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Photons Walking the Line: A quantum walk with adjustable coin operations
A. Schreiber, K. N. Cassemiro, V. Potocek, A. Gabris, P. J. Mosley, E. Andersson, I. Jex, Ch. Silberhorn
TL;DR
The paper addresses the limited experimental evidence for coherent quantum walks by implementing a five-step coined walk with passive optical elements and a fiber feedback loop. The measured distribution is non-Gaussian with a larger spread than the classical walk, while adjustable coin settings support future search algorithms.
Problem
Few experiments had reported quantum walks, motivating an optical implementation that maintains coherence while scaling in reachable steps and position Hilbert space.
Method
A one-dimensional coined quantum walk uses photon polarization as the coin, arrival time as position, and a fiber-network feedback loop to realize repeated steps with adjustable coin operations.
Results
After five steps, the measured distribution showed decreasing central-bin probabilities and growing outer wings, with σ = 3.03(10) versus σQ = 2.83 and σC ≈ 2.24.
Takeaways & Limitations
The compact implementation demonstrates coherent, flexible coined quantum walks and provides a starting point for one- or two-dimensional quantum-walk search algorithms.
Takeaways & Limitations
Beyond five steps, spurious optical reflections introduce systematic distribution errors, while 50/50 beam-splitter losses reduce the signal-to-noise ratio.
Abstract
from arXiv · showhide
We present the first robust implementation of a coined quantum walk over five steps using only passive optical elements. By employing a fiber network loop we keep the amount of required resources constant as the walker's position Hilbert space is increased. We observed a non-Gaussian distribution of the walker's final position, thus characterizing a faster spread of the photon wave-packet in comparison to the classical random walk. The walk is realized for many different coin settings and initial states, which opens the way for the implementation of a quantum walk-based search algorithm.