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arXiv:1309.3202 · Physical Review Letters

Spectral Multiplexing for Scalable Quantum Photonics using an Atomic Frequency Comb Quantum Memory and Feed-Forward Control

Neil E. Sinclair, Erhan Sağlamyürek, Hassan Mallahzadeh, Joshua A. Slater, Mathew George, Raimund Ricken, Morgan P. Hedges, Daniel Oblak, Christoph Simon, W. Sohler, Wolfgang Tittel

Published Jul 29, 2014• 331 citations
arXivHTMLPDF

Abstract

Future multiphoton applications of quantum optics and quantum information science require quantum memories that simultaneously store many photon states, each encoded into a different optical mode, and enable one to select the mapping between any input and a specific retrieved mode during storage. Here we show, with the example of a quantum repeater, how to employ spectrally multiplexed states and memories with fixed storage times that allow such mapping between spectral modes. Furthermore, using a Ti:Tm:LiNbO_{3} waveguide cooled to 3 K, a phase modulator, and a spectral filter, we demonstrate storage followed by the required feed-forward-controlled frequency manipulation with time-bin qubits encoded into up to 26 multiplexed spectral modes and 97% fidelity.

DOI
10.1103/physrevlett.113.053603

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Future multiphoton applications of quantum optics and quantum information science require quantum memories that simultaneously store many photon states, each encoded into a different optical mode, and enable one to select the mapping between any input and a specific retrieved mode during storage. Here we show, with the example of a quantum repeater, how to employ spectrally multiplexed states and memories with fixed storage times that allow such mapping between spectral modes. Furthermore, using a Ti:Tm:LiNbO_{3} waveguide cooled to 3 K, a phase modulator, and a spectral filter, we demonstrate storage followed by the required feed-forward-controlled frequency manipulation with time-bin qubits encoded into up to 26 multiplexed spectral modes and 97% fidelity.
The realization of scalable quantum networks for distribution of entanglement over long distances hinges on quantum repeaters. To outperform the exponential transmission loss in optical fibers, quantum repeaters must employ multiplexing schemes in the temporal, spectral, or spatial domain. The performance of such a multiplexed scheme is contingent on efficient quantum memories offering both extended storage times and large multimode capacities. Towards this goal, we experimentally demonstrate a memory platform operating at telecom wavelength using an Er3+^{3+}3+:Y2_{2}2​SiO5_{5}5​ crystal. We record on-demand storage and recall of weak coherent pulses for up to 111 ms, exceeding by an order of magnitude that of previously reported demonstrations based on Er3+^{3+}3+. The memory exhibits an efficiency of 10.36 % at 300 μμμs storage time with a signal-to-noise ratio of 10.910.910.9, mainly limited by spontaneous emission noise. We also store two spectral modes representing a frequency-bin encoding and retrieve the state with an overlap of 81.8 %. Furthermore, we demonstrate multimode capacity by simultaneously storing 20 temporal and 3 spectral modes with on-demand and selective recall capabilities, serving as a promising step toward a scalable quantum repeater architecture.
In quantum communication with quantum repeaters, multiplexed quantum memory is expected to enhance communication rates. When using an atomic frequency comb (AFC) for on-demand storage, the frequency mode number is often limited by the optical power of the control pulses. Here, using a space-coupled waveguide electro-optic modulator, we increased the output power, allowing us to apply control pulses to multiple modes simultaneously. Further, through enhancement of an experimental setup that increases power density, we increased the number of modes. Consequently, we pioneered, to the best of our knowledge, on-demand storage using five modes of AFC. This technology is a significant achievement toward frequency-multiplexed on-demand quantum memory.
Processing and distributing quantum information using photons through fibre-optic or free-space links are essential for building future quantum networks. The scalability needed for such networks can be achieved by employing photonic quantum states that are multiplexed into time and/or frequency, and light-matter interfaces that are able to store and process such states with large time-bandwidth product and multimode capacities. Despite important progress in developing such devices, the demonstration of these capabilities using non-classical light remains challenging. Here, employing the atomic frequency comb quantum memory protocol in a cryogenically cooled erbium-doped optical fibre, we report the quantum storage of heralded single photons at a telecom-wavelength (1.53 μm) with a time-bandwidth product approaching 800. Furthermore, we demonstrate frequency-multimode storage and memory-based spectral-temporal photon manipulation. Notably, our demonstrations rely on fully integrated quantum technologies operating at telecommunication wavelengths. With improved storage efficiency, our light-matter interface may become a useful tool in future quantum networks.
Extended quantum networks are based on quantum repeaters that often rely on the distribution of entanglement in an efficient and heralded fashion over multiple network nodes. Many repeater architectures require multiplexed sources of entangled photon pairs, multiplexed quantum memories, and photon detection that distinguishes between the multiplexed modes. Here we demonstrate the concurrent employment of (1) spectrally multiplexed cavity-enhanced spontaneous parametric down-conversion in a nonlinear crystal; (2) a virtually-imaged phased array that enables mapping of spectral modes onto distinct spatial modes for frequency-selective detection; and (3) a cryogenically-cooled Tm 3+ :LiNbO 3 crystal that allows spectral filtering in an approach that anticipates its use as a spectrally-multiplexed quantum memory. Through coincidence measurements, we demonstrate quantum correlations between energy-correlated photon pairs and a strong reduction of the correlation strength between all other photons. This constitutes an important step towards a frequency-multiplexed quantum repeater.
A photonic quantum memory that can store multiple qubits and later recall any randomly chosen subset is essential for large-scale quantum networking and computing. This capability, similar to random-access memory in classical computers, has been difficult to achieve due to the lack of a versatile random-access mechanism and the limited multi-mode storage capacity of existing quantum memory protocols. One promising route uses photon-echo protocols, which apply optical rephasing pulses to selectively read out frequency-multiplexed photonic qubits within a broadened optical transition. Here we advance a photon-echo-based quantum memory protocol using linearly swept adiabatic rephasing pulses, which imprint phase shifts to suppress unwanted echoes. Using the optical transitions of a 171Yb3+:Y2SiO5 crystal, we demonstrate storage and retrieval of multiple complex spectro-temporal photonic modes and achieve random-access memory across eight distinct spectral modes. We show enhanced mode-mapping versatility while substantially lowering the required rephasing pulse intensity, providing a more efficient and reliable approach for high-fidelity qubit storage and retrieval. Scalable quantum networks require memories that store photonic qubits while allowing selective access to individual states, a capability not currently available. A chirped-pulse phase-encoding approach provides random access to spectro-temporal-multiplexed information, enabling storage and retrieval across eight spectral modes with lower rephasing pulse intensity.
Quantum memory is a key element for quantum repeaters and linear optical quantum computers. In addition to memory, repeaters and computers also require manipulating quantum states by means of unitary transformations, which is generally accomplished using interferometric optical setups. We experimentally investigate photon-echo type atom-light interaction for the possibility to combine storage with controlled transformation of quantum states. As an example, we demonstrate unambiguous state discrimination of qubits and qutrits in an Ti:Er:LiNbO3_33​ waveguide cooled to 3K using states encoded into large ensembles of identically prepared photons in superposition of different temporal modes. The high robustness and flexibility of our approach makes it promising for quantum communication and computation as well as precision measurements.