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arXiv:1012.5830 · Optics Letters

Photon echo without a free induction decay in a double-Λ system

Sarah E. Beavan, Patrick M. Ledingham, Jevon J. Longdell, Matthew J. Sellars

Published Mar 29, 2011• 17 citations
arXivHTMLPDF

Abstract

We have characterized a novel photon-echo pulse sequence for a double-Λ-type energy level system where the input and rephasing transitions are different from the applied π pulses. We show that, despite having imperfect π-pulses associated with large coherent emission due to free induction decay (FID), the noise added in the echo mode is only 0.2 ± 0.1 photons per shot, compared to 4 × 10⁴ photons in the FID modes. Using this echo pulse sequence in the "rephased amplified spontaneous emission" (RASE) scheme [Phys. Rev. A 81, 012301 (2010)] will allow for generation of entangled photon pairs that are in different frequency, temporal, and potentially spatial modes to any bright driving fields. The coherence and efficiency properties of this sequence were characterized in a Pr(3+):Y₂SiO₅ crystal.

DOI
10.1364/ol.36.001272

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We have characterized a novel photon-echo pulse sequence for a double-Λ-type energy level system where the input and rephasing transitions are different from the applied π pulses. We show that, despite having imperfect π-pulses associated with large coherent emission due to free induction decay (FID), the noise added in the echo mode is only 0.2 ± 0.1 photons per shot, compared to 4 × 10⁴ photons in the FID modes. Using this echo pulse sequence in the "rephased amplified spontaneous emission" (RASE) scheme [Phys. Rev. A 81, 012301 (2010)] will allow for generation of entangled photon pairs that are in different frequency, temporal, and potentially spatial modes to any bright driving fields. The coherence and efficiency properties of this sequence were characterized in a Pr(3+):Y₂SiO₅ crystal.
This thesis explores the concept of photon-echo-rephasing of the spontaneous emission from an ensemble of atoms. This generates entangled photon pairs, with intermediate storage and on-demand recall of one photon of the pair. In the first instance, this has applications as a single photon source. A more advanced application is as a building-block of a quantum repeater protocol, which is essential for expanding the range and versatility of current quantum communication links. The system used for this experimental demonstration of rephased amplified spontaneous emission (RASE) is a rare-earth ion doped crystal; Pr:YSO. Rare earth ion crystals are promising candidates for practical quantum information processing devices, largely because they possess long coherence times on both optical and hyperfine transitions. To achieve the rephasing, a novel pulse sequence utilizing four atomic levels is developed and characterized. This pulse sequence allows for the single-photon signals to be resolved spatially, spectrally, and temporally from any background coherent emission associated with the bright driving fields. It is seen that spontaneous emission can indeed be rephased using this sequence, however the degree of correlation measured was not sufficient to prove a non-classical correlation. This is attributed to an insufficient signal to noise ratio. The sources of background noise are identified, and strategies proposed to enable improved correlation measurements in future RASE experiments. The most significant noise contribution is spontaneous emission that differs only slightly in frequency from the signal. Single-photon counters are used for the RASE detection; appropriate for discrete-variable quantum repeater applications, but lacking any frequency resolution. Therefore auxiliary frequency filtering is required. Narrow-band (~kHz) filtering is performed in the RASE experiment using spectral hole-burning properties of the Pr:YSO sample itself. Furthermore, a concept for a dynamic filter is developed and tested. This uses a combination of hole-burning and Stark-shifting. The demonstration involves switching between two MHz-wide transmission windows, 10.2 MHz apart. Each spectral region is switched from an absorption of ~0.5 dB to 60 dB in a matter of micro-seconds. This technique will be particularly useful in future RASE experiments for tunable frequency discrimination. An additional noise source that is found to fundamentally limit the fidelity of RASE in this system is due to uncorrelated spontaneous emission at the same frequency as the RASE signal. This emission is a direct consequence of the low branching ratio of the optical transition. A means of avoiding this emission is proposed; namely to place the sample in a cavity to effectively enhance the branching ratio. Aside from the quality of the photon correlations, another important aspect of RASE is the storage time. Methods are discussed for achieving long memory times (~1 s) in the Pr:YSO system, using transitions between ground state hyperfine levels. In general this thesis presents a series of experiments that lay the foundation for generating entanglement between photonic and collective-atomic states, through the measurement and rephasing of spontaneous emission from an ensemble of atoms.
In a solid-state ensemble of rare-earth ions, we have experimentally generated a photon-echo of a spontaneous emission event, a process referred to as rephased amplified spontaneous emission (RASE). An enabling step for this demonstration was to use a double-Λ energy level structure for the photon-echo, rather than a standard two-pulse/two-level photon echo sequence, such that the single-photon echo is spectrally and spatially distinguishable from the coherent emission that follows imperfect π-pulses.
In this paper we report the first demonstration of "rephased amplified spontaneous emission" (RASE) with photon-counting detection. This protocol provides an all-in-one photon-pair source and quantum-memory that has applications as a quantum repeater node. The RASE protocol is temporally multimode, and in this demonstration the photon echo was generated in a way that is spatially multimode and includes intermediate storage between two potentially long-lived spin states. A correlation between spontaneous emission and its photon echo was observed, using an ensemble of Pr(3+) ions doped into a Y2SiO5 crystal. Alterations that would allow for the measurement of nonclassical correlations are identified. These should generally apply for future experiments in rare-earth ion crystals, which are promising systems for implementing highly-multiplexed quantum repeater operations.
In this paper we report the first demonstration of “cavity enhanced rephased amplified spontaneous emission”. The rephased amplified spontaneous emission (RASE) protocol provides an all-in-one photon-pair source and quantum memory that has applications as a quantum repeater node. Cavity enhancement of the interaction of the optical mode with the ensemble has the potential to improve the fidelity of the entanglement of the photon pairs. Using heterodyne detection, amplified emission and photon echo induced rephased amplified emission were observed from a Pr3+ doped Y2SiO5 crystal placed in a Fabry Perot cavity with a finesse of 70. Modifications to the experiment to allow non-classical correlations to be observed are discussed.
Over the last two decades, quantum memories have been intensively studied for potential applications of quantum repeaters in quantum networks. Various protocols have also been developed. To satisfy no noise echoes caused by spontaneous emission processes, a conventional two-pulse photon-echo scheme has been modified. The resulting methods include double-rephasing, ac Stark, dc Stark, controlled echo, and atomic frequency comb methods. In these methods, the main purpose of modification is to remove any chance of a population residual on the excited state during the rephasing process. Here, we investigate a typical Gaussian rephasing pulse-based double-rephasing photon-echo scheme. For a complete understanding of the coherence leakage by the Gaussian pulse itself, ensemble atoms are thoroughly investigated for all temporal components of the Gaussian pulse, whose maximum echo efficiency is 26% in amplitude, which is unacceptable for quantum memory applications.
We present a novel quantum memory protocol inspired by the two photon echo that overcomes the main drawbacks of the latter, namely, contamination of the retrieval pulse by spontaneous emission and free induction decay. It makes use of two rephasing pulses instead of one, the second is intended to send the atoms back to the fundamental state so that the echo is emitted in a non-inverted medium. The emission of the echo after the first pulse is prevented by unfulfillment of the spatial phase matching condition. Experimental maximum efficiency is found to be limited to 30% for 0:5 at. % Tm3+:YAG, mainly because of finite coherence lifetime. We have performed a preliminary analysis of noise, finding a significant reduction of spontaneous emission noise in the retrieval mode. However, further improvements are to be done since the signal to noise ratio is still close to one.