Quantum ratchet with photons

Abstract

Quantum simulation is one of the most important applications of quantum computers. Unlike full-scale quantum computing where hundreds of qubits are required, quantum simulation can be performed at small-scale quantum information processing system. In a recent work [1], Zhang et al. from the University of Science and Technology of China (USTC) have experimentally demonstrated a quantum simulation of an interesting phenomenon with photons, the quantum ratchet. A classical ratchet is a spatially periodic rotor in which the directed motion of particles can be realized without a bias force. Similarly, in a quantum ratchet, a quantum particle is directed evolved under apparent no bias force. Quantum ratchet effect has attracted much attention recently. The effect may exist in many quantum systems [2], and it was demonstrated with cold atoms in optical lattice [3]. This quantum effect plays a crucial role in many related studies, such as dynamical localization, quantum resonance, and quantum chaos, and has potential applications in solving the evolution of wave functions. Researchers in USTC elaborately created a delta-kicked Hamiltonian quantum ratchet [4] in an all-optical system, and this quantum rotor is driven by Hamiltonian chaos, in the absence of noise. Directed motion and quantum resonance are observed. They constructed successfully a delta-kicked potential for light by a phase mirror which is the key of the periodically flashing potential in the experiment and the potential is determined by the etched depth of the phase mirror. The quantum particle is the photons which are reflected between a plane mirror and the phase mirror, and these photons encounter the phase mirror again and again with a certain temporal interval. In order to obtain the momentum information, they use skillfully a cylindrical lens to convert the pattern of the position distribution into that of the momentum distribution on the focal plane of the lens. They eventually successfully observed the quantum ratchet and quantum resonance in the experiment in the momentum and kinetic energy space with photons. Since the experiment is performed using classical light, the effects can be even observed with a naked eye for up to 22 steps. Compared to the massive quantum ratchet, photons have longer wavelength which provides longer spatial period and makes the quantum ratchet easier to control and measure. Light is easy to be coherently controlled and detected. The approach and the result provided here are significant to simulate other quantum phenomena.