Phase shift spectra of a fiber-microsphere system at the single photon level

TL;DR

This study measures phase shift spectra of a fiber-microsphere system at the single photon level, demonstrating stable phase measurement and high coherence preservation, with potential applications in quantum phase gates.

Contribution

It presents a novel method for measuring phase shifts at the single photon level with high stability and coherence in a fiber-microsphere system.

Findings

Successfully measured phase shift spectra at the single photon level.

Observed transition between undercoupling and overcoupling regimes.

Achieved high purity spectrum indicating preserved coherence.

Abstract

We succeeded in measuring phase shift spectra of a microsphere cavity coupled with a tapered fiber using a weak coherent probe light at the single photon level. We utilized a tapered fiber with almost no depolarization and constructed a very stable phase shift measurement scheme based on polarization analysis using photon counting. Using a very weak probe light (\bar{n} = 0:41), we succeeded in observing the transition in the phase shift spectrum between undercoupling and overcoupling (at gap distances of 500 and 100 nm, respectively).We also used quantum state tomography to obtain a 'purity spectrum'. Even in the overcoupling regime, the average purity was 0.982 \pm 0.024 (minimum purity: 0.892), suggesting that the coherence of the fiber-microsphere system was well preserved. Based on these results, we believe this system is applicable to quantum phase gates using single light emitters such as diamond nitrogen vacancy centers.