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Nature Communications

A terahertz-driven non-equilibrium phase transition in a room temperature atomic vapour

Christopher G. Wade, Matteo Marcuzzi, Emanuele Levi, Jorge Douglas Massayuki Kondo, Igor Lesanovsky, Charles S. Adams, Kevin J. Weatherill

Published Aug 28, 2018• 48 citations
Publisher

Abstract

Abstract There are few demonstrated examples of phase transitions that may be driven directly by terahertz frequency electric fields, and those that are known require field strengths exceeding 1 MV cm−1. Here we report a non-equilibrium phase transition driven by a weak (≪1 V cm−1), continuous-wave terahertz electric field. The system consists of room temperature caesium vapour under continuous optical excitation to a high-lying Rydberg state, which is resonantly coupled to a nearby level by the terahertz electric field. We use a simple model to understand the underlying physical behaviour, and we demonstrate two protocols to exploit the phase transition as a narrowband terahertz detector: the first with a fast (20 μs) non-linear response to nano-Watts of incident radiation, and the second with a linearised response and effective noise equivalent power ≤1 pW Hz−1/2. The work opens the door to a class of terahertz devices controlled with low-field intensities and operating in a room temperature environment.

DOI
10.1038/s41467-018-05597-4

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Abstract There are few demonstrated examples of phase transitions that may be driven directly by terahertz frequency electric fields, and those that are known require field strengths exceeding 1 MV cm−1. Here we report a non-equilibrium phase transition driven by a weak (≪1 V cm−1), continuous-wave terahertz electric field. The system consists of room temperature caesium vapour under continuous optical excitation to a high-lying Rydberg state, which is resonantly coupled to a nearby level by the terahertz electric field. We use a simple model to understand the underlying physical behaviour, and we demonstrate two protocols to exploit the phase transition as a narrowband terahertz detector: the first with a fast (20 μs) non-linear response to nano-Watts of incident radiation, and the second with a linearised response and effective noise equivalent power ≤1 pW Hz−1/2. The work opens the door to a class of terahertz devices controlled with low-field intensities and operating in a room temperature environment.
Single-photon terahertz (THz) detection is one of the most demanding technologies for a variety of fields and could lead to many breakthroughs. Although significant progress has been made in the past two decades, operating it at room temperature still remains a great challenge. Here, we demonstrate, for the first time, a room temperature THz detector at single-photon levels based on nonlinear wave mixing in thermal Rydberg atomic vapor. The low-energy THz photons are coherently upconverted to high-energy optical photons via a nondegenerate Rydberg state involved in a six-wave mixing process, and therefore, single-photon THz detection is achieved by a conventional optical single-photon counting module. The noise equivalent power of such a detector reaches 9.5 × 10−19 W/Hz1/2, which is more than four orders of magnitude lower than the state-of-the-art room temperature THz detectors. The optimum quantum efficiency of the whole-wave mixing process is about 4.3%, with 40.6 dB dynamic range, and the maximum conversion bandwidth is 172 MHz, which is all-optically controllable. The developed fast and continuous-wave single-photon THz detector at room temperature operation has a great potential for portability and chip-scale integration, and could be revolutionary for a wide range of applications in remote sensing, wireless communication, biomedical diagnostics, and quantum optics.
Abstract 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 μV⋅cm −1 ⋅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.
Mode competition in nonequilibrium Rydberg gases enables the exploration of emergent many-body phases. This work leverages this emergent phase for electric field detection at room temperature. Sensitive frequency-resolved electric field measurements at very low-frequencies (VLF) are of central importance in a wide range of applications where deep-penetration is required in communications, navigation and imaging or surveying. The long wavelengths on order of 10–100 km (3–30 kHz) limit the efficiency, sensitivity, and bandwidth of compact classical detectors that are constrained by Chu’s limit. Rydberg-atom electrometers are an attractive approach for microwave electric-field sensors but have reduced sensitivity at lower-frequencies. Very recent efforts to advance the standard Rydberg-atoms approach is based on DC electric-field (E-field) Stark shifting and have resulted in sensitivities between 67.9 and 2.2 uVcm -1 Hz -1/2 (0.1–10 kHz) by fine optimization of the DC E-field. A major challenge in these approaches is the need for embedded electrodes or plates due to DC E-field Stark screening effect, which can perturb coupling of VLF signals when injected from external sources. In this article, it is demonstrated that state-of-art sensitivity ((1.60 ± 0.23) uVcm -1 Hz -1/2 ) can instead be achieved using limit-cycle oscillations in driven-dissipative Rydberg atoms by using a magnetic field (B-field) to develop mode-competition between nearby Rydberg states. The mode-competition between nearby Rydberg-states develop an effective transition centered at the oscillation frequency capable of supporting external VLF E-field coupling in the ~ 10–15kHz regime without the requirement for fine optimization of the B-field magnitude.
The bandwidth and sensitivity of traditional terahertz detection devices are limited by material properties and working temperature. Here, we proposed a four-level Rydberg atom detection method, which can realize real-time detection for the absolute strength of the terahertz electric field under room temperature with the detection bandwidth of 0-5 THz. By theoretical analysis and formula derivation, the relation among the terahertz electric field strength, excitation energy level, coupling photoelectric field strength, gas cell temperature and electromagnetically induced transparency (EIT) signal are studied in detailed. The NEP of this method is proven to be 10−17W/Hz1/2in the range of 0-1 THz, which is three orders of magnitude smaller than that of the traditional optimal thermal detection system. This research has important reference value for the later high-sensitivity detection and application development of terahertz waves
This thesis investigates the interaction between Rydberg atoms and THz radiation and details development of an atom-based imaging system for THz frequency fields. Room temperature caesium atoms are excited to a Rydberg state using resonant infrared lasers and the response of these atoms to THz radiation is studied in multiple ways. We look at the absorption of the initial excitation laser and show how the observed lineshape changes in response to an applied THz field. We use this to measure the power of a THz field across a range of 350 GHz. These measurements also lead us to determine values for the quantum defects in caesium without reliance on data from any other sources. These values agree within error with those previously reported in literature. We study the optical fluorescence emitted by the laser-excited \nvapour and develop a model to predict this emission. We highlight deviations from this model and suggest possible mechanisms for this discrepancy. We show how this model can be used to determine the optimum transition for atom-based THz imaging within our experimental parameters. We demonstrate a high-speed THz imaging system using Rydberg atoms and show that it has near diffraction-limited \nspatial resolution and is able to capture images at kHz framerates. We measure the minimum THz power that the system can detect and show how it compares to other technologies. This system is then used to perform some proof-of-principle experiments to highlight its usefulness in commercial settings. Using optical elements \ndesigned and produced in-house we create and characterise THz vortex beams with both radial and azimuthal phase. We show how these beams are similar to Laguerre-Gauss modes and highlight some important differences.
Abstract The coupling of microwave and optical systems presents an immense challenge due to the natural incompatibility of energies, but potential applications range from optical interconnects for quantum computers to next-generation quantum microwave sensors, detectors and coherent imagers. Several of the engineered platforms that have emerged are constrained by specific conditions, such as cryogenic environments, impulse protocols or narrowband fields. Here we employ Rydberg atoms that allow the wideband coupling of optical and microwave photons at room temperature with the use of a modest set-up. We present continuous-wave conversion of a 13.9 GHz field to a near-infrared optical signal using an ensemble of Rydberg atoms via a free-space six-wave mixing process designed to minimize noise interference from any nearby frequencies. The Rydberg photonic converter exhibits a conversion dynamic range of 57 dB and a wide conversion bandwidth of 16 MHz. Using photon counting, we demonstrate the readout of photons of free-space 300 K thermal background radiation at 1.59 nV cm −1 rad −1/2 s −1/2 (3.98 nV cm −1 Hz −1/2 ) with a sensitivity down to 3.8 K of noise-equivalent temperature, allowing us to observe Hanbury Brown and Twiss interference of microwave photons.