Terahertz-Driven Phase Transition in a Hot Rydberg Vapour

Christopher G. Wade

Alicja Antczak, Aaditya Mishra, Miguel Navarro‐Cía +1 more

Christopher G. Wade, Matteo Marcuzzi, Emanuele Levi +4 more

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.

Christopher G. Wade, Nikola Šibalić, Kevin J. Weatherill +1 more

Zongkai Liu, Qiming Ren, Chris Nill +12 more

Interacting Rydberg atoms constitute a versatile platform for the realization of non-equilibrium states of matter. Close to phase transitions, they respond collectively to external perturbations, which can be harnessed for technological applications in the domain of quantum metrology and sensing. Owing to the controllable complexity and straightforward interpretability of Rydberg atoms, we can observe and tune the emergent collective amplification. Here, we investigate the application of an interacting Rydberg vapour for the purpose of time series prediction. The vapour is driven by a laser field whose Rabi frequency is modulated in order to input the time series. We find that close to a non-equilibrium phase transition, where collective effects are amplified, the capability of the system to learn the input becomes enhanced. This is reflected in an increase of the accuracy with which future values of the time series can be predicted. Using the Lorenz time series and temperature data as examples, our work demonstrates how emergent phenomena enhance the capability of noisy many-body systems for data processing and forecasting.

Daniel R Häupl

Having a strong interaction between hot alkali vapour and light is important for Rydberg physics and nonlinear optics. In optical fibres, the tight confinement by the core offers high laser intensities for even moderate powers. With the invention of hollow-core photonic crystal fibres (HC-PCFs), the interaction of alkali atoms with high intensities over long distances could be studied. In the past, setups combining alkali atoms and a HC-PCF have suffered from long waiting times until atoms are evenly distributed along the fibre. The large size of the used vacuum chambers entailed technical challenges when trying to heat the system uniformly. In this thesis, an all-glass vapour cell incorporating a hollow-core fibre is presented. Its compact size allows fast and precise control of the temperature. Measurements have shown that this cell drastically reduces the time required for equilibration to only hours. Furthermore, the cell’s all-glass design permits unobstructed optical access from all sides. The optical access can be used to record the fluorescence emitted by atoms along the inside of the fibre core. In a first experiment, the frequency-dependent fluorescence along the fibre position was monitored while alkali vapour was diffusing out of the fibre. The atomic density was successfully retrieved using a numerical model based on the optical Bloch equations. Promising technical improvements of the cell design using fibre tapers are discussed, which could lead to an all-fibre vapour cell with fibre-integrated light delivery. The high laser intensity inside a HC-PCF can easily exceed the linear spectroscopy regime. Staying below the saturation intensity inside a waveguide might not always be easy—or even desired. However, established numerical software packages, such as ElecSus, are limited to the weak-probe regime. For that reason, an open-source software based on the Lindblad master equation was developed in this work. The software allows predicting the absolute value of the electric susceptibility for many experimental parameters of hot alkali vapours, including the isotope ratio, temperature or magnetic field strength. Most distinctly, it includes the laser intensity and effects such as optical pumping and transit-time broadening. To verify the model accuracy, spectra of 87Rb-filled cells were measured in a magnetic field of 0.6 T. Entering the hyperfine Paschen-Back regime, individual atomic resonances can be addressed, allowing further benchmarking of the model based on open and closed transitions. Comparing the experimental data with simulated spectra, very good agreement for three different beam diameters and for laser intensities over a range of more than five orders of magnitude was found. Finally, Rydberg states of rubidium—with their multitude of possible transitions—are discussed as a possible source of light in the terahertz (THz) region. In contrast to some other existing sources, THz from hot atomic vapour is expected to be very narrowband. In one report, the successful generation of THz in a four-level-system in hot rubidium vapour was observed. Here, a very similar setup was employed, but unfortunately no THz emission was detected. It is unclear if the reason is of fundamental or experimental nature. One potential route to enhance the THz signal strength at the detector is to channel the signal as much as possible by guiding it inside a waveguide. Appropriate waveguides were investigated, which in contrast to optical frequencies use polymers as a material. Using polymers, waveguides using the 3D printing technique were fabricated and guidance of THz light was observed.