The quantum resources of entanglement and single photons are key to a range of quantum applications. These resources are challenging to produce cleanly and efficiently. This thesis investigates and improves methods of producing entanglement and single photons using measurement and control. To robustly produce entanglement, this thesis extends work by Carvalho et al., who developed a method for creating an entangled state of two atoms. This is done by coupling the atoms to a damped cavity, and using measurement of the output of the cavity to trigger a feedback pulse on the atoms. The robustness of this scheme against imperfect localisation of the two atoms is tested in this thesis, and limits are placed on the size of a trap that will still allow an entangled state to be produced. Additionally, using three-level (Lambda configuration) atoms slows the rate at which the system evolves, allowing more time for measurement and feedback to be applied, though it does not counteract the influence of spontaneous emission. In extending this work to multiple atoms we found that the system rapidly becomes more complex, and we developed a strategy for choosing a feedback pulse that stabilises a specific, highly entangled steady state of multiple particles. In addition to this general strategy, we developed a local and separable feedback for generating entanglement in a four-partite system. This thesis also investigates the production of single photons through the rephased amplified spontaneous emission (RASE) protocol. This protocol offers the promise of a stream of precisely shaped single photons, though it is plagued by efficiency issues. We develop a model of RASE and show that it overcomes the trade-off between efficiency and photon spacing by using different optical depths for the amplified spontaneous emission (ASE) preparation of the ensemble, and the RASE emission of single photons. By tailoring the density profile of the ensemble to mode-match the RASE emission of a single photon, we also able to eliminate reabsorption of the emitted photon, which would otherwise have limited the efficiency to 70%.
The quantum resources of entanglement and single photons are key to a range of quantum applications. These resources are challenging to produce cleanly and efficiently. This thesis investigates and improves methods of producing entanglement and single photons using measurement and control. To robustly produce entanglement, this thesis extends work by Carvalho et al., who developed a method for creating an entangled state of two atoms. This is done by coupling the atoms to a damped cavity, and using measurement of the output of the cavity to trigger a feedback pulse on the atoms. The robustness of this scheme against imperfect localisation of the two atoms is tested in this thesis, and limits are placed on the size of a trap that will still allow an entangled state to be produced. Additionally, using three-level (Lambda configuration) atoms slows the rate at which the system evolves, allowing more time for measurement and feedback to be applied, though it does not counteract the influence of spontaneous emission. In extending this work to multiple atoms we found that the system rapidly becomes more complex, and we developed a strategy for choosing a feedback pulse that stabilises a specific, highly entangled steady state of multiple particles. In addition to this general strategy, we developed a local and separable feedback for generating entanglement in a four-partite system. This thesis also investigates the production of single photons through the rephased amplified spontaneous emission (RASE) protocol. This protocol offers the promise of a stream of precisely shaped single photons, though it is plagued by efficiency issues. We develop a model of RASE and show that it overcomes the trade-off between efficiency and photon spacing by using different optical depths for the amplified spontaneous emission (ASE) preparation of the ensemble, and the RASE emission of single photons. By tailoring the density profile of the ensemble to mode-match the RASE emission of a single photon, we also able to eliminate reabsorption of the emitted photon, which would otherwise have limited the efficiency to 70%.
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 addresses challenges in the experimental realisation of optical quantum computing with atom-cavity platforms and develops techniques for experimental quantum control that generalise across a range of quantum modalities. Within the framework of cavity quantum electrodynamics (cQED), methods are developed and numerically validated for fast and precise control of single atoms to generate photonic states of interest. In particular, we consider the generation of high-repetition-rate n-photon bursts and time-bin entangled states. These photonic resources are essential for applications such as boson sampling, multi-photon interference, optical quantum computing, and quantum-enhanced metrology. Experimental improvements in coherent population transfer are demonstrated using adaptive pulse shaping with acousto-optic modulators. Beyond photon generation, the thesis introduces general-purpose pulse-level techniques, including pulse distortion correction, memory-efficient pulse compression, and machine learning-based optimisation. These methods are shown to be transferable to other platforms, such as neutral atom arrays and superconducting qubits. A particular contribution is the implementation of scalable pulse generation using fixed-point compressed pulse representations on FPGA hardware, achieving substantial memory savings while maintaining high fidelity in simulations of quantum operations. Additionally, reinforcement learning is presented as a powerful tool for optimising complex quantum dynamics, with state-of-the-art results demonstrated across various simulated systems. Throughout the thesis, theoretical developments are supported by numerical simulations and partial experimental validation, providing important insights for the development of future quantum technologies.
Abstract The central technological appeal of quantum science resides in exploiting quantum effects, such as entanglement, for a variety of applications, including computing, communication and sensing 1 . The overarching challenge in these fields is to address, control and protect systems of many qubits against decoherence 2 . Against this backdrop, optical photons, naturally robust and easy to manipulate, represent ideal qubit carriers. However, the most successful technique so far for creating photonic entanglement 3 is inherently probabilistic and, therefore, subject to severe scalability limitations. Here we report the implementation of a deterministic protocol 4–6 for the creation of photonic entanglement with a single memory atom in a cavity 7 . We interleave controlled single-photon emissions with tailored atomic qubit rotations to efficiently grow Greenberger–Horne–Zeilinger (GHZ) states 8 of up to 14 photons and linear cluster states 9 of up to 12 photons with a fidelity lower bounded by 76(6)% and 56(4)%, respectively. Thanks to a source-to-detection efficiency of 43.18(7)% per photon, we measure these large states about once every minute, which is orders of magnitude faster than in any previous experiment 3,10–13 . In the future, this rate could be increased even further, the scheme could be extended to two atoms in a cavity 14,15 or several sources could be quantum mechanically coupled 16 , to generate higher-dimensional cluster states 17 . Overcoming the limitations encountered by probabilistic schemes for photonic entanglement generation, our results may offer a way towards scalable measurement-based quantum computation 18,19 and communication 20,21 .
The possibility of creating a scaleable quantum network by interconverting photonic and atomic qubits shows great promise. The fundamental requirement for such a network is deterministic control over the emission and absorption of photons from single atoms. This thesis reports on the experi-mental construction of a photon source that can emit single-photons with arbitrary spatio-temporal shape, phase, and frequency. The photon source itself is a strongly-coupled atom cavity system based on a single 87 Rb atom within a macroscopic high-finesse Fabry-Perot cavity. It operates intermittently for periods of up to 100µs, with single-photon repetition rates of 1.0 MHz and an efficiency of almost 80%. Atoms are loaded into the cavity using an atomic fountain, with the upper turning point near the centre of the cavity mode. This ensures long interaction times without any disturbances introduced by trapping potentials. The photons’ indistinguishability was tested, with a two-photon Hong-Ou-Mandel visibility of 87%. This ability to both generate, and control, the photons’ properties, for example producing photons with symmetric or multi-peaked spatio-temporal shapes, allows for the production of photons in an n-time-bin superposition state where each time-bin has an arbitrarily defined amplitude and phase. These photons can be used as photonic qubits, qutrits and qquads, and their properties have been tested using a small linear-optics network.
We describe how feedback methods can be implemented to produce and stabilize entangled states of two atoms inside a cavity. Feedback control is used to engineer an effective environment dynamics, such that the entangled state to be protected is a steady state of the system. The control strategy is therefore robust with respect to external noises and fundamental decoherence sources, as spontaneous emission, for example. We also show that the scheme is mostly insensitive to practical imperfections, as detection inefficiencies and laser fluctuations.
The production of single photons using rephased amplified spontaneous emission is examined. This process produces single photons on demand with high efficiency by detecting the spontaneous emission from an atomic ensemble, then applying a population-inverting pulse to rephase the ensemble and produce a photon echo of the spontaneous emission events. The theoretical limits on the efficiency of the production are determined for several variants of the scheme. For an ensemble of uniform optical density, generating the initial spontaneous emission and its echo using transitions of different strengths is shown to produce single photons at 70% efficiency, limited by reabsorption. Tailoring the spatial and spectral density of the atomic ensemble is then shown to prevent reabsorption of the rephased photon, resulting in emission efficiency near unity.