Observation of parity-time symmetry breaking transitions in a dissipative Floquet system of ultracold atoms (1608.05061v2)

Abstract: Open physical systems with balanced loss and gain, described by non-Hermitian parity-time ($\mathcal{PT}$) reflection symmetric Hamiltonians, exhibit a transition which could engenders modes that exponentially decay or grow with time and thus spontaneously breaks the $\mathcal{PT}$-symmetry. Such $\mathcal{PT}$-symmetry breaking transitions have attracted many interests because of their extraordinary behaviors and functionalities absent in closed systems. Here we report on the observation of $\mathcal{PT}$-symmetry breaking transitions by engineering time-periodic dissipation and coupling, which are realized through state-dependent atom loss in an optical dipole trap of ultracold $^6$Li atoms. Comparing with a single transition appearing for static dissipation, the time-periodic counterpart undergoes $\mathcal{PT}$-symmetry breaking and restoring transitions at vanishingly small dissipation strength in both single and multiphoton transition domains, revealing rich phase structures associated to a Floquet open system. The results enable ultracold atoms to be a versatile tool for studying $\mathcal{PT}$-symmetric quantum systems.

• The paper experimentally realizes PT-symmetry breaking transitions in ultracold lithium atoms using tailored state-dependent dissipation and RF coupling.
• The paper employs Floquet engineering to induce time-periodic modulations that reveal multiphoton resonances and complex phase transitions at minimal dissipation levels.
• The paper establishes a robust platform for non-Hermitian physics, paving the way for future studies on topological effects and many-body quantum dynamics.

Observation of Parity-Time Symmetry Breaking Transitions in a Dissipative Floquet System of Ultracold Atoms

This paper presents an in-depth paper of parity-time ($\mathcal{PT}$) symmetry breaking transitions within a system of ultracold lithium ($^6$Li) atoms confined in an optical dipole trap. The investigation focuses on a novel approach to engineering $\mathcal{PT}$-symmetric Hamiltonians, which have traditionally been a subject of interest due to their non-Hermitian yet real-valued eigenvalue spectra under certain conditions. The authors explore the dynamics of these transitions in the presence of time-periodic dissipation, thus contributing significantly to the experimental realization of complex quantum phenomena previously limited to theoretical explorations.

Key Contributions

The authors' primary contribution is the realization of $\mathcal{PT}$-symmetry breaking in an experimentally accessible open quantum system. By leveraging the interaction between hyperfine spin states in ultracold $^6$Li, together with controlled state-dependent dissipation, they engineer a $\mathcal{PT}$-symmetric environment. The coupling between these states is maintained using radio-frequency (RF) fields, and dissipation is induced using optical light resonant with specific atomic transitions. Such a set-up provides a versatile platform to scrutinize the intriguing properties of $\mathcal{PT}$-symmetric physics.

Methodology and Results

The researchers utilize Floquet engineering, a method involving time-periodic modulation of system parameters, to explore transitions in $\mathcal{PT}$-symmetry. Through experiments with both static dissipation and time-periodic coupling, they observe $\mathcal{PT}$-symmetry breaking transitions at exceptionally small dissipation strengths. Furthermore, these transitions manifest complex phase structures typical of Floquet systems.

For static dissipation, the breakdown of $\mathcal{PT}$ symmetry is indicated by a transition point at which eigenvalues of the system's Hamiltonian shift from real to complex. The authors note that while earlier studies connected static dissipation with quantum Zeno effects, this research delineates the distinct regime of $\mathcal{PT}$ symmetry breaking.

In the context of time-periodic modulations, experimental observations reveal a rich phase diagram. Notably, they demonstrate that the $\mathcal{PT}$ symmetry breaking and restoration transitions can occur even with minimal dissipation strength, and they document the existence of multiphoton resonances. These resonances, characterized by their power broadening effects, offer new insights into the complex dynamics of non-Hermitian systems.

Implications and Future Directions

This work carries significant implications for future studies in open quantum systems and non-Hermitian physics. First, ultracold atoms emerge as a powerful tool for manipulating $\mathcal{PT}$-symmetric Hamiltonians, paving the way for experimental exploration of theoretical models. Second, this system's flexibility allows for future inquiries into topics such as topological properties associated with exceptional points or dynamics in strongly interacting systems beyond the noninteracting regime detailed here.

Looking forward, the research opens avenues to integrate and explore interparticle interactions’ effects within the $\mathcal{PT}$ symmetry framework. Such investigations, especially into how such transitions behave under varying interaction strengths (e.g., BEC-BCS crossover), could contribute to a deeper understanding of many-body physics under open quantum conditions.

In conclusion, this paper not only marks significant experimental progress in understanding $\mathcal{PT}$-symmetric systems but also sets the stage for future explorations that could harness their distinctive properties for potentially novel quantum technologies.