Quantum interference of a single spin excitation with a macroscopic atomic ensemble

TL;DR

This paper demonstrates quantum interference between a single spin excitation and a large atomic ensemble, revealing macroscopic quantum effects and advancing quantum information processing capabilities.

Contribution

It reports the first observation of quantum interference involving a single spin excitation in a macroscopic atomic ensemble, using quantum non-demolition measurements.

Findings

Detection of a single photon creates a single spin excitation.

A macroscopic difference in the collective spin distribution is observed.

The results enable generation of exotic quantum states for various applications.

Abstract

We report on the observation of quantum interference of a collective single spin excitation with a spin ensemble of $N_{\text{atom}} =10^5$ atoms. Detection of a single photon scattered from the atoms creates the single spin excitation, a Fock state embedded in the collective spin of the ensemble. The state of the atomic ensemble is then detected by tomography via a quantum non-demolition measurement of the collective spin. A macroscopic difference of the order of $\sqrt{N_{\text{atom}}}$ in the marginal distribution of the collective spin state arises from the interference between the single excited spin and $N_{\text{atom}}$ atoms. The hybrid discrete-continuous processing of the collective spin pave the road towards generation of even more exotic states for quantum information processing, precision measurements and communication.