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Quantum Electronics

Off-resonant Raman quantum memory in impurity crystals: signal-to-noise ratio analysis

A D Berezhnoi, А. А. Калачев

Published Sep 1, 2017• 6 citations
Publisher

Abstract

The possibility of implementing an off-resonant Raman scheme of optical quantum memory on the basis of an ensemble of three-level atoms is investigated under the condition of equal polarisations of resonant transitions forming the Λ-scheme. The developed model is used to analyse the signal-to-noise ratio at the output of an optical quantum memory device in 143 Nd 3+ : Y 7 LiF 4 . It is shown that this ratio can significantly exceed unity for single-photon input pulses. The required values of the parameters can be obtained by using an impurity crystal in the form of a whispering-gallery mode ring resonator.

DOI
10.1070/qel16465

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The possibility of implementing an off-resonant Raman scheme of optical quantum memory on the basis of an ensemble of three-level atoms is investigated under the condition of equal polarisations of resonant transitions forming the Λ-scheme. The developed model is used to analyse the signal-to-noise ratio at the output of an optical quantum memory device in 143 Nd 3+ : Y 7 LiF 4 . It is shown that this ratio can significantly exceed unity for single-photon input pulses. The required values of the parameters can be obtained by using an impurity crystal in the form of a whispering-gallery mode ring resonator.
The possibility of implementation of optical quantum memory via off-resonant Raman absorption and emission of single-photon pulses in rare-earth-ion-doped crystals is theoretically analysed taking into account signal-to-noise ratio at the output of the memory device. The crystal 143Nd3+:Y7LiF4 is considered as an example. It is shown that the signal-to-noise ratio can exceed unity for single-photon input pulses provided that storage and retrieval of them is performed in the doped crystals forming a microcavity such as whispering gallery mode resonator.
A scheme for creating subradiant states in an extended system of atoms, based on the use of an external inhomogeneous electric field, is proposed. It is shown that the maximum signal-to-noise ratio at the output of a quantum memory device using such subradiant states for data storage is obtained when the temporal shape of recorded single-photon wave packets (quantum information carriers) is a time-reversed pulse characteristic of a resonant atomic system. In this case, the quantum memory efficiency tends to unity in the limit of large optical thickness of the resonant medium.
This thesis is about the experimental implementation of a high-speed and robust quantum memory for light. A novel far off-resonant Raman approach to ensemble-based quantum memories in a room-temperature environment is developed and demonstrated. Storage and retrieval of sub-nanosecond, weak coherent light pulses at the single-photon-level with total efficiencies exceeding 30% and storage times of up to 4 μs are achieved. The coherence of the memory is shown by directly interfering a copy of the incident signal with the retrieved signal from the memory. The unconditional noise floor of the memory is found to be low enough to operate the memory in the quantum regime at room temperature. Multiple readout of a single stored excitation is demonstrated, suggesting that 100% readout is possible in different temporal modes. Furthermore, first results regarding the storage and retrieval of polarisation encoded qubits are obtained. This and the memory’s ability to operate in the quantum regime at room temperature with a low unconditional noise floor illustrate its potential usefulness for real world applications.
This thesis is about the experimental implementation of a high-speed and robust quantum memory for light. A novel far off-resonant Raman approach to ensemble-based quantum memories in a room-temperature environment is developed and demonstrated. Storage and retrieval of sub-nanosecond, weak coherent light pulses at the single-photon-level with total efficiencies exceeding 30% and storage times of up to 4 μs are achieved. The coherence of the memory is shown by directly interfering a copy of the incident signal with the retrieved signal from the memory. The unconditional noise floor of the memory is found to be low enough to operate the memory in the quantum regime at room temperature. Multiple readout of a single stored excitation is demonstrated, suggesting that 100% readout is possible in different temporal modes. Furthermore, first results regarding the storage and retrieval of polarisation encoded qubits are obtained. This and the memory’s ability to operate in the quantum regime at room temperature with a low unconditional noise floor illustrate its potential usefulness for real world applications.
Quantum memories are a key technology enabling arbitrarily-scalable optical quantum information processing architectures via active synchronisation and multiplexing of quantum information. Optically active confined impurities, called colour centres, in a spin-free and highly transparent diamond host material are promising systems to realise such devices in the solid-state, suitable for integration in a scalable guided-wave on-chip photonic platform. In this thesis, the transition from optical Raman quantum memories in atomic vapours to solid-state systems is prepared, exploring two diamond-based systems: On the basis of ensembles of neutrally charged nitrogen vacancy centres, desirable properties and criteria of solid-state systems used for optical quantum memories are developed. These include non-vanishing off-resonant Raman coupling, variable acceptance bandwidth for broadband pulses and cross-linear polarised optical transitions to prevent the generation of noise during the Raman memory protocol, which have been realised experimentally by applying external uniaxial stress to the system. However, attempts to measure the very short orbital ground state lifetime affirmed unfavourable theoretical estimations, rendering this centre unsuitable for memory applications. As a potential alternative, the negatively charged silicon vacancy centre has been investigated, which offers a large ground state splitting with extended life and coherence times and narrow inhomogeneous spectral lines due to its inversion symmetry. In an ensemble of silicon vacancy centres, the ground state lifetime is measured and coherent manipulation via resonant Ramsey interference and Hahn echo is demonstrated. In a second step, off-resonant two-photon transitions are achieved implementing stimulated Raman adiabatic passage between the orbital ground states. Finally, strong light-matter interaction of a weak signal field with the ensemble via phase-sensitive four-wave mixing is shown forming the basis for integrated Raman-based memories and single photon nonlinearities in ensembles of silicon vacancy centres and closely related defects.