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arXiv:1706.05978 · Physical Review A

Storage of polarization-entangled THz-bandwidth photons in a diamond quantum memory

Kent Bonsma-Fisher, Duncan G. England, Jean-Philippe W. MacLean, Philip J. Bustard, Khabat Heshami, Kevin J. Resch, Benjamin Sussman

Published Jul 18, 2017• 23 citations
arXivHTMLPDF

Abstract

Bulk diamond phonons have been shown to be a versatile platform for the generation, storage, and manipulation of high-bandwidth quantum states of light. Here we demonstrate a diamond quantum memory that stores, and releases on demand, an arbitrarily polarized $\ensuremath{\sim}250$ fs duration photonic qubit. The single-mode nature of the memory is overcome by mapping the two degrees of polarization of the qubit, via Raman transitions, onto two spatially distinct optical phonon modes located in the same diamond crystal. The two modes are coherently recombined upon retrieval and quantum process tomography confirms that the memory faithfully reproduces the input state with average fidelity $0.784\ifmmode\pm\else\textpm\fi{}0.004$ with a total memory efficiency of $(0.76\ifmmode\pm\else\textpm\fi{}0.03)%$. In an additional demonstration, one photon of a polarization-entangled pair is stored in the memory. We report that entanglement persists in the retrieved state for up to 1.3 ps of storage time. These results demonstrate that the diamond phonon platform can be used in concert with polarization qubits, a key requirement for polarization-encoded photonic processing.

DOI
10.1103/physreva.96.012324

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Bulk diamond phonons have been shown to be a versatile platform for the generation, storage, and manipulation of high-bandwidth quantum states of light. Here we demonstrate a diamond quantum memory that stores, and releases on demand, an arbitrarily polarized \ensuremath∼250\ensuremath{\sim}250\ensuremath∼250 fs duration photonic qubit. The single-mode nature of the memory is overcome by mapping the two degrees of polarization of the qubit, via Raman transitions, onto two spatially distinct optical phonon modes located in the same diamond crystal. The two modes are coherently recombined upon retrieval and quantum process tomography confirms that the memory faithfully reproduces the input state with average fidelity 0.784\ifmmode±\else\textpm\fi0.0040.784\ifmmode\pm\else\textpm\fi{}0.0040.784\ifmmode±\else\textpm\fi0.004 with a total memory efficiency of (0.76\ifmmode±\else\textpm\fi0.03)(0.76\ifmmode\pm\else\textpm\fi{}0.03)%(0.76\ifmmode±\else\textpm\fi0.03). In an additional demonstration, one photon of a polarization-entangled pair is stored in the memory. We report that entanglement persists in the retrieved state for up to 1.3 ps of storage time. These results demonstrate that the diamond phonon platform can be used in concert with polarization qubits, a key requirement for polarization-encoded photonic processing.
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±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.
This thesis presents demonstrations of the storage and manipulation of single photons in \na room-temperature diamond quantum memory using a Raman memory protocol. We \nreport on results from four experiments. \n \nIn the first we demonstrate single photon storage and, upon retrieval, verify the quantum \nnature of the light with a Hanbury Brown Twiss measurement of g^(2)(0) = 0.65±0.07. \nA measurement of g^(2)(0) < 1 is indicative of quantum light. This is the first demonstration \nof single photon storage where the bandwidth of the stored light is greater than 1 THz. The \ndiamond memory stores light for over 13 times the duration of the input wavepacket. In \nthe second experiment, we report the storage and retrieval of polarization-encoded qubits \nand demonstrate qubit storage above a classical bound. We also verify that entanglement \nbetween the input photon and an auxiliary persists through storage and retrieval. \nWe then turn to additional uses of a Raman quantum memory. We demonstrate that \na photon stored in the diamond memory can, upon retrieval, have its frequency and bandwidth \nconverted. We report frequency conversion over a range of 4.2 times the bandwidth of \nthe input photon (4.1 nm, 2.3 THz), and bandwidth modulation between 0.5 to 1.9 times \nthe bandwidth of the input. We verify that the output light from storage and spectral \nmanipulation is still non-classical in nature. \n \nFinally, we demonstrate both single- and two-photon quantum interference mediated \nby the diamond memory, where the memory acts as a beamsplitter between photon and \noptical phonon modes in the diamond lattice. In a first experiment, a single photon is split \ninto two time-bins. The first time-bin is stored in the memory, then recalled and made to \ninterfere with the second time-bin producing fringes. In a second experiment, a photon \nfrom a weak coherent state is stored in the memory and, upon retrieval, undergoes Hong- \nOu-Mandel interference with a second photon. We measure Hong-Ou-Mandel interference \nwith a visibility of 59% giving a signature of non-classical interference (> 50%). \nThis collection of experiments establishes the diamond memory as a prime candidate for \ncertain quantum communication and processing applications. These results demonstrate \nthe potential for the diamond memory to be an integrated platform for photon storage, \nspectral conversion, and information processing.
The spectral manipulation of photons is essential for linking components in a quantum network. Large frequency shifts are needed for conversion between optical and telecommunication frequencies, while smaller shifts are useful for frequency-multiplexing quantum systems, in the same way that wavelength division multiplexing is used in classical communications. Here we demonstrate frequency and bandwidth conversion of single photons in a room-temperature diamond quantum memory. Heralded 723.5 nm photons, with 4.1 nm bandwidth, are stored as optical phonons in the diamond via a Raman transition. Upon retrieval from the diamond memory, the spectral shape of the photons is determined by a tunable read pulse through the reverse Raman transition. We report central frequency tunability over 4.2 times the input bandwidth, and bandwidth modulation between 0.5 and 1.9 times the input bandwidth. Our results demonstrate the potential for diamond, and Raman memories in general, as an integrated platform for photon storage and spectral conversion.
We report on the quantum storage and retrieval of photonic polarization quantum bits onto and out of a solid state storage device. The qubits are implemented with weak coherent states at the single photon level, and are stored for a predetermined time of 500 ns in a praseodymium doped crystal with a storage and retrieval efficiency of 10%, using the atomic frequency comb scheme. We characterize the storage by using quantum state tomography, and find that the average conditional fidelity of the retrieved qubits exceeds 95% for a mean photon number μ=0.4. This is significantly higher than a classical benchmark, taking into account the poissonian statistics and finite memory efficiency, which proves that our crystal functions as a quantum storage device for polarization qubits. These results extend the storage capabilities of solid state quantum light matter interfaces to polarization encoding, which is widely used in quantum information science.
Light, when reduced to the level of individual quanta, can possess, besides its familiar properties of wavelength, direction, and polarization, a set of correlations irreducible to classical correlations, among other peculiar behaviour. These correlated states are intrinsically interesting, and are also useful for quantum-enhanced information processing. In this thesis, I use a high-bandwidth, far-off-resonant Raman memory to implement two quantum information primitives -- entanglement generation and light storage -- at room temperature and ambient conditions. Specifically, I show, for the first time, the entanglement of two solid-state objects at room temperature and, also, the storage of light in a hollow-core optical fibre. In the first part, I show that the optical phonon modes of two diamonds can be entangled -- the prototypical non-classical correlation -- at room temperature. The entanglement was generated by spontaneous Raman scattering with projective measurements using single-photon detectors. The degree of entanglement was rigorously quantified by measuring the concurrence -- an entanglement monotone -- of the joint state of the scattered optical fields. In the second part, I store light in the coherent superposition of cesium atoms confined within a kagome-structured hollow-core photonic crystal fibre at room temperature using a far-off-resonant stimulated Raman interaction. The storage efficiency of the memory was 27$pm$1% and the noise level was sufficiently low such that single-photon-level pulses could be stored. Taken together, these results highlight the potential of Raman memories for quantum information tasks in noisy systems with short coherence times.
We demonstrate a dual-rail optical Raman memory inside a polarization interferometer; this enables us to store polarization-encoded information at GHz bandwidths in a room-temperature atomic ensemble. By performing full process tomography on the system, we measure up to 97 ± 1% process fidelity for the storage and retrieval process. At longer storage times, the process fidelity remains high, despite a loss of efficiency. The fidelity is 86 ± 4% for 1.5 μs storage time, which is 5000 times the pulse duration. Hence, high fidelity is combined with a large time-bandwidth product. This high performance, with an experimentally simple setup, demonstrates the suitability of the Raman memory for integration into large-scale quantum networks.