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Journal of Modern Optics

Refractive index control for optical quantum storage

А. А. Калачев, Olga A. Kocharovskaya

Published Aug 1, 2011• 5 citations
Publisher

Abstract

Manipulating refractive index in time allows one to implement quantum storage in an extended resonant atomic system without recourse to inhomogeneous broadening of optical transitions or amplitude modulation of control fields, which when combined with other storage techniques provides the possibility to improve mode or channel capacity of optical storage devices. We discuss different methods of refractive index manipulation in the context of quantum storage applications, giving special attention to a simple frequency shift of an absorption structure relative to the resonant energy levels. We show that a linear modulation of refractive index in time without significant losses can be realized in doped solid-state materials such as Er3+ :Y2SiO5 crystal, to provide optical quantum storage.

DOI
10.1080/09500340.2011.599500

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Manipulating refractive index in time allows one to implement quantum storage in an extended resonant atomic system without recourse to inhomogeneous broadening of optical transitions or amplitude modulation of control fields, which when combined with other storage techniques provides the possibility to improve mode or channel capacity of optical storage devices. We discuss different methods of refractive index manipulation in the context of quantum storage applications, giving special attention to a simple frequency shift of an absorption structure relative to the resonant energy levels. We show that a linear modulation of refractive index in time without significant losses can be realized in doped solid-state materials such as Er3+ :Y2SiO5 crystal, to provide optical quantum storage.
We show the possibility to periodically modulate the refractive index in a homogeneous resonant atomic medium in space or/and time while simultaneously keeping vanishing absorption or gain. Such modulation is based on periodic resonant enhancement of the refractive index, controlled by an external optical field, and opens the way to produce coherently controllable photonic structures. We suggest the possible implementation of the proposed scheme in rare-earth doped crystals with excited state absorption.
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Off-resonant Raman interaction of a single-photon wave packet and a classical control field in an atomic medium with controlled refractive index is investigated. It is shown that a continuous change of refractive index during the interaction leads to the mapping of a single photon state to a superposition of atomic collective excitations (spin waves) with different wave vectors and vice versa. The suitability of refractive-index control for developing multichannel quantum memories is discussed and possible schemes of implementation are considered.
We examine a quantum memory scheme based on controllable dephasing of atomic coherence of a non-resonant, inhomogeneously broadened Raman transition. We show that it generalizes the physical conditions for time-reversible interaction between light and atomic ensembles from weak to strong fields and from linear to non-linear interactions. We also develop a unified framework for different realizations exploiting either controlled reversible inhomogeneous broadening or atomic frequency combs, and discuss new aspects related to storage and manipulation of quantum states.
This thesis presents on-chip quantum storage of telecommunication wavelength light using nanophotonic resonators coupled to erbium ions. Storage of light in an optical quantum memory has applications in quantum information and quantum communication. For example, long distance quantum communication using quantum repeater protocols is enabled by quantum memories. Efficient and broadband quantum memories can be made from resonators coupled to ensembles of atoms. Like other rare earth ions, erbium is appealing for quantum applications due to its long optical and hyperfine coherence times in the solid state at low temperatures. However, erbium is unique among rare earth ions in having an optical transition in the telecommunication C band (1540 nm), making it particularly appealing for quantum communication applications. In this work, we use nano-scale resonators coupled to erbium-167 ions in yttrium orthosilicate crystals (167Er 3+:Y2SiO5). We demonstrate quantum storage in two types of resonators. In a nanobeam photonic crystal resonator milled directly in 167Er 3+:Y2SiO5, we show storage of weak coherent states using the atomic frequency comb protocol. The storage fidelity for single photon states is estimated to be at least 93.7% ± 2.4% using decoy state analysis, Storage of up to 10 μs and multimode storage are demonstrated. Using a hybrid amorphous silicon 167Er 3+:Y2SiO5 resonator and on-chip electrodes, we demonstrate a multifunctional memory using the atomic frequency comb protocol with DC Stark shift control. In addition dynamic control of memory time, Stark shift control allows modifications to the frequency and bandwidth of stored light. We show tuning of the output pulse by ± 20 MHz relative to the input pulse, and broadening of the pulse bandwidth by more than a factor of three. The storage efficiency in both devices was limited to < 1%. On the way to these results, we describe 167Er 3+:Y2SiO5 spectroscopy measurements including optical coherence times and hyperfine lifetimes below 1 K, and we estimate the linear DC stark shift along two crystal directions. The design and fabrication of the on-chip resonators is presented. We discuss the limitations to storage time and efficiency, including superhyperfine coupling and resonator parameters, and we outline a path forward for improving the storage efficiency in these types of devices.