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Quantum Information and Computation

Modular quantum memories using passive linear optics and coherent feedback

Hendra I. Nurdin, John Edward Gough

Published Sep 1, 2015• 11 citations
Publisher

Abstract

In this paper, we show that quantum memory for qudit states encoded in a single photon pulsed optical field has a conceptually simple modular realization using only passive linear optics and coherent feedback. We exploit the idea that two decaying optical cavities can be coupled in a coherent feedback configuration to create an internal mode of the coupled system which is isolated and decoherence-free for the purpose of qubit storage. The qubit memory can then be switched between writing/read-out mode and storage mode simply by varying the routing of certain freely propagating optical fields in the network. It is then shown that the qubit memories can be interconnected with one another to form a qudit quantum memory. We explain each of the phase of writing, storage, and read-out for this modular quantum memory scheme. The results point a way towards modular architectures for complex compound quantum memories.

DOI
10.26421/qic15.11-12-9

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In this paper, we show that quantum memory for qudit states encoded in a single photon pulsed optical field has a conceptually simple modular realization using only passive linear optics and coherent feedback. We exploit the idea that two decaying optical cavities can be coupled in a coherent feedback configuration to create an internal mode of the coupled system which is isolated and decoherence-free for the purpose of qubit storage. The qubit memory can then be switched between writing/read-out mode and storage mode simply by varying the routing of certain freely propagating optical fields in the network. It is then shown that the qubit memories can be interconnected with one another to form a qudit quantum memory. We explain each of the phase of writing, storage, and read-out for this modular quantum memory scheme. The results point a way towards modular architectures for complex compound quantum memories.
In this paper, we show that quantum memory for qudit states encoded in a single photon pulsed optical field has a conceptually simple modular realization using only passive linear optics and coherent feedback. We exploit the idea that two decaying optical cavities can be coupled in a coherent feedback configuration to create an internal mode of the coupled system which is isolated and decoherence-free for the purpose of qubit storage. The qubit memory can then be switched between writing/read-out mode and storage mode simply by varying the routing of certain freely propagating optical fields in the network. It is then shown that the qubit memories can be interconnected with one another to form a qudit quantum memory. We explain each of the phase of writing, storage, and read-out for this modular quantum memory scheme. The results point a way towards modular architectures for complex compound quantum memories.
The scalability of quantum technologies will depend on the ability to interconnect independent quantum systems through photonic channels. However, heterogeneous quantum platforms emit and absorb photons with widely differing properties, severely limiting inter-node interference and modular connectivity. Here we demonstrate a cold-atom optical quantum memory that simultaneously achieves near-unity storage-and-retrieval efficiency and deterministic temporal adaptation of single photons between arbitrary and programmable input and output pulse waveforms. Operating at high optical depth and within a fully integrated architecture, the system can store photons with durations spanning over three orders of magnitude and reshape them arbitrarily without compromising efficiency, achieving compatibility with many current platforms. By augmenting the role of a quantum memory from a passive storage element to an active programmable photonic interface, our results establish a key building block for scalable entanglement-based quantum networks and modular quantum computing architectures.
We have performed a proof-of-principle experiment in which qubits encoded in the polarization states of single photons from a parametric down-conversion source were coherently stored and read out from a quantum memory device. The memory device utilized a simple free-space storage loop, providing a cyclical readout that could be synchronized with the cycle time of a quantum computer. The coherence of the photonic qubits was maintained during switching operations by using a high-speed polarizing Sagnac interferometer switch.
This thesis presents progress towards two complementary goals. The first part discusses our aim of building high-efficiency quantum memories for quantum optical states and exploring the potential of the memory for performing operations as well as for passive storage. The second part presents our work towards developing a compact source of quantum light by fabricating optical resonators from non-linear optical crystals. Our high-efficiency memories operate by storing the information carried by a weak laser field in the coherence between the hyperfine-split electronic energy levels in a vapour of atomic rubidium. We analyse the performance of a three-level gradient echo memory protocol that is used to store and retrieve optical information from the atomic coherence using an off-resonant Raman transition. Using a warm rubidium vapour, we find that the memory preserves the quantum state of the input light better than any classical means of storage could achieve. We also present results showing that trapped, cold atomic vapours provide an increased storage lifetime and analyse the contribution that four-wave-mixing makes to the retrieved light. Both the warm- and cold-atom implementations of the memory are shown to store and recall light with over 80% efficiency. We also present a number of extensions to the basic memory protocol. We demonstrate that the light-atom coupling can be controlled by the bright coupling field that is used to complete the Raman transition. This control is used to perform a beam-splitting operations between two optical pulses that are separated in time. Using frequency-multiplexed states, we show that beam-splitting operations are also possible between optical modes that are separated in frequency. This operation can be extended to perform arbitrary operations by using a series of memories. Finally, we present a high-fidelity memory for dual-rail q-bits encoded across two frequency modes. The resonators that were fabricated to explore compact sources of quantum light demonstrated many of the properties that are required to obtain the stable operation of a quantum source. We demonstrated that non-linear optical crystals can be machined into monolithic resonators that use total internal reflections to confine light. Two geometries were tested, whispering gallery resonators and square travelling wave cavities. Because both types of resonators use total internal reflection instead of dielectric coatings, they feature very low loss rates. Lithium niobate resonators of both types exhibited cavity decay rates that were dominated by bulk absorption in the crystal. The resonators were shown to be frequency tunable using an applied voltage and could be coupled to free-space modes with a tunable coupling rate. We present preliminary results of non-linear frequency conversion in the resonators. We also investigated calcium fluoride and magnesium fluoride resonators for use as high-finesse reference cavities. We report a finesse of 5000 in a calcium fluoride whispering gallery resonator. A square calcium fluoride monolithic resonator was also produce to have a low insertion loss when coupled to a Gaussian free-space mode. We demonstrate a power coupling efficiency of 94% to that resonator. We also present a proposal to use a square lithium niobate resonator as a tunable coupler for a high-finesse calcium fluoride whispering gallery resonator. The coupled resonator system could be used as a compact high-efficiency optical quantum memory.
We present a protocol to construct an arbitrary quantum circuit. The quantum bits (qubits) are encoded in polarization states of single photons. They are stored in spatially separated dense media deposed in an optical cavity. Specific sequences of pulses address individually the storage media to encode the qubits and to implement a universal set of gates. The proposed protocol is decoherence-free in the sense that spontaneous emission and cavity damping are avoided. We discuss a coupling scheme for experimental implementation in neon atoms.
In this paper, an optical memory based on an ensemble of cold Cesium atoms interacting with light in the electromagnetically induced transparency (EIT) regime is studied in view of spatially multimode quantum storage. The interfacing protocol involves a control field, generating EIT for the very weak field that carries the quantum signal to be stored and enabling its mapping into the collective hyperfine coherence of the cold atomic ensemble. For the demonstration of multimode storage, the signal pulse at the single photon level is prepared in a mode with (OAM), while the control field is in a TEM00 mode. After the writing process, the information is kept in the memory register. In the retrieval phase, the information is read back from atoms to light and we show that these pulses at the single photon level with OAM can be stored and retrieved with a preserved spatial phase and a good efficiency.