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Physical Review Letters

From Photons to Phonons and Back: A THz Optical Memory in Diamond

Duncan G. England, Philip J. Bustard, J. Nunn, Rune Lausten, Benjamin Sussman

Published Dec 9, 2013• 73 citations
Publisher

Abstract

Optical quantum memories are vital for the scalability of future quantum technologies, enabling long-distance secure communication and local synchronization of quantum components. We demonstrate a THz-bandwidth memory for light using the optical phonon modes of a room temperature diamond. This large bandwidth makes the memory compatible with down-conversion-type photon sources. We demonstrate that four-wave mixing noise in this system is suppressed by material dispersion. The resulting noise floor is just 7×10(-3) photons per pulse, which establishes that the memory is capable of storing single quanta. We investigate the principle sources of noise in this system and demonstrate that high material dispersion can be used to suppress four-wave mixing noise in Λ-type systems.

DOI
10.1103/physrevlett.111.243601

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Optical quantum memories are vital for the scalability of future quantum technologies, enabling long-distance secure communication and local synchronization of quantum components. We demonstrate a THz-bandwidth memory for light using the optical phonon modes of a room temperature diamond. This large bandwidth makes the memory compatible with down-conversion-type photon sources. We demonstrate that four-wave mixing noise in this system is suppressed by material dispersion. The resulting noise floor is just 7×10(-3) photons per pulse, which establishes that the memory is capable of storing single quanta. We investigate the principle sources of noise in this system and demonstrate that high material dispersion can be used to suppress four-wave mixing noise in Λ-type systems.
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.
Photonic quantum information processing is a key element for scalable quantum technologies, and has applications in secure long-distance quantum communication and connecting nodes of a quantum computation network. However, logical photon-photon gates and state-of-the-art single photon sources rely on probabilistic processes. Quantum memories are devices that enable storage and on-demand recall of quantum states of light, and have been highlighted as a vital component in photonic networks to overcome the scaling problem by synchronising probabilistic processes. The Raman memory has a large storage bandwidth and high synchronising capacity, and is an ideal candidate for local synchronisation. However, previous demonstrations of the Raman memory suffer from four-wave mixing noise, which prohibits quantum level operation. In this thesis I investigate methods to increase the signal to noise ratio in the Raman memory. I investigate increasing the light-matter coupling strength to boost the memory efficiency, and then explore two different methods to suppress four-wave mixing noise. I demonstrate that operating the Raman memory in a cavity is successful in reducing four-wave mixing, but it is technically challenging to maintain a high memory efficiency. I investigate a new method of noise suppression by introducing an absorption feature at the frequency of the unwanted noise field. This technically simple method is successful in reducing the noise by an order of magnitude, and will be applicable to many quantum memory protocols. In the final section of this thesis I explore the temporal mode properties of the Raman memory. I demonstrate that the Raman memory is single mode and can be used to separate and manipulate temporal modes of light. This positions the Raman memory as a key device for enabling high-dimensional photonic quantum information processing, and enhancing light-matter interactions. These results pave the way towards an efficient, low-noise, mode-selective quantum memory.
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.