NEW: We are recruiting - two vacancies are available for PDRAs to start Feb. 2019. Please contact Matt Jones for more details.
Laser cooling and laser spectroscopy are powerful techniques for controlling the motion and quantum state of individual atoms. In this project we aim to extend this level of control to the interactions between the atoms. By exciting laser-cooled strontium atoms to a very high lying electronic energy state - known as a Rydberg state, we can switch on strong, long-range van der Waals interactions between the atoms, which completely dominate the behaviour of the atom cloud. Uniquely, our experiment uses strontium atoms, which also have a second valence electron that can be used to probe and manipulate the Rydberg gas. So far, we have shown that this can provide information on collisions that can transfer atoms from one Rydberg state to another, with very high temporal resolution. We have also extended these techniques to provide spatial information, enabling us to probe spatial correlations in this strongly interacting system. We have most recently coupled the excited state of a strontium narrow-line MOT to a high-lying Rydberg state, known as Rydberg dressing, to create a many-body system with long-range interactions and active cooling.
Current Research Directions:
A promising approach to control both interacting and dissipative properties of a Rydberg gas is to off-resonantly couple the atomic ground state to a Rydberg state, allowing to admix some interacting Rydberg character to a long lived state. We are currently using a novel dressing scheme where we dress the upper state of the spin forbidden 5s2 1S0 → 5s5p 3P1 narrow-line MOT transition to a high lying Rydberg state. Unlike previous Rydberg-dressing experiments, this upper state is dissipative which facilitates active cooling rather than just conservative trapping of a Rydberg-dressed ensemble. We allow atoms to acquire the special properties of Rydberg states, i.e. sensitivity to DC electric field and long-range dipolar interactions, whilst still being laser-cooled on the closed MOT transition. Active cooling of a Rydberg dressed gas remains a relatively unexplored area and one could expect interesting physics to arise from the presence of cooling and mechanical effects of the interactions.
We have recently developed a Monte-Carlo simulation to gain further insights into narrow-line MOT's, both in the presence and absence of the dressing laser. This model is able to quantitatively reproduce the spatial, temporal and thermal dynamics of narrow-line MOT's. The figure below is from our latest arXiv submission and shows excellent agreement between the experimental (top row) and theoretical (bottom row) absorption images for a variety of dressing laser detunings.
Narrow line Rydberg spectroscopy
Most Rydberg spectroscopy in strontium has been performed on singlet Rydberg states that are relatively accessible with blue wavelengths (461nm and 413nm) and a broad intermediate transition (30.2MHz). We are currently exploring previously unobserved triplet Rydberg states accessible via the 7.5kHz spin-forbidden 5s5p 3P1 state, using 689nm and 319nm lasers, both frequency stabilised to better than 40kHz. These states offer a wide range of properties, with isotropic, anisotropic, attractive and repulsive Rydberg interactions.
Using a femtosecond optical frequency comb to measure energy levels to better than 50kHz absolute precision, observing coupling strength using Autler Townes spectroscopy, measuring the polarizability of the Rydberg states, and using autoionisation spectroscopy of the Rydberg states as a measure of both quantum defects and blackbody transfer rates, we can perform very detailed high resolution spectroscopy of a range of Rydberg states.
Spin squeezing in Rydberg lattice clocks
Atomic clocks based on an extremely narrow optical transition in strontium are currently leading the field in stability and accuracy, enabling frequency measurements at the 10-18 level. In an ongoing collaboration with the group of Thomas Pohl at the Max Planck Institute for Complex Systems in Dresden, we are investigating the use of Rydberg states to improve the performance of the clock even further.
This work was supported by the EU STREP HAIRS, a network of researchers from Durham, Nottingham, Tubingen, Dresden and Palaiseau with the aim of using these techniques to create a hybrid quantum system of atoms and superconducting circuits.
The figure on the right is from our publication where we propose to create very strongly squeezed states in a lattice clock using Rydberg dressing, with implications for quantum-enhanced frequency metrology.