Quantum technologies are evolving at a rapid pace and rely on the understanding and control of quantum systems. By obtaining high levels of control, quantum sensors such as interferometers and clocks have seen their performance improved by many orders of magnitude, quantum computation is achieving ever higher fidelities, and quantum simulators are able to mimic a wide range of systems whose evolution cannot realistically be simulated on classical computers. The use of collective interactions in optical cavities has seen several new developments in recent years. It is becoming an important tool for quantum simulators and is pushing the development of ultra-stable active optical clocks based on superradiant lasers. In the Dark Cavities project the aim is to use collective interactions to push the boundaries of these systems.
Neutral atoms represent a platform in which interaction with a two-level system, or qubit, can be enhanced and modified e.g. by an optical resonator. This has made resonators a ubiquitous tool across a wide range of platforms spanning quantum dots, superconducting qubits, NV-centres, and atomic interfaces. The resonators allow photon-mediated couplings between all constituents of the ensemble, which can result in emergent behaviour and can be used to study phase transitions to sub- or superradiant behaviour such as superradiant lasing or fast quantum state readout. Retaining memory in the quantum system or improving its coherence remains a challenge. The presence of multiple energy levels increases the degrees of freedom in these systems and their manipulation allows for more exotic behaviour. The combination of infinite-range interactions mediated by an optical cavity and such multi-level structures has so far received very little attention. The overall objective of this project is to generate and characterize controllable and long-lived collective atomic dark states in large ensembles of atoms that can be used to improve the performance of superradiant lasers and atomic clocks or as memory and spin-interaction in quantum simulators. I will exploit these dark states to generate coherence controlled superradiant lasing and to protect narrow atomic states against decay and prolong their lifetime. To realize the overall objective of the project I will need to push the state of the art forward in two essential ways.
A) Coherence time of superradiant lasing – By controlling the magnetic field seen by an ensemble of cold strontium atoms coupled to an optical resonator, I can generate a dark superposition state which allows dynamic control of the superradiant lasing coherence time. The realization of such a system will strengthen the performance of superradiant lasers as active atomic clocks.
B) Extend the lifetime of excited collective atomic states – Collective dark states can emerge from quantum interference of decay channels in a multi-level atomic system. I will use the qudit nature of atomic strontium to gain access to these collective states and extend the lifetime of the atoms. This allows the dynamic generation of dark or bright states, which is of interest for the protection of atomic states, realizing spin squeezing protocols demonstrating controlled quantum memories or investigating the competition between long- and short-range interactions in atomic ensembles. The use of qudit systems with tens of energy levels will open the path to much larger many-level systems with hundreds of levels, necessary for computation of gauge theories.
