This thesis investigates an entangled light source with an in-built quantum memory based on the protocol of rephased amplified spontaneous emission (RASE). RASE has promising applications as a building-block of a quantum repeater: a device essential for extending the range of current quantum communication links. To be useful RASE must be able to produce high fidelity non-classical light with high efficiency, and be able to store multimode entanglement for long times. This thesis characterises the RASE source and determines to what degree these requirements can be met. The experimental RASE demonstration was conducted in a rare-earth ion doped crystal. Rare-earth ions provide a particularly promising platform for developing quantum technologies as they possess long coherence times on both the optical and hyperfine transitions. In the RASE protocol an inverted ensemble of two-level atoms amplifies the vacuum fluctuations resulting in amplified spontaneous emission (ASE). This results in entanglement between the output optical field and the collective modes of the amplifying ensemble. The collective atomic state dephases due to the inhomogeneous broadening of the ensemble but this can be rephased using photon echo techniques. When the ensemble rephases, a second optical field, the rephased amplified spontaneous emission (RASE), is emitted and is entangled with the ASE. In this thesis, a modified four-level rephasing scheme is used that allows the single photon signals to be spectrally resolved from any coherent background emission associated with the bright driving fields. In addition, four-level RASE incorporates storage on the long-lived hyperfine ground states. Two experiments are described in this thesis. First, a free-space four-level RASE demonstration using continuous-variable detection. In this experiment the different sources of noise were characterised and low noise operation was shown to be possible. Entanglement of the ASE and RASE was confirmed by violating the inseparability criterion with 98.6% confidence. In addition, entanglement was demonstrated after storage of the collective atomic state on the spin states and RASE was shown to be temporally multimode, with almost perfect distinguishability between two temporal modes demonstrated. The degree of entanglement between the ASE and RASE was limited by the rephasing efficiency, which saturated at 3%. It was determined that distortion of the rephasing pulses as they propagate through the optically thick ensemble was the probable cause of the low efficiency. The second experiment was a preliminary cavity-enhanced RASE demonstration. Theoretically perfect rephasing efficiency can be obtained by placing the crystal in an impedance-matched optical cavity. The initial cavity design showed encouraging evidence of an enhancement in the rephasing efficiency, with a 4-fold improvement over the free-space experiment. Improvements to the cavity design were proposed to allow a further increase in the rephasing efficiency of RASE. In summary, this thesis provides an extensive characterisation of an entangled light source with an in-built quantum memory based on rephasing spontaneous emission from an ensemble of ions. Importantly, the RASE scheme allows generation and storage of entanglement in a single protocol, which holds great promise for the development of integrated quantum networks.
This thesis investigates an entangled light source with an in-built quantum memory based on the protocol of rephased amplified spontaneous emission (RASE). RASE has promising applications as a building-block of a quantum repeater: a device essential for extending the range of current quantum communication links. To be useful RASE must be able to produce high fidelity non-classical light with high efficiency, and be able to store multimode entanglement for long times. This thesis characterises the RASE source and determines to what degree these requirements can be met. The experimental RASE demonstration was conducted in a rare-earth ion doped crystal. Rare-earth ions provide a particularly promising platform for developing quantum technologies as they possess long coherence times on both the optical and hyperfine transitions. In the RASE protocol an inverted ensemble of two-level atoms amplifies the vacuum fluctuations resulting in amplified spontaneous emission (ASE). This results in entanglement between the output optical field and the collective modes of the amplifying ensemble. The collective atomic state dephases due to the inhomogeneous broadening of the ensemble but this can be rephased using photon echo techniques. When the ensemble rephases, a second optical field, the rephased amplified spontaneous emission (RASE), is emitted and is entangled with the ASE. In this thesis, a modified four-level rephasing scheme is used that allows the single photon signals to be spectrally resolved from any coherent background emission associated with the bright driving fields. In addition, four-level RASE incorporates storage on the long-lived hyperfine ground states. Two experiments are described in this thesis. First, a free-space four-level RASE demonstration using continuous-variable detection. In this experiment the different sources of noise were characterised and low noise operation was shown to be possible. Entanglement of the ASE and RASE was confirmed by violating the inseparability criterion with 98.6% confidence. In addition, entanglement was demonstrated after storage of the collective atomic state on the spin states and RASE was shown to be temporally multimode, with almost perfect distinguishability between two temporal modes demonstrated. The degree of entanglement between the ASE and RASE was limited by the rephasing efficiency, which saturated at 3%. It was determined that distortion of the rephasing pulses as they propagate through the optically thick ensemble was the probable cause of the low efficiency. The second experiment was a preliminary cavity-enhanced RASE demonstration. Theoretically perfect rephasing efficiency can be obtained by placing the crystal in an impedance-matched optical cavity. The initial cavity design showed encouraging evidence of an enhancement in the rephasing efficiency, with a 4-fold improvement over the free-space experiment. Improvements to the cavity design were proposed to allow a further increase in the rephasing efficiency of RASE. In summary, this thesis provides an extensive characterisation of an entangled light source with an in-built quantum memory based on rephasing spontaneous emission from an ensemble of ions. Importantly, the RASE scheme allows generation and storage of entanglement in a single protocol, which holds great promise for the development of integrated quantum networks.
It has been proposed that secure communication over large distances can be achieved using quantum repeater protocols. Two key components which are required for a repeater are an entangled light source and a quantum memory. The approach investigated here, is to use a protocol which incorporates both a memory and entangled light source using rephased amplified spontaneous emission (RASE). To be a viable repeater, RASE must achieve second long storage times, and efficiencies exceeding 90%. The focus of this thesis was to investigate the mechanisms limiting the efficiency of RASE. In this work, RASE was performed in the rare-earth ion-doped crystal Pr3+:Y2SiO5, and the coherence was stored on the ground hyperfine transition for 5 µs. Rare-earth ion-doped crystals are an ideal platform for RASE because they offer long optical and hyperfine coherence times. The hyperfine transitions are of particular interest, because the memory storage times required for repeater can be engineered. RASE works by inverting an ensemble of two-level atoms to create the optical gain necessary for amplified spontaneous emission (ASE). Detection of ASE heralds the ensemble into a collective state which is entangled with the emitted field. Because the ensemble is inhomogeneously broadened, the collective state will then begin to dephase. The state can be rephased, after a tunable delay time, and the entanglement readout as a second optical field (RASE). In this work, the metric chosen to confirm entanglement was the inseparability criterion. In previous work, the two emitted fields of light were proven to be entangled with significant confidence, however, the rephasing efficiency was limited to 3% [1]. The explanation for this was a mismatch between the first and second pass of the beam through the crystal, which was the consequence of a retroreflective mirror being positioned incorrectly. In this thesis, the beam mismatch, caused by the position of the mirror, was shown to not affect the RASE experiment. Instead, the RASE experiment is more likely to be affected by a spatial mode mismatch which is caused by the distortion of the control pulses. The effect of these distortions was observed in the efficiency measurement of RASE. According to the theory, the efficiency of RASE should consistently improve with optical depth. Experimentally, however, in the regime where the two fields were entangled, the efficiency was maximized at 8.3% when the optical depth was 0.8. The performance then began to deteriorate when the optical depth exceeded 1. This result, therefore, informs how the RASE experiment should be designed in the future. Specifically, care should be taken to correct for the effects of any spatial mode distortions, in order for RASE to operate at the threshold efficiency required for a quantum repeater.
Here, we demonstrate generating and storing entanglement in a solid-state spin-wave quantum memory with on-demand readout using the process of rephased amplified spontaneous emission (RASE). Amplified spontaneous emission (ASE), resulting from an inverted ensemble of Pr^{3+} ions doped into a Y_{2}SiO_{5} crystal, generates entanglement between collective states of the praseodymium ensemble and the output light. The ensemble is then rephased using a four-level photon echo technique. Entanglement between the ASE and its echo is confirmed and the inseparability violation preserved when the RASE is stored as a spin wave for up to 5 μs. RASE is shown to be temporally multimode with almost perfect distinguishability between two temporal modes demonstrated. These results pave the way for the use of multimode solid-state quantum memories in scalable quantum networks.
We studied the recall efficiency as a function of optical depth of rephased amplified spontaneous emission (RASE), a protocol for generating entangled light. The experiments were performed on the 3H4→1D2 transition in the rare-earth doped crystal Pr3+:Y2SiO5, using a four-level echo sequence between four hyperfine levels to rephase the emission. Rephased emission was observed for optical depths in the range of αL = 0.8 to 2.0 with a maximum rephasing efficiency of 14 % observed while incorporating spin storage. This efficiency is a significant improvement over the previously reported non-classical result but is well short of the predicted efficiency. We discuss the possible mechanisms limiting the protocol's performance, and suggest ways to overcome these limits.
Quantum memories could benefit many devices in quantum information processing. For a quantum to be useful in real-world applications, the quantum memory must have a high recall efficiency. Here we demonstrate an efficient (up to 80%) spin-storage quantum memory in Er:YSO, using the 4-level rephased amplified spontaneous emission protocol. We show non-classical correlations between the ASE and RASE fields produced by the quantum memory. Also, we demonstrate the storage of 70 temporal modes, with a write time of 150 us, and a storage time of 25 us. Finally, a clear pathway is presented to improve the efficiency, storage time, and mode capacity. Such a device would have applications in quantum networking and measurement-based quantum computing.
Atomic ensembles in the form of rare-earth-doped crystals offer significant potential for implementing a quantum memory due to their long coherence lifetimes, enhanced light-matter interaction, and the potential for integration. We use a double Lambda configuration based on hyperfine levels in praseodymium doped yttrium orthosilicate. In this scheme, a probabilistically generated collective excitation of the internal atomic states in the form of a spin-wave is signaled by emission of a Stokes-shifted herald photon via spontaneous Raman scattering. After a storage delay, a read field addresses the appropriate transition, mapping the spin wave back to a photon, resulting in the emission of an anti-Stokes shifted retrieved photon. Momentum conservation, as dictated by phase-matching conditions, ensure that the herald and retrieved photons are emitted in a well-defined spatial modes relative to the co-propagating write and read optical fields. Inhomogeneous broadening of the absorption profile precludes straightforward implementation of the above scheme, and we resort to spectral hole-burning techniques to prepare a sub-ensemble of atoms with a narrow range of optical transition energy in the ground state. Using a second identical crystal, we also implement an optical spectral filter that suppresses the powerful write and read beams, and spontaneous-emission noise, while transmitting the herald and retrieved photons. We report on the second-order optical correlations between the herald and retrieved photons that are the signature of generation and retrieval of a stored spin-wave excitation. Demonstration of such a quantum memory is an enabling step towards distributing entanglement for a quantum repeater scheme.
(English) Future networks for quantum communication will require a new form of technology in order to operate over long distances. The no-cloning theorem precludes the use of amplification, so quantum repeater schemes have been proposed to achieve long-distance quantum communication through the distribution of entanglement. A key device for this setup is a quantum memory, which is required in order to store entanglement and synchronise its distribution. Among the many physical systems than can be used as a quantum memory, rare-earth ion-doped crystals stand out due to their long coherence times and potential for multiplexing and integration. This thesis reports the use of one such system, Pr3+:Y2SiO5, as a quantum memory using the atomic frequency comb (AFC) protocol. Building on previous work, we improved the performance of the memory to be able to demonstrate light-matter entanglement with a telecommunications wavelength photon and the quantum memory, for both a bulk memory and an integrated version. To measure light-matter entanglement, we use photon pairs generated through cavity-enhanced spontaneous parametric down conversion, where one photon (the signal) is stored in the memory and the other (the idler) is at a telecommunications wavelength. These photon pairs exhibit energy-time entanglement, which we analyse in the time basis using fibre- and AFC-based Mach-Zehnder interferometers. In the first experiment, we demonstrate light-matter entanglement where the bulk quantum memory is used with on-demand retrieval via storage in a spin-wave. We measure interference fringes with average visibilities of 89(2)% for AFC storage, and 70(3)% for spin-wave storage, sufficient to demonstrate entanglement. This is the first such demonstration using an on-demand multimode solid-state memory and telecom photon. We show that entanglement is maintained for up to 47.7 µs of storage. These results were made possible through improvements in the AFC efficiency and the noise filtering. Furthermore, we perform on-demand storage and retrieval of up to 30 temporal modes. We take the next step towards implementing our system in a real world scenario by measuring entanglement over a distance. To do so, we send the idler photons through both optical fibre spools and deployed fibre in the metropolitan area of Barcelona, Spain, while the photon detection still occurs in the same lab. The resulting visibilities (80% or higher) demonstrate that the photonic qubit does not decohere during transmission through the optical fibre. We then decouple the photon creation and detection by measuring non-classical correlations between signal photons detected in the lab, and idler photons detected 16 km away (44 km in fibre) in a different location. The final experiment uses a fibre-integrated memory for light-matter entanglement. We use a Pr3+:Y2SiO5 crystal containing a Type I laser-written waveguide, interfaced directly with optical fibre. The fibre-integration allows for improved transmission and AFC efficiency compared to a free-space coupled waveguide memory. To analyse the performance of the memory, we demonstrate non-classical correlations for storage times up to 28 µs. We perform qubit tomography in the time basis for storage times of 3 and 10 µs, measuring two-qubit fidelities of 86(2)% and 86(4)%, respectively, demonstrating storage of entanglement. These experiments demonstrate the potential of our system for use as part of a quantum repeater, with many opportunities for further improvement to the storage time, efficiency, and multiplexing capability.