Quantum walk comb in a fast gain laser. I. Heckelmann, M. Bertrand, A. Dikopoltsev, M. Beck, G. Scalari and J. Faist in Science 382:434 (2023). What the paper says!?
This work studies a random walk in a synthetic dimension (not in real space), implemented with a frequency comb in a ring resonator. The synthetic dimension formed here involves the modes of this resonator, specifically frequency ladders in optical ring cavities whose couplings through external modulation (through electro-optical effect of the laser pump) lead to supermodes. For high-enough gain system evolves toward a high-energy one, which is considered surprising (expectations are dissipations into the low-energy one, but why? if it's driven?)
It is unclear to me what the main achievement is, either that it «offers a promising platform to generate broadband, tunable, and stable frequency combs» or that it implements a quantum walk in an abstract space (frequency, or what they call a "synthetic dimension") as the title suggests.
One nice feature is indeed the striking mapping to the harmonic oscillator, cf. their Fig. 1, the exact way this comes about being unclear to me. It might also have something to do with self-interference propagation[1] rather than quantum walks (or maybe SIP has to do with quantum walk?)
The paper was passed to me by Sven Höfling regarding intractability and boson sampling, although the connection is not obvious to me, especially as they comment that:
The quantum walk comb
exhibits high predictability
