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Memory-enhanced photonic quantum information processing

Final Report Summary - MEQUIP (Memory-enhanced photonic quantum information processing)

Optical fields provide a unique resource for quantum information science. In theory, quantum states of light can be used to transmit quantum information over long distances, construct sensors with unmatched precision, and achieve universal quantum computation. Despite this potential, the realized complexity of optical quantum processors to date is very modest. This complexity is critically restricted by the scalability of quantum photonics. MEQUIP addresses the two main challenges facing scalable quantum photonics. First, effective interactions using linear optics are measurement based, and thus inherently probabilistic. MEQUIP develops the use of quantum memories as a way to implement multiplexing with feed-forward control, a method that renders probabilistic routines deterministic. Second, practical scalability requires robust, high-performance components that are easy to fabricate and occupy a small footprint. To this end, MEQUIP develops efficient guided-wave implementations of essential photonic components: sources of single photons, programmable multiport interferometers, and single-photon detectors. In parallel with these scientific aims, MEQUIP provides a training framework for the researcher to gain expertise in key areas of research and technology and acquire skills and experience needed to develop a career as an independent and mature research scientist.

The research and training components of MEQUIP are directed to three training objectives and four science objectives. The training objectivess are for the researcher to (T1) gain expertise in quantum memories and their applications to quantum information processing (QIP), (T2) acquire broad expertise in quantum photonics, and (T3) attain extensive training in diverse applications of quantum optics to quantum information science. The research objectives concern (S1) synchronisation of heralded single-photon sources with quantum memories, (S2) construction of a guided-wave quantum photonics platform, (S3) demonstration of light-matter interference in a Raman memory, and (S4) development of QIP protocols that use quantum memories and time-frequency encoding.

Throughout MEQUIP, the research fellow (RF) has been integrated into the Ultrafast Quantum Optics Group (UFQO), led by the scientist in charge (SIC) Prof. Ian Walmsley. Within the UFQO, the RF has led the Quantum Photonics Subgroup, a team of currently 11 researchers and students, and collaborated closely with the Quantum Memories team led by Dr Josh Nunn. Activities have covered all aspects of experimental research, including formulating research goals, experimental design, apparatus construction, data analysis, and results dissemination. This has been carried out by the RF both at an individual and collaborative level, including co-supervision of doctoral students with the SIC. Collaboration with external research partners has also played a significant role in the project. Primary examples involve custom fabrication of photonic devices with Prof. Smith at the University of Southampton and developing the theory of multi-parameter quantum metrology with Prof. Kim at Imperial College London.

The training outcomes of MEQUIP have prepared the RF for an evolving research career in quantum optics and optical quantum information. An expertise in quantum memories (T1) has been gained through work with the Quantum Memories research team, including the design, construction, and use of two novel Raman memories. Uses of quantum memories in QIP have also been explored, leading to two publications on novel, and efficient, use of time-frequency encoding for QIP. A broad expertise in quantum photonics (T2) has been acquired through work leading the Quantum Photonics research team, including engagement with complementary research projects such as the EPSRC Programme BLOQS and the UK Quantum Research Hub NQIT. These activities, along with research toward objective S2, have led to a wide understanding of quantum photonics by the RF, as indicated by publications on integrated light sources, optical circuits, and photodetectors. Furthermore, training in the diverse applications of quantum optics to quantum information science (S3) has been achieved through wide participation in the UFQO, Quantum Science research community at Oxford, as well as collaborative research projects on optical quantum information that extend beyond Oxford. Through these activities, the RF has learned various aspects of quantum computation and metrology, as indicated by publications spanning both of these subfields of quantum information science.

The scientific outcomes of MEQUIP have provided new tools and methods for scalable quantum photonics and its application to quantum information science. Two key results address fundamental and practical challenges to scalability. First, a demonstration of the synchronisation of a heralded single photon with a quantum memory (S1), shows the potential for quantum memories to effectively scale probabilistic quantum routines. Second, the development of integrated components for optical quantum information – including guided-wave memories, sources, circuits, and detectors – has contributed to a practical platform for larger systems (S2). Beyond their role in synchronisation, the potential for processing information directly in memories has been investigated by studying the quantum interference of optical and material excitations during the memory operation (S3). Lastly, two novel QIP protocols that efficiently leverage the manipulation of time-frequency modes, made possible by quantum memories, have been developed (S4).

The primary impact of MEQUIP concerns academic research. The project has delivered tools and methods that enable new research in quantum photonics and optical quantum information. Advances made in MEQUIP have already led to scientific opportunities being explored by new collaborative projects. These include the use of quantum memories within the EPSRC Programme BLOQS, the use of a SFWM source arrays within the EC Project QUCHIP, the characterisation of photon sources in ANR Project SemiQuantRoom, and the investigation of spectral multiplexing within the UK Quantum Hub NQIT. A second direct impact comes from training elements of MEQUIP. The researcher is now in a strengthened position to develop a research career in quantum photonics with increasing independence. Within MEQUIP, the researcher also co-supervised doctoral students, which contributes to the future workforce in both quantum optics research but also the emerging quantum technology industry. Lastly, outreach activities of MEQUIP have focused on public engagement and school education. These activities have contributed to the public understand of quantum science.