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Frontiers in Optics

Towards a Quantum Memory: Effects of Hyperfine Structure on Atomic Frequency Combs in Pr:YSO

Aditya N. Sharma, Martin Ritter, Robinjeet Singh, Elizabeth A. Goldschmidt, Alan L. Migdall

Published Jan 1, 2020• 0 citations
Publisher

Abstract

We experimentally prepare 120 MHz-bandwidth atomic frequency combs. High-quality comb formation – and thus high-efficiency retrieval of stored pulses – only occurs for certain tooth spacings, due to the hyperfine energy-level spacings.
DOI
10.1364/fio.2020.fm5a.5

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Frontiers in Optics•Jan 1, 2021•0 citations
Quantum memory will be a key component in future quantum networks, and atomic frequency combs (AFCs) in rare-earth-doped crystals are one promising platform for realizing this technology. We theoretically and experimentally investigate the formation of AFCs in Pr3+:Y2SiO5, with an overall bandwidth of 120 MHz and tooth spacing ranging from 0.1 MHz to 20 MHz, showing agreement between our calculations and measurements. We observe that the echo efficiency depends crucially on the AFC tooth spacing. Our results suggest approaches to developing a high-efficiency AFC quantum memory.
Photon echoes in rare-earth-doped crystals are studied to understand the challenges of making broadband quantum memories using the atomic frequency comb (AFC) protocol in systems with hyperfine structure. The hyperfine structure of Pr 3+ poses an obstacle to this goal because frequencies associated with the hyperfine transitions change the simple picture of modulation at an externally imposed frequency. The current work focuses on the intermediate case where the hyperfine spacing is comparable to the comb spacing, a challenging regime that has recently been considered. Operating in this regime may facilitate storing quantum information over a larger spectral range in such systems. In this work, we prepare broadband AFCs using optical combs with tooth spacings ranging from 1 MHz to 16 MHz in fine steps, and measure transmission spectra and photon echoes for each. We predict the spectra and echoes theoretically using the optical combs as input to either a rate equation code or a density matrix code, which calculates the redistribution of populations. We then use the redistributed populations as input to a semiclassical theory using the frequency-dependent dielectric function. The two sets of predictions each give a good, but different account of the photon echoes.
We describe a quantum memory spectral preparation strategy that optimizes memory efficiency and bandwidth in materials such as 167 Er 3+ :Y 2 SiO 5 in a high-field regime, where the hyperfine structure is resolved.We demonstrate the method in 167 Er 3+ :Y 2 SiO 5 by preparing spectrally isolated 18-dB-absorbing features on a <1 dB background.Using these features we create an atomic frequency comb and show a quantum storage of 200-ns pulses with 22% efficiency, limited by the background absorption which arises from laser instability.We describe the experimental improvements needed to reach the material limits: O(1)-s spin-state storage, O(100) MHz bandwidth, and >90% efficiency.
Quantum memories are used for storing the quantum mechanical states of photons and release identical photons at a later time. This has many applications within the field of quantum computing and is also essential in order to achieve long range quantum communication and cryptography. In this thesis Atomic frequency comb (AFC) quantum memories were investigated. In particular, three-level memory schemes were investigated in detail and a program for simulating these protocols was developed. These schemes make use of control pulses to transfer the population from the optically excited state to a ground state hyperfine level, as well as radio frequency fields to swap the population in the two ground state hyperfine levels. In this work control and RF pulses, tailored to give good transfer rates, are suggested and effects due to off resonant excitation was investigated. A memory protocol which could give efficiencies up to 60% for Praseodymium doped Yttrium Silicate based AFC memories is proposed. The main bottleneck for reaching higher efficiencies is that the AFC peak spacing needs to be small in order to give enough time to transfer the population to the spin state with the control pulses. With the control pulses suggested in this work and the current technology of tailoring AFC peaks in Pr-based memories, this gives a maximum finesse of 6 of the AFC structure.