Approaching single-photon pulses

TL;DR

This paper derives bounds on how closely physical states can approximate ideal single-photon pulses, revealing limitations in ultrafast optics and demonstrating localized states near single photons.

Contribution

It provides the first theoretical bounds on the fidelity of single-photon pulse approximations and introduces localized states close to single photons.

Findings

Maximum fidelity is low for few-cycle pulses.

Fidelity increases with slower envelope variation.

Strictly localized states can closely approximate single photons.

Abstract

Single-photon pulses cannot be generated on demand, due to incompatible requirements of positive frequencies and positive times. Resulting states therefore contain small probabilities for multiphotons. We derive upper and lower bounds for the maximum fidelity of realizable states that approximate single-photon pulses. The bounds have implications for ultrafast optics; the maximum fidelity is low for pulses with few cycles or close to the onset, but increases rapidly as the pulse envelope varies more slowly. We also demonstrate strictly localized states that are close to single photons.