Christopher G. Wade

Nonequilibrium phase transition in a dilute Rydberg ensemble.

We demonstrate a nonequilibrium phase transition in a dilute thermal atomic gas. The phase transition, between states of low and high Rydberg occupancy, is induced by resonant dipole-dipole interactions between Rydberg atoms. The gas can be considered as dilute as the atoms are separated by distances much greater than the wavelength of the optical transitions used to excite them. In the frequency domain, we observe a mean-field shift of the Rydberg state which results in intrinsic optical bistability above a critical Rydberg number density. In the time domain, we observe critical slowing down where the recovery time to system perturbations diverges with critical exponent α=-0.53±0.10. The atomic emission spectrum of the phase with high Rydberg occupancy provides evidence for a superradiant cascade.

Real-time near-field terahertz imaging with atomic optical fluorescence

A time-averaged intensity distribution of terahertz waves is imaged by converting terahertz waves to optical fluorescence. The conversion becomes possible by exciting Cs atoms to a Rydberg state. The image acquisition time is 40 ms. Terahertz (THz) near-field imaging is a flourishing discipline1,2, with applications from fundamental studies of beam propagation3 to the characterization of metamaterials4,5 and waveguides6,7. Beating the diffraction limit typically involves rastering structures or detectors with length scale shorter than the radiation wavelength; in the THz domain this has been achieved using a number of techniques including scattering tips8,9 and apertures10. Alternatively, mapping THz fields onto an optical wavelength and imaging the visible light removes the requirement for scanning a local probe, speeding up image collection times11,12. Here, we report THz-to-optical conversion using a gas of highly excited Rydberg atoms. By collecting THz-induced optical fluorescence we demonstrate a real-time image of a THz standing wave and use well-known atomic properties to calibrate the THz field strength.

Driven-dissipative many-body systems with mixed power-law interactions: Bistabilities and temperature-driven nonequilibrium phase transitions

We investigate the nonequilibrium dynamics of a driven-dissipative spin ensemble with competing power-law interactions. We demonstrate that dynamical phase transitions as well as bistabilities can emerge for asymptotic van der Waals interactions, but critically rely on the presence of a slower decaying potential core. Upon introducing random particle motion, we show that a finite gas temperature can drive a phase transition with regards to the spin degree of freedom and eventually leads to mean-field behavior in the high-temperature limit. Our work reconciles contrasting observations of recent experiments with Rydberg atoms in the cold-gas and hot-vapor domain, and introduces an efficient theoretical framework in the latter regime.

Intrinsic optical bistability in a strongly driven Rydberg ensemble.

We observe and characterize intrinsic optical bistability in a dilute Rydberg vapor. The bistability is characterized by sharp jumps between states of low and high Rydberg occupancy with jump-up and -down positions displaying hysteresis depending on the direction in which the control parameter is changed. We find that the shift in frequency of the jump point scales with the fourth power of the principal quantum number. Also, the width of the hysteresis window increases with increasing principal quantum number, before reaching a peak and then closing again. The experimental results are consistent with predictions from a simple theoretical model based on semiclassical Maxwell--Bloch equations including the effects of interaction-induced broadening and level shifts. These results provide insight into the dynamics of driven dissipative systems.

Piezoelectrically actuated time-averaged atomic microtraps

We present a scheme for creating tight and adiabatic time-averaged atom-traps through the piezoelectric actuation of nanomagnetic structures. We show that potentials formed by the circular translation of magnetic structures have several advantages over conventional rotating-field techniques, particularly for high trap frequencies. As the magnitude of the actuation is changed, the trapping potential can be changed adiabatically between harmonic 3D confinement and a toroidal trap.

Probing an excited-state atomic transition using hyperfine quantum-beat spectroscopy.

We describe a method to observe the dynamics of an excited-state transition in a room-temperature atomic vapor using hyperfine quantum beats. Our experiment using cesium atoms consists of a pulsed excitation of the D2 transition and continuous-wave driving of an excited-state transition from the 6P3/2 state to the 7S1/2 state. We observe quantum beats in the fluorescence from the 6P3/2 state which are modified by the driving of the excited-state transition. The Fourier spectrum of the beat signal yields evidence of Autler-Townes splitting of the 6P3/2, F=5 hyperfine level and Rabi oscillations on the excited-state transition. A detailed model provides qualitative agreement with the data, giving insight to the physical processes involved.

Observation of interference effects via four-photon excitation of highly excited Rydberg states in thermal cesium vapor

We report on the observation of electromagnetically induced transparency (EIT) and absorption (EIA) of highly excited Rydberg states in thermal Cs vapor using a four-step excitation scheme. The advantage of this four-step scheme is that the final transition to the Rydberg state has a large dipole moment and one can achieve similar Rabi frequencies to two- or three-step excitation schemes using two orders of magnitude less laser power. This scheme enables new applications such as dephasing free Rydberg excitation. The observed lineshapes are in good agreement with simulations based on multilevel optical Bloch equations.

Terahertz Electrometry with Rydberg EIT

We use three-photon Rydberg electromagnetically induced transparency (EIT) to perform Rydberg electrometry at 0.634 THz.

Terahertz-Driven Phase Transition in a Hot Rydberg Vapour

We use a weak terahertz-frequency field (\(I_{\mathrm{{T}}} \ll 1\) Wm\(^{-2}\)) to drive a non-equilibrium phase transition in a hot caesium Rydberg vapour. We measure a phase diagram in the parameter space of terahertz field and laser detuning, and we find a linear shift of the critical laser detuning with coefficient \(-179\pm 2\) MHzW\(^{-1}\)m\(^{-2}\). Considering the system as a terahertz detector we calculate sensitivity, \(S \approx 90\) \(\upmu \)Wm\(^{2}\)Hz\(^{-1/2}\). The phase transition is accompanied by hysteresis and bistability, allowing the system to act as a latch controlled by a 1 ms terahertz pulse with energy of order 10 fJ.

Probing an Excited State Transition Using Quantum Beats

We observe the dynamics of an excited-state transition in a room temperature atomic vapour using hyperfine quantum beats. The experiment consists of a pulsed excitation of the caesium D\(_2\) transition, and continuous-wave driving of an excited-state transition from the 6P\(_{3/2}\) state to the 7S\(_{1/2}\) state.