A Thouless Quantum Pump with Ultracold Bosonic Atoms in an Optical Superlattice (1507.02225v1)

Abstract: More than 30 years ago, Thouless introduced the concept of a topological charge pump that would enable the robust transport of charge through an adiabatic cyclic evolution of the underlying Hamiltonian. In contrast to classical transport, the transported charge was shown to be quantized and purely determined by the topology of the pump cycle, making it robust to perturbations. On a fundamental level, the quantized charge transport can be connected to a topological invariant, the Chern number, first introduced in the context of the integer quantum Hall effect. A Thouless quantum pump may therefore be regarded as a 'dynamical' version of the integer quantum Hall effect. Here, we report on the realization of such a topological charge pump using ultracold bosonic atoms that form a Mott insulator in a dynamically controlled optical superlattice potential. By taking in-situ images of the atom cloud, we observe a quantized deflection per pump cycle. We reveal the genuine quantum nature of the pump by showing that, in contrast to ground state particles, a counterintuitive reversed deflection occurs when particles are prepared in the first excited band. Furthermore, we were able to directly demonstrate that the system undergoes a controlled topological phase transition in higher bands when tuning the superlattice parameters.

• The paper demonstrates the experimental realization of quantized particle transport using a Mott insulator of 87Rb atoms in a dynamically controlled optical superlattice.
• It reveals that atoms in the first excited band show a reversed deflection, highlighting the uniquely quantum nature of Berry curvature-driven anomalous velocities.
• It establishes a versatile platform for exploring topological phase transitions, linking measured center-of-mass displacement directly to the Chern number.

An Examination of a Thouless Quantum Pump with Ultracold Bosonic Atoms in an Optical Superlattice

The paper presents a significant advancement in the experimental realization of a Thouless quantum pump using ultracold bosonic atoms in an optical superlattice. Proposed over three decades ago, the Thouless quantum pump enables the quantized transport of particles through an adiabatic cyclic evolution of the Hamiltonian, derived from a topological invariant known as the Chern number. This work successfully demonstrates the underlying quantum mechanical principle enabling the robust transport of bosons without classical analogs, providing a dynamic version of the integer quantum Hall effect.

Experimental Realization and Observations

The experiment utilized a Mott insulator of ultracold $^{87}$Rb atoms in a dynamically controlled optical superlattice to observe quantized particle transport. Through in-situ imaging, the researchers confirmed a quantized deflection per pump cycle, which varied depending on the energy band the atoms were prepared in. Notably, the findings revealed a counterintuitive reversed deflection when particles were excited into the first excited band, thereby illustrating the distinctly quantum nature of the pump process.

The potential for controlled topological phase transitions enabled by varying the superlattice parameters was also demonstrated, indicating that adjustments in these parameters could transition the system between different topological phases in higher energy bands. This feature allows for exploration of different topological regimes, establishing the platform as a versatile tool for topological quantum simulation.

Theoretical Underpinning

Ultracold atoms in optical superlattices, formed by superimposing two lattices with different periodicities, provide a versatile system for implementing topological charge pumps. The cyclic adiabatic variation of the superlattice phase $\varphi$ traverses a closed trajectory in the parameter space, reminiscent of encircling a degeneracy point that leads to the transport characterized by a Chern number. This reveals the relationship between 1D quantum pumping and the 2D integer quantum Hall effect, where $\varphi$ acts akin to threading a magnetic flux, inducing quantized motion orthogonally to the field.

The paper elucidates the origin of this topological nature through a transition from a quantum sliding lattice limit to a Wannier tunneling limit. This underpins the observed phenomena and emphasizes that such quantized transport emerges from the fundamentally quantum mechanical effect of Berry curvature-driven anomalous velocities, rather than from classical forces.

Implications and Future Prospects

The authors have shown that the measured center-of-mass displacement of the atom cloud is directly linked to the Chern number, a result that profoundly underscores the robustness of the topological transport against various perturbations such as interaction effects, disorder, and non-adiabatic transitions. This makes the system an ideal candidate for future explorations of topological physics using quantum simulations.

The implications of this research are vast, paving the way for investigating topological phenomena, quantum nonequilibrium states, and developing new quantum technologies. Future endeavors may include the development of higher-dimensional topological pumps, exploration of Z_2 spin pumps by introducing internal degrees of freedom, and simulating more complex topological models like the four-dimensional quantum Hall effect. The methods used here offer new avenues for direct observation and manipulation of edge states, potentially revolutionizing our understanding of topological insulators and superfluids, along with their practical applications in quantum computing and beyond.

In conclusion, this paper presents a detailed and compelling experimental realization of a concept that bridges the fundamental principles of quantum mechanics and topological invariants, offering a promising platform for future research in quantum technologies. The quantized nature and the manipulation of topological phase transitions demonstrated here may hold the key to developing robust quantum systems resistant to environmental noise and imperfections.