Cavity Mediated Collective Spin Exchange Interactions in a Strontium Superradiant Laser (1711.03673v1)

Abstract: Laser cooled and quantum degenerate atoms are widely being pursued as quantum simulators that may explain the behavior of strongly correlated material systems, and as the basis of today's most precise sensors. A key challenge towards these goals is to understand and control coherent interactions between the atoms. Here, we observe long-range exchange interactions mediated by an optical cavity, which manifest as tunable spin-spin interactions on the pseudo spin-1/2 system composed of the millihertz linewidth clock transition in strontium. We observe the so-called one axis twisting dynamics, the emergence of a many-body energy gap, and signatures of gap protection of the optical coherence against certain sources of decoherence. These effects manifest in the output of a pulsed, superradiant laser operating on the millihertz linewidth transition. Our observations will aid in the future design of versatile quantum simulators that take advantage of the unique control and probing capabilities of cavity QED and the rich internal structure of long-lived Sr atoms. They also open a route for the next generation of atomic clocks that utilize quantum correlations for enhanced metrology.

Cavity Mediated Collective Spin Exchange Interactions in a Strontium Superradiant Laser

The research paper "Cavity Mediated Collective Spin Exchange Interactions in a Strontium Superradiant Laser" presents an exploration of cavity-mediated spin-exchange interactions in a strontium-based atomic system. This paper underscores the utilization of strontium atoms, boasting ultra-long-lived optical dipoles, within a quantum simulation framework that highlights collective spin dynamics primarily driven by an optical cavity. The coupling of atoms to the cavity mode facilitates exchange interactions, which are realized between effective spins encoded in the ground and excited states of the millihertz transition in strontium. In particular, the paper focuses on the execution and detection of one-axis twisting dynamics (OAT) and the emergence of many-body energy gaps, which offer protection against sources of atomic decoherence.

In their experimental setup, Norcia et al. perform collective spin manipulations that invoke an XX-Heisenberg model—all within the confines of cavity quantum electrodynamics (QED). The approach leverages the significant impact of exchanging cavity photons, which entails spin flips in the atoms interacting within the cavity. The authors’ choice of $^{87}$Sr forms a robust basis because of its narrow optical clock transition linewidth, offering a propitious base for high precision atomic clocks and superradiant lasers.

Key findings in this research demonstrate how atom-cavity interactions can lead to inversion-dependent frequency shifts and support a robust theoretical framework that explains many-body spin dynamics. Norcia et al. utilized a coherent state preparation to establish tunable inversion levels and showed that the superradiant laser frequency is significantly influenced by these dynamics. Notably, the one-axis twisting observed as a spin precession is dependent on inversion and cavity detuning, aligning with predictions from the effective Hamiltonian derived in their work.

One compelling result of this paper is the elucidation of the many-body energy gap's protective powers against single-particle dephasing. The manipulation of atomic coherence through controlled dephasing and rephasing within the cavity indicates how populations can transition between states of different symmetry. This transition results in a measurable discrepancy in frequency between ‘bright’ and ‘dark’ atomic states, thereby providing direct spectroscopic insight into the energetics imparted by the collective spin Hamiltonian.

The work of Norcia et al. extends beyond theoretical exploration, offering significant implications for the development of atomic clocks that exceed current precision limits by capitalizing on quantum correlations. The coupling of strontium atoms to a cavity mode establishes a pathway for quantum simulation of long-range spin models, drawing upon strong photon-mediated interactions. Importantly, these findings, by interlinking collective atomic dynamics with cavity QED, suggest enhanced possibilities for metrology through the achievable control of quantum entanglement and the accuracy of phase measurement in novel timekeeping systems.

Overall, by marrying the detailed paper of spin-exchange interactions with the emergent properties of atom-cavity systems, the research contributes to both the theoretical and practical advancements in atomic clocks and precision measurement devices. Future directions may involve extending this robust control of quantum states to even broader classes of quantum many-body systems, thereby enhancing our understanding of quantum correlations and their practical applications in the evolving field of quantum metrology and simulation.