United Kingdom

Quantum networks are a new paradigm of complex networks, allowing us to harness networked quantum technologies and to develop a quantum internet. But how robust is a quantum network when its links and nodes start failing? We show that quantum networks based on typical noisy quantum-repeater nodes are prone to discontinuous phase transitions with respect to the random loss of operating links and nodes, abruptly compromising the connectivity of the network, and thus significantly limiting the reach of its operation. Furthermore, we determine the critical quantum-repeater efficiency necessary to avoid this catastrophic loss of connectivity as a function of the network topology, the network size, and the distribution of entanglement in the network. In particular, our results indicate that a scale-free topology is a crucial design principle to establish a robust large-scale quantum internet.

Bristol Quantum Information Technologies Workshop

Robustness of Noisy Quantum Networks

percolation theory

phase transitions

complex quantum networks

quantum internet

**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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poster

The premise for this study is that quantum computing/simulation techniques can be integrated with the latest artificial intelligence (AI)/machine learning (ML) methods to deliver hybrid quantum/classical solutions for global crisis response management and war-gaming tasks. This capability would enable not only unparalleled fidelity in characterising global crisis response scenarios, but also the ability to 'war-game' vast numbers of scenarios, to find optimum responses to rapidly changing crisis situations. By exploiting these quantum techniques, we believe that these hybrid solutions could eventually help crisis responders/managers to refine optimised response plans dynamically, not just for the current crisis (e.g. global pandemic), but also for subsequent global economic and security crises that might result; enabling critical resources to be deployed more effectively to maximise operational benefits. They could also help crisis policy makers to refine policy to better prepare for global crises by war-gaming huge numbers of crisis response scenarios, and to help them determine how best to prevent and mitigate future global crises (e.g. public health, economic, security).

This feasibility study will explore which parts of the complex crisis management process could benefit from quantum algorithms, especially in representing the space of possible response plans to global crisis scenarios, and wargaming which responses deliver the most optimum response to diverse global scenarios. The premise is that quantum computers could deliver orders of magnitude improvement in processing power, that could enable global crises to be managed more effectively, before they happen!

Global crisis response management and wargaming using quantum computing/simulation techniques

We present the simulations of optical waveguide arrays used as photonic quantum simulators of a Heisenberg spin-chain in a silicon nitride photonic chip. The photonic lattices were manufactured using electron beam lithography and reactive ion etching process to obtain ridge waveguides. The photonic devices are currently in process of characterization with a CW laser of 780 nm, where the light is coupled vertically with a single mode fiber into Bragg gratings optimized for a wavelength of 800 nm. Finally, the dynamics of the system can be determined by measuring the output of the lattice and choosing the initial state with the input port of the array of waveguides.

Integrated Nanophotonic Waveguide Lattices as Photonic Quantum Simulators

We proposed a new constrained quantum optimization algorithm (CQOA) inspired by the quantum extreme value Searching algorithm (QEVSA) [1]. The QEVSA finds the extreme value (minimum or maximum) of an unconstraint goal function. It combines the well-known binary searching algorithm and the quantum existence testing [QET] which is a special case of a quantum counting algorithm. if one is interested in finding the extreme value of a constraint goal function, an extension of the functionalities of the QEVSA is required, in other words, a new extended version of the QET is needed, we name it constraint quantum relation testing (CQRT). The QET exploits the power of the quantum phase estimation (QPE) [2] for computing the necessary qubits for classical and quantum certainty of the physical implementation. Therefore, involving the constraint and the index relation which determines the type of optimization (minimum or maximum) in the QET does not modify the working mechanism of the QPE. Consequently, the optimum number of qubits necessary for the CQRT is similar to the QET function i.e., the computational complexities of QET and CQRT are equal. The CQOA merges the binary search algorithm and the CQRT function.

[1] S. Imre, ”Quantum Existence Testing and Its Application for Finding Extreme Values in Unsorted Databases” IEEE Transactions on Computers, Vol: 56, Issue: 5, May 2007.
[2] Imre, S.; Balázs, F.: Quantum computing and communications: An engineering approach (Wiley, Chichester, UK) 2005 ISBN 0-470-86902-X, DOI: 10.1002/9780470869048.


Constrained Quantum Optimization Strategy

Quantum machine learning is a research subject that is becoming increasingly relevant within the near-term quantum computation. Quantum neural networks are a well-known example for learning tasks in both qubit and qumode-based systems. Stochastic Gradient Descent (SGD) methods have been introduced in recent work for various variational quantum circuits but only in qubit-based architectures. In this work, we present different variants of the SGD method in the continuous-variable framework via the example application of photonic quantum neural networks for regression models. Using only a low number of shots, we see a nicely converging training process using simulated circuits on StrawberryFields [3] photonic simulator and its GPU enabled TensorFlow backend. Regression results are found best on 2-qumode circuits for the presented test function approximations and the effect of further stochastically sampling with mini-batches during the training process.​

Continuous-variable stochastic gradient descent methods applied for
a photonic quantum neural network

Delivering quantum optical technologies at scale will also require the scaling and integration of classical readout and control hardware. To date, silicon quantum photonics has not fully harnessed the performance of the complimentary metal-oxide-semiconductor (CMOS) electronics industry and typically relies on discrete, PCB scale, packaged hardware. We report an integrated homodyne detector consisting of a silicon photonic integrated circuit wirebonded directly to a dedicated silicon-germanium transimpedance amplifier die. We measure a bandwidth of 1.7 GHz, 84% efficiency for on-chip states, shot noise clearance across 9 GHz with a device footprint of 0.84 mm2. Using a fibre coupled PPLN source, we measure up to 0.89 dB of quadrature squeezing on-chip and characterise the squeezing across the entire 9 GHz bandwidth available. In addition, we present some new results further demonstrating the performance advances available from photonic-electronic integration.

Interfacing silicon photonics and
electronics for high performance
squeezed light detection

We present the numerical simulations for the design and manufacture of the basic components to develop a fully integrated photonic circuit on a silicon nitride platform for the generation of a frequency-degenerate four-photon entangled state, centering the attention on the design of thermal phase tuners. Finally, we manufactured the devices using electron beam lithography for waveguide-based structures. Additionally, the thermal phase tuners were implemented in a Mach-Zehnder interferometer.



Basic component design for a photonic integrated circuit that generate a four photon entangled state

Integrated quantum photonics is a promising avenue for scaling up quantum optics beyond the proof of principle demonstrations. Such a future requires on-chip reconfigurable photonics where the integration with single-photon detectors remains a significant challenge. Heat as a standard method to tune photonic circuits makes superconducting detectors challenging. This work shows the cointegration of microelectromechanical reconfiguration with on-chip superconducting single-photon detectors (SSPD). As a demonstration of principle, we perform high-extinction routing, high-dynamic-range single-photon detection and power stabilization. This platform allows for heat-load free reconfiguration and all-integrated quantum optics experiments.

Published Work:
Reconfigurable photonics with on-chip single-photon detectors (doi.org/10.1038/s41467-021-21624-3) (2021)
Samuel Gyger*, Julien Zichi*, Lucas Schweickert*, Ali W. Elshaari*, Stephan Steinhauer*, Saimon F. Covre da Silva^, Armando Rastelli^, Val Zwiller*, Klaus D. Jöns*, Carlos Errando-Herranz*
* KTH Royal Institute of Technology
^ JKU Linz

Reconfigurable photonics with on-chip single-photon detectors

We give a protocol for ghost imaging in a way that is always counterfactual - while imaging an object, no light interacts with that object. This extends the idea of counterfactuality beyond communication, showing how this interesting phenomenon can be leveraged for metrology. Given, in the infinite limit, no photons ever go to the imaged object, it presents a method of imaging even the most light-sensitive of objects without damaging them. Even when not in the infinite limit, it still provides a many-fold improvement in visibility and signal-to-noise ratio over previous protocols, with over an order of magnitude reduction in absorbed intensity.

Ghost Imaging Counterfactually

We study the entanglement of formation and the quantum discord contained in even and odd multipartite j-spin coherent states. The key element of this investigation is the fact that a single j-spin coherent state is viewed as comprising 2j qubit states. We compute the quantum correlations present in the n even and odd j-spin coherent states by considering all possible bipartite splits of the multipartite system. We discuss the dierent bipartition schemes of quantum systems and we examine in detail the conservation rules governing the distribution of quantum correlations between the dierent qubits of the multipartite system. Finally, we derive the explicit expressions of quantum correlations present in even and odd spin coherent states decomposed in four spin sub-systems. We also analyze the properties of monogamy and we show in particular that the entanglement of the formation and the quantum discord obey the relation of monogamy only for even multipartite j-spin coherent states.

Entanglement of formation and quantum discord in multipartite j-spin coherent states

Satellite-based quantum key distribution enables quantum-secured communication on a global scale. There are international efforts underway to build and launch satellites capable of Quantum Key Distribution (‘QKD’) e.g. the QUARC, QEYSSat, Micius and SpooQy-1 missions. 
These QKD satellites work with a network of Optical Ground Stations (OGSs) to receive optical signals and distribute cryptographic keys to end users. However, OGSs near metropolitan centres are often subject to high levels of light pollution and suffer from restricted clear lines of sight to the satellite. Further, static ground stations are severely limited by local meteorological conditions. A mobile OGS is not subject to these restrictions and can be used to distribute keys flexibly. This may be useful when performing site surveys in order to derive realistic data on expected key rates and meteorological/geographic conditions. We present our progress towards field-demonstrations of QKD using our mobile OGS to receive both a beacon signal and quantum signal. This OGS is designed to work with our industry lead 6U and research focused 12 U QKD CubeSats.


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Robustness of Noisy Quantum Networks

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