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New Journal of Physics

Generation of time-bin-encoded photons in an ion-cavity system

T. V. Ward, Matthias Keller

Published Dec 1, 2022• 11 citations
Publisher

Abstract

Abstract We investigate two single-photon generation schemes and compare their suitability for use in time-bin entanglement encoding. A trapped ion coupled to an optical cavity produces single photons through a cavity-assisted Raman transition. By manipulating the phase relationship between time-bins of successive photons, distinct features in the interference pattern of a Hong–Ou–Mandel measurement emerge. Through careful selection of the initial state, detrimental effects of spontaneous emission can be significantly reduced. We demonstrate that this reduction allows us to impart a measurable phase profile onto the emitted photons making time-bin entanglement encoding feasible with an ion-cavity system.

DOI
10.1088/1367-2630/aca9ee

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Abstract We investigate two single-photon generation schemes and compare their suitability for use in time-bin entanglement encoding. A trapped ion coupled to an optical cavity produces single photons through a cavity-assisted Raman transition. By manipulating the phase relationship between time-bins of successive photons, distinct features in the interference pattern of a Hong–Ou–Mandel measurement emerge. Through careful selection of the initial state, detrimental effects of spontaneous emission can be significantly reduced. We demonstrate that this reduction allows us to impart a measurable phase profile onto the emitted photons making time-bin entanglement encoding feasible with an ion-cavity system.
We investigate two schemes for generating indistinguishable single photons, a key feature of quantum networks, from a trapped ion coupled to an optical cavity. Through selection of the initial state in a cavity-assisted Raman transition, we suppress the detrimental effects of spontaneous emission on the photon's coherence length, measuring a visibility of 81(2)81(2)%81(2) without subtraction of background counts in a Hong-Ou-Mandel interference measurement, the highest reported for an ion-cavity system. In comparison, a visibility of 50(2)% was measured using a more conventional single-photon scheme. We demonstrate through numerical analysis of the single-photon generation process that our scheme produces photons of a given indistinguishability with a greater efficiency than the conventional one. Single-photon schemes such as the one demonstrated here have applications in distributed quantum computing and communications, which rely on high-fidelity entanglement swapping and state transfer through indistinguishable single photons.
We present a scheme to realize a deterministic solid state source of time-bin entangled photon pairs using cavity-assisted stimulated Raman adiabatic passage (STIRAP) in a single quantum dot. The quantum dot is embedded inside a semiconductor cavity, and the interaction of a coherent superposition of two temporally separated input pulses and the cavity mode leads to a two-photon Raman transition, which produces a time-bin entangled photon pair through the biexciton-exciton cascade. We show that the entanglement of the generated state can be measured using triple coincidence detection, and the degree of entanglement is quantified as the visibility of the interference. We also discuss the effect of pure dephasing on entanglement of the generated photon pair. Pronounced interference visibility values of greater than 1/21/\sqrt{2}1/2​ are demonstrated in triple coincidence measurement using experimentally achievable parameters, thus demonstrating that the generated photons are suitable for applications with Bell's inequality violation and quantum cryptography.
Control over the internal states of trapped ions makes them the ideal system to generate single and two-photon states. Coupling a single ion to an optical cavity enables efficient emission of single photons into a single spatial mode and grants control over their temporal shape, phase and frequency. Using the long coherence time of the ion's internal states and employing a scheme to protect the coherence of the ion-cavity interaction, we demonstrate the generation of a two-photon entangled state with full control over the phase. Initially, ion-photon entanglement is generated. A second photon is subsequently generated, mapping the ion's state onto the second photon. By adjusting the drive field the phase of the entangled state can be fully controlled. We implement this scheme in the most resource efficient way by utilizing a single 40^{40}40Ca+^++ ion coupled to an optical cavity and demonstrate the generation of a two-photon entangled stated with full phase control with a fidelity of up to 82\%.
Quantum networking is an established and growing field of research, with applications in many other fields of quantum technology, as well as fundamental research in quantum mechanics. A quantum network consists of multiple quantum devices that can interchange quantum information or be entangled with each other. Trapped ions coupled to optical cavities are among the most promising platforms in quantum technology broadly and networking specifically, with the ability to initialise and store quantum states in the ions and, via the cavity, transfer those states onto photons, which can interact with distant systems. This thesis describes two experiments using an ion trap with integrated high finesse optical cavity as a source of single photons. In the first, the photons were converted to a standard telecommunications wavelength through quantum frequency conversion and transmitted over 10 km of optical fibre. The attenuation experienced by the photons in optical fibre without frequency conversion would make transmission over such a long distance impractical. The converted photons were shown to maintain their nonclassical statistics, meaning such a system could be used to create quantum networks over long distances. A novel scheme for producing single photons from the ion-cavity system was demonstrated. This scheme significantly reduces the impact of decoherence in the system compared to commonly used schemes through selection of the initial state of the ion. This scheme may be directly applied to ion-photon entanglement and would lead to improved fidelity in ion-ion entanglement or state transfer. Both experiments represent significant steps towards the building of quantum networks based on ions coupled to optical cavities. They lay the groundwork for future experiments using the same system to entangle ions with photons and eventually with ions in other traps.
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.
Quantum states encoded in the time-bin degree of freedom of photons represent a fundamental resource for quantum information protocols. Traditional methods for generating and measuring time-bin-encoded quantum states face severe challenges due to optical instabilities, complex setups, and timing resolution requirements. To circumvent these issues, we leverage an approach based on Hong-Ou-Mandel interference and we propose and demonstrate a robust and scalable protocol to generate and measure arbitrary high-dimensional time-bin quantum states. We experimentally implement the protocol in a photonic setup reaching high-fidelity quantum state tomographies of two- and three-dimensional quantum states encoded in time bins with short temporal separation. We also certify intrasystem polarization-time entanglement of single photons through a nonclassicality test. The demonstrated approach enables access to high-dimensional states and tasks that are practically inaccessible with standard schemes, thereby advancing fundamental quantum information science and opening applications in quantum communication.