United Kingdom

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.



Bristol Quantum Information Technologies Workshop

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

electron-beam lithography

thermal tuners

photonic integrated circuits

**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

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.


Field Demonstration of A Mobile Optical Ground Station for CubeSat Quantum Key Distribution

A workflow for evaluation of entanglement source quality is proposed. Based on quantum state density matri-ces obtained from theoretical models and experimental data, we make an estimate of a potential performance of a quantum entanglement source in quantum key distribution protocols. This workflow is showcased for continuously pumped spontaneous parametric down-conversion (SPDC) source, where it highlights the trade-off between entangled pair generation rate and entanglement quality caused by multiphoton nature of the generated quantum states. We employ this characterization technique to show that secure key rate of down-converted photon pairs is limited to 0.029 bits per detection window due to intrinsic multiphoton contributions. We also report that there exists one optimum gain for continuous-wave down-conversion sources. We find a bound for secure key rate extracted from SPDC sources and make a comparison with perfectly single-pair quantum states, such as those produced by quantum dots.

Effect of source statistics on utilizing photon entanglement in quantum key distribution

We studied the behavior of tunable one-dimensional discrete-time quantum walk (DTQW) in the presence of decoherence, modeled as a noisy quantum channel with bit-flip, phase-flip, and general amplitude damping effects. By tuning the noise level for the respective noise channel, we observed the emergence of the broad class of transport behavior: quantum walk, quantum-classical walk, and classical walk. The robustness of the walk under noise is captured by the scaling exponents. We showed the walk under maximum decoherence from the general damping amplitude channel maintained the robust ballistic spread while retaining the Gaussian-like distribution. This work may be applicable in the optimization or tuning of the quantum walk under noisy channels. 


Scaling exponent analysis of the tunable discrete quantum walk in the noisy channel

Recently, the first integrated Measurement Device Independent Quantum Key Distribution (MDI QKD) system has been implemented here in Bristol. To build on this result and work towards improved security and key rates, a new indium phosphide (InP) transmitter chip has been designed for a second-generation MDI QKD implementation. The new chip contains two laser sources, including a distributed feedback laser to allow for faster pulsing and high-speed phase modulators with a bandwidth of up to 30 GHz. With the new lasers and phase modulators a higher pulse rate will be achieved, leading to better key rates. Additionally, an on-chip photodiode can be used to monitor incoming light. This makes the chip much more resilient against hacking attacks on the receiver side. Since MDI QKD is intrinsically protected against detector attacks, this means that this new MDI QKD system will show great security overall.

An improved chip for MDI QKD

Photons are natural carriers of high-dimensional quantum information, and the encoded qudits can benefit from higher quantum information capacity and noise-resilience. However, schemes to generate the resources needed for high-dimensional quantum computing have so far not been demonstrated for linear optics. Here, we show how to generate GHZ states of arbitrary numbers of photons in arbitrary dimensions using destructive interference in linear optical circuits described by Fourier matrices. We combine our results with recent schemes for qudit Bell measurements to show that universal linear optical quantum computing can be performed in arbitrary dimensions.

A scheme for universal high-dimensional quantum computation with linear optics

Natural language processing (NLP) problems are ubiquitous in classical computing, where they often require millions of dollars on computational resources and billions of parameters to infer sentence meanings. It is not efficient classically to improve these state-of-the-art language models and this means that the required computational resources are expected to scale exponentially with the increase in parameters. With the appearance of quantum computing hardware and simulators, it is worth developing methods to examine such problems on these platforms. 

Quantum Natural Language Processing (QNLP) deals with the design and implementation of NLP models intended to be run on quantum hardware.

We are introducing a version of the popular transformer model in which by the application quantum phenomena we are trying to find a way to bring down the training cost in NISQ computers. We are using the DisCoCat language modeling framework to encode text into quantum gates. The quantum transformer algorithm is created in Qiskit by adding variational quantum circuit layers to the classical transformer. We are also experimenting with different ansatz and cost functions to prevent the barren plateau issue. We will evaluate the model on NLP tasks like classification, machine translation, and next-word prediction.

Quantum Transformer

Entanglement between remote matter systems is the cornerstone of quantum information transfer, crucial for establishing quantum secure communication between distant parties as well as for distributed quantum information processing architectures. This can be done via the implementation of cooperative light-matter coupling between coherently controllable matter qubits. This can manifest in collective rate enhancements, such as those at the heart of super radiance [1], but also includes effects such as measurement-induced remote entanglement via path erasure [2]. Here, we report coherent control of cooperative emission from two indistinguishable solid-state emitters, brought into resonance via Stark tuning. We characterize the indistinguishability of the emitters and the degree of correlation in the initial state of the joint system by observing the emergence of the bunched correlation peak in the intensity correlation experiment [3], the primary signature of cooperative emission. We find that due to the substantial spatial separation between the emitters, the cooperative emission lifetime is unchanged from the spontaneous emission lifetimes of the uncorrelated emitters. This highlights the fact that the bunched correlation peak in the intensity correlation is not a sufficient witness of superradiant behavior. Finally, in a stark departure from a pair of uncorrelated emitters, we observe photon statistics resembling that of a weak coherent state in Hong-Ou-Mandel type two-photon interference measurements. These experiments establish a new way to control and characterize cooperative behavior between distant matter qubits using the full quantum optics toolbox, a key stepping stone on the route to realizing large-scale quantum photonic network.

[1] R. H. Dicke, Phys. Rev. 93, 99 (1954). [2] S. Wolf et. Al., Phys. Rev. Lett. 124, 063603 (2020). [3] A. Sipahigil et. al., Science 354, 847-850 (2016).

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Reconfigurable photonics with on-chip single-photon detectors

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