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Quantum Broadband Optical Solid-State Memories for Large-Scale Photonic Networks

Periodic Reporting for period 1 - Quantum BOSS (Quantum Broadband Optical Solid-State Memories for Large-Scale Photonic Networks)

Reporting period: 2016-11-01 to 2018-10-31

Harnessing quantum physics has promised powerful and advanced technologies, such as guaranteed secure communications and computational processing beyond that of any classical computer. Photons – single quanta of light – present a promising platform to realise these quantum technologies. Common to all quantum platforms, the non-deterministic nature of generating, manipulating and measuring photons results in the ‘scaling catastrophe’ severely limiting the size of any composite system.

We address this scaling issue with the creation of a quantum optical memory. Such a device is capable of storing and recalling photons on demand, allowing for the synchronisation of heralded probabilistic processes via temporal multiplexing. The quantum memory can be used to store photons output from successful operations while the remaining components of the composite system are run. Once each component has successfully executed, the photons can be released from the QMs simultaneously and the next quantum task can proceed.

The main objective of this action was to produce resources for the building of a large-scale quantum photonic network. The proposed resources were (1) heralded single-photon sources, and (2) integrated chip-scale solid-state quantum memories. Importantly, the memories focused on efficiency as well as telecommunication compatibility – wavelength (1550nm), bandwidth (GHz) and propagation time (ms).

Benefits to society:
Large-scale quantum-based networks are well placed to benefit academic, industrial and commercial sectors, and indeed our society. Firstly, global area quantum networks would provide guaranteed secure communications based on quantum key distribution and quantum repeating. Therefore, this technology will revolutionise online security, allowing for unconditionally secure monetary transactions in banking, retail, and the stock market. With cybercrime and identity theft a continental threat, this technology would have a palpable impact on people's quality of life and furthermore, would have a major impact on the effectiveness of intelligence services in combating terrorism.

Secondly, local area photonic networks could lead to quantum computers that offer exponential speed-ups over classical devices. This technology has the potential to transform methods of research in the healthcare, pharmaceutical and green energy sectors. It could assist epidemiology and genetic research, cut costs in medication design, and help improve artificial light-harvesting devices for alternative energy sources, for example by permitting simulations of efficient photosynthesis. This technology will have an immediate impact on any research and innovation at the molecular level, where classical computers are ineffectual and time-consuming, leaving expensive empirical methods the only option. In the next coming decades, these emergent quantum-enhanced technologies will foster the generation of IP, the creation of spin-out companies and jobs, contributing to the productivity of the UK economy, and competitiveness globally.
The report covers the entire period of the project. The action produced three main outcomes toward the realisation of an efficient, telecommunication compatible, integrated quantum memory. Below is a brief overview of the main results, more details found in the technical report.

(1) Noise-free Quantum Memory: The storage and recall of single photons was achieved with a ladder protocol using a warm caesium vapour platform. The quantum characteristics of the single photons were retained after storage. These results resulted in a publication (Physical Review A 97, 042316 (2018)), and conference several conference presentations (QIM2017, CLEO Europe 2017, CEWQO2017, FIO2017).

(2) Modified Broadband Raman Memory: An order of magnitude improvement on the noise performance of the Raman memory with caesium vapour was achieved with a simple modification of the protocol. Measurements of the second-order auto-correlation function with weak coherent state inputs, together with a theoretical model, suggest nonclassicality can be retained when storing single photons that have been heralded at the input of the memory with 27% efficiency. These results are to be published.

(3) Broadband Solid-State Memory: A broadband atomic frequency comb protocol was implemented using the inhomogeneously broadened line of praseodymium doped yttrium orthosilicate. Pulses in the GHz band were stored and recalled with a fixed delay. These results have been presented at ICAP2018 are to be published
The three main results that are mentioned above all go beyond the state of the art.
(1) The quantum memory was measured to have the lowest noise floor of light-mater based memories with on demand recall. The noise was measured to be consistent with the dark counts of the detection system. This performance allowed for the preservation of the second-order autocorrelation function of the heralded single photon source. To my knowledge there is no other light-matter based quantum memory that has this noise performance.
(2) The order of magnitude improvement on the noise performance of the Raman memory can allow of the storage and recall of single photons while retaining the nonclassicality. This goes well beyond the previous implementations of the Raman protocol.
(3) Storing broadband light pulses using the atomic frequency comb protocol in praseodymium doped yttrium orthosilicate is a world’s first. The ultimate goal would be to combine this protocol with the inherently noise-free one of the first project.

These results have been exploited to deliver impact to the scientific community through publications and conference presentations. While the socio-economic impact of these results are so-far minimal, these results pave the way for an efficient and telecommunication compatible quantum memory, of which the potential impacts are large and summarised in the first section of this report.
Schematic of a Quantum Memory for Light.