United Kingdom

High-dimensional quantum entanglement can give rise to stronger forms of nonlocal correlations compared to qubit systems. Beyond being of fundamental interest, this offers significant advantages for quantum information processing. The problem of certifying these stronger correlations, however, remains an important challenge, in particular in an experimental setting. Here we theoretically formalise and experimentally demonstrate a notion of genuine high-dimensional quantum nonlocal steering. We show that high-dimensional entanglement combined with judiciously chosen local measurements can lead to a stronger form of steering, provably impossible to obtain via entanglement in lower dimensions. Exploiting the connection between steering and incompatibility of quantum measurements, we derive simple two-setting steering inequalities for certifying the presence of genuine high-dimensional steering. We report the experimental violation of these inequalities using macro-pixel photon-pair entanglement certifying genuine high-dimensional steering in dimensions up to d=15. Our work paves the way for the characterisation and certification of quantum nonlocal correlations in high-dimensional systems.

Bristol Quantum Information Technologies Workshop

Genuine High-Dimensional Quantum Steering

quantum steering

high-dimensional entanglement

quantum information

**BQIT is a three day annual workshop** aimed at enabling leading UK and international academics and industrial partners to come together; to explore and discuss future ambitions and challenges in the field of **Quantum Information Technologies.** Since our conception in 2014 we have had 230 speakers and panellists who have presented their work and opinions on a range of topics, from quantum theory to innovation in industry.


### Welcome Message
The BQIT:21 team welcomes you to the eighth Bristol Quantum Information Technologies workshop! 

Last year was the first time in our workshop's history that we had gone fully virtual. The global circumstances that continue to impact us all professionally and personally have forced BQIT once again to be delivered as an all-virtual workshop for 2021. The access this creates to colleagues across the world who would otherwise be unable to join us, as well as being able to make the workshop free for all attendees due to the lower costs involved in delivering an online workshop, are just a couple of the extraordinary benefits of meeting online. This also means that we can welcome three times as many attendees to BQIT than previous years.   

We take this opportunity to thank each of our speakers and poster presenters for contributing their technical content, this year's sponsors, whose support this year enabled BQIT:21 to be free to all attendees, and to all of our attendees for joining us online this year. We recognise the extra energy required to stay connected and motivated during these difficult times, and are hugely encouraged to see so many of you willing to continue to connect with us on an online platform. 

It is our hope that you have a productive and enjoyable time while you engage and interact with BQIT:21, and to everyone that has helped take BQIT:21 online, thank you! 

**The BQIT board and organisation team **


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### This year's workshop agenda is available here: [Virtual BQIT:21](https://drive.google.com/file/d/1QwsSM0ztnxC_lgUKf3TbO3GvnI44AQkY/view?usp=sharing)
For further information, please contact us at: **bqit-admin@bristol.ac.uk**

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technical paper

To support the development of a U.K. quantum technology industry, NPL has been developing techniques to perform accurate and precise measurements of key parameters of components used in quantum photonic systems. This resulted in the first standardised methods for quantifying the performance of quantum communications hardware and is important for security evaluation.
Attenuated coherent states are used in current hardware, but a single-photon source based on a single emitter could be more efficient. The emission from these single-photon sources is frequently characterised by the second-order correlation, g2(τ). A g2(0) < 0.5 indicates the channel consists of mostly single photons. Reported values of g2(0) never reach zero, as expected for a perfect single-photon source; these measurements cannot definitively ascribe the additional coincidences at τ = 0 to an uncorrelated background, or a second dimmer single-photon source. Higher order correlation functions provide additional discrimination between these contributions. 
We have performed g3(τ) measurements on NV centres in nanodiamond. We have developed a model which predicts the form of the measured histograms and have found that it fits the data well. In one example these results show no three-fold coincidences detected over an 18-hour period. This allows us to ascribe what fraction of the additional coincidences at g2(0) are due to a second single-photon source. 
This technique has immediate applicability for optimising single-photon sources, which can enable a range of photonic quantum technologies.

Photon statistics of single photon sources

Quantum dots embedded in photonic nanostructures offer a highly efficient and coherent deterministic photon-emitter interface [1]. It constitutes an on-demand single-photon source for quantum-information applications, enables single-photon nonlinear, optics and the constructing of deterministic quantum gates for photons [2]. We review recent experimental progress, and demonstrate that the current technology can be scaled up to reach quantum advantage [3] with the demonstration of near-transform-limited emitters in high-cooperativity planar nanophotonic waveguides [4]. The coherent control of a single spin in the quantum dot [5, 6] offers additional opportunities of generating advanced multi-photon entangled states [7]. We discuss potential applications of these novel deterministic photonic hardware in quantum computing and quantum communication [8], e.g., for constructing a resource efficient one-way quantum repeater [9].

References

[1] Lodahl et al., Rev. Mod. Phys. 87, 347 (2015).
[2] Lodahl, Quantum Science and Technology 3, 013001 (2018).
[3] Uppu et al., Science Advances 6, eabc8268 (2020).
[4] Pedersen et al., ACS Photonics (2020).
[5] Javadi et al., Nature Nanotechnology 13, 398 (2018).
[6] Appel et al., Phys. Rev. Lett. 126, 013602 (2021). 
[7] Tiurev et al., Arxiv: 2007.09295.
[8] Uppu et al., Arxiv: 2103.01110.
[9] Borregaard et al., Phys. Rev. X 10, 021071 (2020).


Single-photon quantum hardware: towards scalable photonic quantum technology with a quantum advantage

Solid state atom-like systems have great promise for building quantum networks at scale but are burdened by phonon sidebands and broadening due to surface charges. Nevertheless, coupling to a small mode volume cavity would allow high rates of extraction from even highly dephased emitters. Recently, low-noise silicon nitride has been demonstrated as platform compatible with single spin and single photon manipulation [1]. We consider the nitrogen vacancy centre in diamond, a system understood to have a poor quantum optics interface with highly distinguishable photons, and design a silicon nitride cavity that allows 99 % efficient extraction of photons at 200 K with an indistinguishability of > 50 %, improvable by external filtering. We analyse our design using FDTD simulations, and treat optical emission using a cavity QED master equation valid at and beyond strong coupling and which includes both ZPL broadening and sideband emission [2]. The design is compact (< 10 um), and owing to its planar geometry, can be fabricated using standard silicon processes. Our work therefore points towards scalable fabrication of non-cryogenic atom-like efficient sources of indistinguishable photons.

[1] JA Smith, J Monroy-Ruz, JG Rarity, KC Balram. "Single photon emission and single spin coherence of a nitrogen vacancy centre encapsulated in silicon nitride" Appl. Phys. Lett. 116, 134001 (2020)

[2] JA Smith, C Clear, KC Balram, DPS McCutcheon, JG Rarity. "NV centre coupled to an ultrasmall mode volume cavity: a high efficiency source of indistinguishable photons at 200 K" accepted for publication in Physical Review Applied (2021)

Hybridising NV centres with silicon nitride photonics for high efficiency sources of indistinguishable photons at 200 K

Integrated micro- and nanostructures allow for the efficient generation of photon pairs via parametric fluorescence thanks to the enhancement of the light-matter interaction associated with the spatial and temporal light confinement. These systems grant an unprecedented control over the properties of the generated non-classical light, as their spectral properties can be designed to enhance or suppress specific nonlinear processes. An interesting case is that of structures composed of resonant elements that are linearly independent but are coupled through the nonlinear interaction. In this case one is able to control the spectral position of one or more resonances independently, resulting in a great flexibility and resilience. In this talk I will provide some examples connected with the generation of photon pairs by spontaneous four-wave mixing or spontaneous parametric down conversion.

Parametric fluorescence in linearly uncoupled resonators

Nonlinear nanophotonic technologies can enable an unprecedented ability to connect regions of the optical spectrum together. In this talk, I will discuss our laboratory’s efforts in developing nonlinear photonic resonators in the silicon nitride platform for both quantum and classical photonic links. I will review the basic design and engineering principles behind these devices, and discuss their application in quantum frequency conversion of single photons, visible-telecom entangled photon pair generation, the generation of visible laser light from near-infrared pump sources, and the use of nonlinear resonators in optical atomic clocks.

Connecting widely separated optical frequencies using chip-integrated nonlinear microresonators

To investigate the fundamental nature of matter and its interactions, particles are accelerated to very high energies and collided within detectors, producing a multitude of other particles that are scattered in all directions. These particles leave signals of their passage while transversing the detector. The problem of track reconstruction is to recover the original trajectories from these signals. This represents a very demanding data analysis problem, and it will become even more so as the luminosity of the next generation accelerators keeps increasing.
Approaches to track reconstruction based on quantum information technologies have recently been put forward. In order to access the potential for a quantum speedup for this problem, we start by elaborating a computational complexity analysis of a standard tracking algorithm, the Combinatorial Track Finder. First, we find that it is possible to improve the classical complexity of the original algorithm.
We further show that we can use quantum search algorithms to reach a lower quantum computational complexity. To the best of our knowledge, this is the first computational complexity-level demonstration of a quantum advantage for a state-of-the-art track reconstruction method.
Work developed in collaboration with A.Ambainis, P.Bargassa, M.Dimitrijevs, A.Glos, M.Kālis, A.Kumar, A.Locāns, Y.Omar, S.Pratapsi, G.Quinta, A.Rivošs, J.Seixas.

Investigating Quantum Speedup for Track Reconstruction: Classical and Quantum Computational Complexity Analysis

Time-correlated single-photon counting (TCSPC) has been established as the preferred detection approach for high performance lidar and depth profiling due to its excellent surface to surface resolution and high optical sensitivity. 
This presentation will show the results of laboratory-based experiments obtained using different optical transceiver configurations. Particular attention will be given to underwater depth imaging using an individual commercially available silicon single photon avalanche diode (Si-SPAD) detectors, and prototypes of Si-SPAD detector arrays fabricated in complementary metal-oxide semiconductor (CMOS) or in custom fabrication technology. 
Depth and intensity profiles of stationary and moving targets were obtained under several underwater scattering conditions. The spatial resolution of each system was investigated in order to provide an estimation of the image degradation as the level of scattering increased. Data were analyzed using a pixel wise cross correlation approach, however additional bespoke image processing approaches will be discussed.

Underwater single-photon depth imaging

On-chip integration of single-photon detectors and reconfigurable optical circuits is a crucial step toward a fully scalable approach to quantum photonic technologies. By confining light inside lithographically patterned waveguides, single-photons can be actively routed and interfered in miniaturized reconfigurable optical networks, and their state can be read out with on-chip detectors. Integration of these two key elements on a common platform enhances the scalability of quantum photonic devices by minimizing their footprint and eliminating the need of lossy interconnects between separated optical systems. I will present recent progress on realizing cryo-compatible reconfigurable photonic circuits using hybrid integration. Our design is based on superconducting nanowire detectors for measuring single photon events combined with either electro-optomechanical or electro-optical phase shifters embedded in on-chip interferometers. These concepts enable the realization of programmable photonic circuits that feature low dissipation loss.

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