Quantum coherence induced correction to Thouless pumping

30 May 2018. NUS physicists have discovered and experimentally shown that quantum coherence effects can be used to control particle transport in lattice structures.Thousands of years ago, the Archimedes screw emerged as a superb design to pump water from a low-lying body into irrigation ditches, and is believed to have been of critical historical importance to the ancient world. Similar to the Archimedes screw, the Thouless pump (proposed by theoretical physicist DJ THOULESS, Physics Nobel Laureate 2016), transports subatomic particles (such as electrons) by slowly and periodically changing some parameters of a lattice structure that hosts the particles. Thouless discovered that the number of particles transported over an arbitrary cycle is always given by the same integer multiple of some basic unit. As this integer is connected with the underlying topology invariant of the physical system, the Thouless pump provides a way to mathematically connect the characteristic of the underlying topology of quantum states with observed physical phenomena.

In 2015, a research team led by Prof GONG Jiangbin from the Department of Physics, NUS discovered that quantum coherence effects can control the behaviour of a Thouless pump such that the average amount of pumped matter per cycle need not be quantised but becomes continuously tuneable. Quantum coherence concerns the wave-like behaviour of matter and refers to the definite phase relationship between the different components of quantum states that comprise a superposition (resulting in, for example, the constructive or destructive interference of waves). In collaboration with a team from University of Science and Technology of China, Prof Gong’s research team has now experimentally confirmed the theory and demonstrated that the rate at which the pump is switched on can be used to extensively control the amount of transported matter.

Prof Gong said, “The experimental results have verified one fascinating aspect of the Thouless pump which we first discovered in 2015. This has advanced our fundamental understanding of a quantum pump. We plan to further explore the use of quantum coherence effects as a resource for controlling quantum transport.”

Schematic of Thouless pumping in the absence (left) and presence (right) of inter-band coherence effects. Q_a and Q_b represent pumping effects contributed by individual bands, and Q_IBC represents the inter-band coherence induced correction to the conventional Thouless pumping. (Left) For an initial state being an incoherent mixture of states from different energy bands, the population correction factor is of the order of 1/T². The pumping outcome Q_a+Q_b is the sum of the contribution from each band. (Right) For an initial state having quantum coherence, the pumping operation induces a population correction factor of the order of 1/T. This is reflected as an additional term Q_IBC that is continuously tunable.

References

MW Ma; LW Zhou; Q Zhang; M Li; CY Cheng; JP Geng; X Rong; FZ Shi*; JB Gong*; JF Du*, “Experimental observation of a generalized Thouless pump with a single spin” PHYSICAL REVIEW LETTERS Volume: 120 Issue: 12 Article Number: 120501 DOI: 10.1103/PhysRevLett.120.120501 Published: 2018.

HL Wang, LW Zhou, and JB Gong*, “Interband coherence induced correction to adiabatic pumping in periodically driven systems”, PHYSICAL REVIEW B Volume: 91 Issue 8 Article Number 085420 DOI:10.1103/PhysRevB.91.085420 Published 2015.