Storage and Retrieval of THz-Bandwidth Single Photons Using a Room-Temperature Diamond Quantum Memory

We report the storage and retrieval of single photons, via a quantum memory, in the optical phonons of a room-temperature bulk diamond. The THz-bandwidth heralded photons are generated by spontaneous parametric down-conversion and mapped to phonons via a Raman transition, stored for a variable delay, and released on demand. The second-order correlation of the memory output is $g^{(2)}(0)=0.65\pm 0.07$, demonstrating a preservation of nonclassical photon statistics throughout storage and retrieval. The memory is low noise, high speed and broadly tunable; it therefore promises to be a versatile light-matter interface for local quantum processing applications.

Figure 1

Experimental concept, energy level diagram, and setup. (a) The memory protocol. A horizontally ($H$) polarized single photon (green, 723 nm) is written into the quantum memory with a vertically ($V$) polarized write pulse (red, 800 nm). After a delay $\tau$, an $H$-polarized read pulse recalls a $V$-polarized photon. (b) Energy levels in the memory. The ground state $|0⟩$ and the storage state $|1⟩$ correspond to the crystal ground state and an optical phonon, respectively. The signal photon and the read-write pulses are in two-photon resonance with the optical phonon (40 THz) and are far detuned from the conduction band $|2⟩$. (c) The experimental setup. The laser output is split to pump the photon source and to produce the orthogonally polarized read and write beams. The photons are produced in pairs with one (signal) at 723 nm and the other (herald) at 895 nm. The signal photon is stored in, and recalled from, the quantum memory. The herald and signal photons are detected using APDs and correlations between them are measured using a coincidence logic unit.

Figure 2

Measured coincidences between the signal and herald photons as a function of read-write delay (blue bars). An exponential decay of half-life 3.5 ps (solid blue fit) is characteristic of the optical phonon lifetime. The background noise (red bars) shows a SNR of 3.8:1 for single photon retrieval. The estimated noise due to thermal phonon population is shown by the dotted line. Inset: Transmission through the diamond in the presence of the write pulse, showing memory absorption. The profile width of 326 fs (solid red fit) indicates the large bandwidth of the photons stored in the memory. All error bars are from poissonian counting statistics. Signal and herald detection events are defined as coincident if the time delay between them falls within a 1 ns window.

Figure 3

Detection coincidences between the herald photon and the signal photon retrieved from the memory as a function of electronic delay (blue). A peak of 5 times the accidental rate at zero delay demonstrates strong correlations between the herald and signal photons after readout from the memory. The memory noise (red, offset by 2 ns for clarity) shows no increase at zero delay. Signal and herald detection events are defined as coincident if the time delay between them falls within a 156 ps window. The width of the peaks is due to the timing jitter ($\sim 500\mathrm{ps}$) of the APDs.

Figure 4

The heralded second-order correlation function of the memory output as a function of storage time. Values below the classical limit of $g_{\mathrm{out}}^{(2)}(0)=1$ demonstrate the quantum characteristics of the output field. Nonclassical statistics are observed for storage times up to $\sim 3\mathrm{ps}$. Error bars are from Poissonian counting statistics.