Controllable optical phase shift over one radian from a single isolated atom

TL;DR

This paper demonstrates that a single trapped atom can induce a controllable optical phase shift exceeding one radian, validating atomic theory and enabling new quantum information applications.

Contribution

It provides the first direct measurement of a large optical phase shift from a single atom, confirming microscopic models of refractive index at the quantum level.

Findings

Measured a phase shift of 1.3 radians from a single atom

Validated the atomic theory model of phase shifts

Enabled potential quantum information protocols

Abstract

Fundamental optics such as lenses and prisms work by applying phase shifts to incoming light via the refractive index. In these macroscopic devices, many particles each contribute a miniscule phase shift, working together to impose a total phase shift of many radians. In principle, even a single isolated particle can apply a radian-level phase shift, but observing this phenomenon has proven challenging. We have used a single trapped atomic ion to induce and measure a large optical phase shift of $1.3 \pm 0.1$ radians in light scattered by the atom. Spatial interferometry between the scattered light and unscattered illumination light enables us to isolate the phase shift in the scattered component. The phase shift achieves the maximum value allowed by atomic theory over the accessible range of laser frequencies, validating the microscopic model that underpins the macroscopic phenomenon of the refractive index. Single-atom phase shifts of this magnitude open up new quantum information protocols, including long-range quantum phase-shift-keying cryptography [1,2] and quantum nondemolition measurement [3,4].