inside an atomic vapor cell. The phase difference at these two locations is related to the direction of arrival of the incident RF wave. To demonstrate this approach, we measure phase differences of an incident 19.18 GHz wave at two locations inside a vapor cell filled with cesium atoms for various incident angles. Comparisons of these measurements with both the full-wave simulation and the plane wave theoretical model show that these atom-based sub-wavelength phase measurements can be used to determine the angle of arrival of an RF field.

Determining the angle-of-arrival of a radio-frequency source with a Rydberg atom-based sensor

Rydberg atoms have attracted significant interest recently as electric field sensors. In order to assess potential applications, detailed understanding of relevant figures of merit is necessary, particularly in relation to other, more mature, sensor technologies. Here we present a quantitative analysis of the Rydberg sensor's sensitivity to oscillating electric fields with frequencies between 1 kHz and 1 THz. Sensitivity is calculated using a combination of analytical and semi-classical Floquet models. Using these models, optimal sensitivity at arbitrary field frequency is determined. We validate the numeric Floquet model via experimental Rydberg sensor measurements over a range of 1-20 GHz. Using analytical models, we compare with two prominent electric field sensor technologies: electro-optic crystals and dipole antenna-coupled passive electronics.

https://iopscience.iop.org/article/10.1088/1361-6455/ab6051/pdf

Assessment of Rydberg atoms for wideband electric field sensing

The recent developments of Rydberg atom-based sensors with the ability to measure phase have made it possible to receive digital phase-modulated signals. In this letter, we demonstrate the reception of binary phase-shift keying, quadrature phase-shift keying, and quadrature amplitude modulation signals with an atom-based receiver. We present measured values of error vector magnitude as a function of symbol rate for various modulation schemes and discuss the bandwidth of a Rydberg atom-based receiver system. The results show that we can now interrogate ensembles of atoms to such extent that we can use them to receive data from a phase-modulated communication signal.

Detecting and Receiving Phase-Modulated Signals With a Rydberg Atom-Based Receiver

Up to now, the measurement of radio-frequency (RF) electric field achieved using the electromagnetically-induced transparency (EIT) of Rydberg atoms has proved to be of high-sensitivity and shows a potential to produce a promising atomic RF receiver at resonance between two chosen Rydberg states. In this paper, we study the extension of the feasibility of digital communication via this quantum-based antenna over a continuously tunable RF-carrier at off-resonance. Our experiment shows that the digital communication at a rate of 500 kbps can be performed reliably within a tunable bandwidth of 200 MHz near a 10.22 GHz carrier. Outside of this range, the bit error rate (BER) increases, rising to, for example, 15% at an RF-detuning of ±150 MHz. In the measurement, the time-varying RF field is retrieved by detecting the optical power of the probe laser at the center frequency of RF-induced symmetric or asymmetric Autler-Townes splitting in EIT. Prior to the digital test, we studied the RF-reception quality as a function of various parameters including the RF detuning and found that a choice of linear gain response to the RF-amplitude can suppress the signal distortion. The modulating signal can be decoded at speeds up to 500 kHz in the tunable bandwidth. Our test consolidates the physical basis for reliable communication and spectral sensing over a wider broadband RF-carrier, which paves a way for the concurrent multi-channel communications founded on the same pair of Rydberg states.

Rydberg-atom-based digital communication using a continuously tunable radio-frequency carrier.

High-sensitive measurement of radio-frequency (RF) electric field is available via the electromagnetically induced transparency (EIT) effect of Rydberg atom at room-temperature, which has been developed to be a promising atomic RF receiver. In this Letter, we investigate the credibility of the digital communication via this quantum-based antenna over the entire continuously tunable RF-carrier. Our experiment shows that digital communication at a rate of 500 kbps performs reliably within a tunable bandwidth of 200 MHz at carrier 10.22 GHz and a bit error rate (BER) appears out of this range, for example, the BER runs up to 15 % at RF-detuning $\pm150$ MHz. In the measurement, the time-variant RF field is retrieved by detecting the density of the probe laser at the center frequency of RF-induced symmetric or asymmetric Autler-Townes splitting in EIT. Prior to the digital test, we have studied the RF-receiving quality versus the physical ambiance and found that a choice of linear gain response to the RF-amplitude can suppress the signal distortion and the modulating signal is able to be decoded as fast as up to 500 kHz in the tunable bandwidth. Our checkout consolidates the physical foundation for a reliable communication and spectrum sensing over the broadband RF-carrier.

The credibility of Rydberg atom based digital communication over a continuously tunable radio-frequency carrier

In this study, we demonstrate the direct measurement of a microwave (MW) magnetic field through the detection of atomic Rabi resonances with Cs vapor cells in a free-space low-Q cavity. The line shape (amplitude and linewidth) of detected Rabi resonances is investigated versus several experimental parameters such as the laser intensity, cell buffer gas pressure, and cell length. The specially designed low-Q cavity creates a suitable MW environment allowing easy testing of different vapor cells with distinct properties. Obtained results are analyzed to optimize the performances of a MW magnetic field sensor based on the present atom-based detection technique.

Microwave magnetic field detection based on Cs vapor cell in free space.

We describe a microwave (MW) magnetic-field detection technique based on the Rabi resonance of a Cs-buffer gas vapor cell. Inside the vapor cell, alkali atoms interact with MWs radiated from an antenna through an open-ended rectangular waveguide. We obtained the Rabi resonance line shape, which determines precisely the Rabi frequency of the MW magnetic field with a phase modulation frequency. For various vapor cells, a detailed characterization of several experimental parameters and a theoretical analysis were performed. The optimal laser intensity and the cell temperature that maximize the Rabi resonance amplitude are obtained. Moreover, the cell-temperature-based frequency shift of the Rabi resonance was investigated. These experimental results are important in developing future International System of Units-traceable MW magnetic field sensors and an MW power standard.

Rabi Resonances in Buffer-Gas-Filled Cs-Vapor Cells for SI-Traceable Microwave Magnetic Field Detection

We experimentally investigated continuously frequency-tunable microwave (MW) magnetic field detection based on Rabi resonance with a single cesium-rubidium hybrid vapor cell. The multispecies atomic systems, with their tunable abilities in transition frequencies, enabled this atomic sensing head to cover a broader detectable MW field scope compared to the use of a single alkali atom. Here, we demonstrated the simultaneous observation of atomic Rabi resonance signals with 85Rb, 87Rb, and 133Cs in the same vapor cell. Using an experimentally feasible static magnetic field (DC field) below 500 G, we realized a MW magnetic field strength detection with a bandwidth of 4.8 GHz around 8.1 GHz. The use of a hybrid cell system cell also enabled the detection of an identical MW field for different atomic species with the help of the DC field, allowing us to perform a perfect comparison for different applications that require the same electromagnetic environment. The results may be useful for the realization and application of many atomic detectors based on different physical principles.

Xiaochi Liu

Papers

Rydberg-atom-based digital communication using a continuously tunable radio-frequency carrier.

The credibility of Rydberg atom based digital communication over a continuously tunable radio-frequency carrier

Microwave magnetic field detection based on Cs vapor cell in free space.

Rabi Resonances in Buffer-Gas-Filled Cs-Vapor Cells for SI-Traceable Microwave Magnetic Field Detection

Tunable microwave magnetic field detection based on Rabi resonance with a single cesium-rubidium hybrid vapor cell