Demonstrating backflow in classical two beams’ interference

Abstract

The well-known interference pattern of bright and dark fringes was first observed for light beams back in 1801 by Thomas Young. The maximum visibility fringes occur when the irradiance of the two beams is equal, and as the ratio of the beam intensities deviates from unity, fringe visibility decreases. An interesting outcome that might not be entirely intuitive, however, is that the wavefront of such unequal amplitude beams’ superposition will exhibit a wavy behavior. In this work, we experimentally observe the backflow phenomenon within this wavy wavefront. Backflow appears in both optics (retro- propagating light) and in quantum mechanics, where a local phase gradient is not present within the spectrum of the system. It has become an interesting subject for applications as it is closely related to superoscillations whose features are used in super resolution imaging and in a particle’s path manipulations. The first successful attempt to observe backflow was made only recently in an assembly of optical fields, by synthesizing their wavefront in a complex manner. Yet, backflow is perceived as hard to detect. Here, by utilizing interference in its most basic form, we reveal that backflow in optical fields is robust and surprisingly common, more than it was previously thought to be.

Quantum advantages for transportation tasks - projectiles, rockets and quantum backflow

“…All such effects are therefore manifestations of the same mathematical phenomenon, seen through different coordinate systems. In the light of the recent interest in experimentally demonstrating quantum backflow [9][10][11][12][13] , we argue that projectile scenarios are more experimentally friendly and operationally interesting.…”

Simulating quantum backflow on a quantum computer

“…It utilizes two system qubits, S 1 and S 2 , and seven two-qubit operators k  , with k = 1,K,7. According to equation (13), the (scaled)…”

“…In this analogy, the momentum and probability current of a quantum particle are represented by the transverse wave vector and Poynting vector of the light beam, respectively. Thus, the optical experiments reported in [12,13] can be regarded as simulating QB for a particle on a line, whereas the experiment in [14] is the optics counterpart of QB for a two-dimensional rotational motion.…”

“…While a direct experimental observation of QB remains an open challenge, there has been exciting progress in simulating the QB effect using classical light [12][13][14]. The simulations utilize the analogy between the dynamics of quantum particles and the transverse spreading of light beams.…”

Design of quantum backflow in the complex plane

“…As such, backflowing wave functions suffer from significant weakness of magnitude in the interesting region, which poses difficulties in their experimental realization. Nevertheless, backflow has been observed with optical wave functions, such as in superoscillatory focusing [14], complex modulated beams [15] and even in two-beam interference [16], while ideas for observations in the quantum regime have also been formulated [17,18].…”