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arXiv:1205.0704 · Physical Review Letters

Experimental Realization of Light with Time-Separated Correlations by Rephasing Amplified Spontaneous Emission

Patrick M. Ledingham, William R. Naylor, Jevon J. Longdell

Published Aug 29, 2012• 27 citations
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Abstract

Amplified spontaneous emission is a common noise source in active optical systems, it is generally seen as being an incoherent process. Here we excite an ensemble of rare earth ion dopants in a solid with a π pulse, resulting in amplified spontaneous emission. The application of a second π pulse leads to a coherent echo of the amplified spontaneous emission that is correlated in both amplitude and phase. For small optical thicknesses, we see evidence that the amplified spontaneous emission and its echo are entangled.

DOI
10.1103/physrevlett.109.093602

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Physical Review Letters•Aug 29, 2012•30 citations
Amplified spontaneous emission is a common noise source in active optical systems, it is generally seen as being an incoherent process. Here we excite an ensemble of rare earth ion dopants in a solid with a π pulse, resulting in amplified spontaneous emission. The application of a second π pulse leads to a coherent echo of the amplified spontaneous emission that is correlated in both amplitude and phase. For small optical thicknesses, we see evidence that the amplified spontaneous emission and its echo are entangled.
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.
Amplified spontaneous emission is usually treated as an incoherent noise process. Recent theoretical and experimental work using rephasing optical pulses has shown that rephased amplified spontaneous emission (RASE) is a potential source of wide bandwidth time-delayed entanglement. Due to poor echo efficiency the plain RASE protocol does not in theory achieve perfect entanglement. Experiments done to date show a very small amount of entanglement at best. Here we show that RASE can, in principle, produce perfect multimode time-delayed two mode squeezing when the active medium is placed inside a Q-switched cavity.
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.
This thesis explores the use of cryogenic rare earth ion doped solids for applications in quantum information science. A theoretical analysis of optically rephased memory protocols, namely the two pulse photon echo, is developed. This analysis is a fully quantum mechanical analysis. The two pulse photon echo suffers from collective amplified spontaneous emission in the output mode due to gain along the direction of propagation. This noise can be 'rephased' with a pi pulse, much like a two pulse photon echo, resulting in time separated nonclassical correlations. This thesis aims to characterize this noise and experimentally realize these nonclassical correlations. A theoretical analysis is presented for a hybrid photon echo rephasing protocol. The protocol uses strong optical rephasing pulses as well as external electric fields to rephase coherence in a two level ensemble of atoms. The Stark effect is utilized to reduce the collective spontaneous emission caused by inversion. We find that the protocol is noise free and has a theoretical maximum efficiency of 54% in the forward direction and 100% in the backward direction. The necessary task of laser frequency stabilization is presented in this thesis. The laser is locked using electronic and optical feedback, named the 'hybrid' lock. A theoretical analysis is presented as well as experimental results. The laser locking was sufficient for phase stable photon echo experiments to be conducted. Three experiments are presented in this thesis. The first experiment involved the characterization of the detection mode and the two pulse photon echo. Benchmark values are given to the photon echo efficiency, echo lifetime and Rabi frequency. The second experiment is the characterization of the amplified spontaneous emission from inverting a two level ensemble of cryogenic rare earth ions doped in a solid. These experiments were conducted on different optical depths on the inhomogeneous line of Tm:YAG. We find that indeed there is phase independent noise after the inversion with a bandwidth given by the Fourier width of the inverting pulse. It was seen that the noise measured increased exponentially with the optical depth, as expected from the theoretical analysis. The third experiment was the measurement of rephased amplified spontaneous emission and the nonclassical time separated correlation between the noise and the rephased noise. As outlined by the theory, the amplified spontaneous emission can be rephased with a pi pulse. This experiment is implemented on different optical depths on the inhomogeneously broadened line of Tm:YAG. For optical depths of 0.25 and greater, it was seen that the correlations were classical. For an optical depth of 0.046, the correlation was seen to violate the inseparability criterion. This violation of the criterion means that the ASE and RASE fields can not be described as a separable state. This correlation measurement was made with a confidence interval of 95.15%.
We present a fully quantum mechanical treatment of optically rephased photon echoes. These echoes exhibit noise due to amplified spontaneous emission; however, this noise can be seen as a consequence of the entanglement between the atoms and the output light. With a rephasing pulse one can get an ``echo'' of the amplified spontaneous emission, leading to light with nonclassical correlations at points separated in time, which is of interest in the context of building wide bandwith quantum repeaters. We also suggest a wideband version of DLCZ protocol based on the same ideas.
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.