Heralded Single-Magnon Quantum Memory for Photon Polarization States

We demonstrate a heralded quantum memory where a photon announces the mapping of a light polarization state onto a single collective-spin excitation (magnon) shared between two atomic ensembles. The magnon can be converted at a later time into a single polarized photon with polarization fidelity over 90(2)% for all fiducial input states, well above the classical limit of $\frac{2}{3}$. The process can be viewed as a nondestructive quantum probe where a photon is detected, stored, and regenerated without touching its—potentially undetermined—polarization.

Figure 1

(a) Setup. The small arrows indicate beam polarizations. OP is the optical pumping beam. NPBS, PBS, QWP, and HWP denote a nonpolarizing beam splitter, a polarizing beam splitter, a quarter wave plate, and a half wave plate, respectively. D1, D2, D3 are single-photon counting modules for herald detection and polarization analysis. A static magnetic field induces magnon precession. (b) Energy levels. Ensembles $A$ and $B$ are initially prepared in $|g_{\mp }⟩\equiv |F=3$, $m_F=\mp 3⟩$. The write (green) and the read (red) processes are ${\sigma }^{\pm }\mathrm{\text{−}}\pi$ and $\pi \mathrm{\text{−}}{\sigma }^{\pm }$ spontaneous Raman transitions, respectively. (c) Precession of the two macroscopic spins, as measured via cavity transmission, and timing of the optical-pumping ($t_{\mathrm{op}}$), write ($t_w$), and read ($t_r$) processes.

Figure 2

(a) Density matrices ${\rho }_{\mathrm{meas}}$ of the retrieved single photons for fiducial input states $H$, $L$, and $S$. (b) The measured degrees of polarization ($p_{\mathrm{out}}$) and fidelities ($\mathcal{F}$) of the retrieved single photons.

Figure 3

Polarization fidelity of the stored photon as a function of $\theta$ for $\varphi =0$ [Eq. (1)]. Insets (i)–(iii): The results of projection measurements of the output field in three mutually orthogonal bases, $H\mathrm{\text{−}}V$, $L\mathrm{\text{−}}R$, and $S\mathrm{\text{−}}T$. The solid curves are a simultaneous fit for all 60 data points. No backgrounds have been subtracted.