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Bristol Quantum Information Technologies Workshop

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**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 **


![](https://assets.underline.io/uploads/markdown_image/1/image/a12e0c1ad3410144cb48557690e4ecaf.png)
### 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**

![](https://assets.underline.io/uploads/markdown_image/1/image/a12e0c1ad3410144cb48557690e4ecaf.png)

In this talk, I will present the main product of QuiX: a quantum photonic processor, i.e. a large-scale tunable multimode optical interferometer, for use in photonic quantum information processing applications. QuiX has recently demonstrated the world's largest photonic processor, and I will present results on the characterization, calibration and operation of this device.

QuiX Quantum Photonic Processors

EPJ Quantum Technology is part of the well known European Physical Journal (EPJ) series, indexed in all major citation databases, which covers the entire spectrum of pure and applied physics. We will give a brief introduction to the journal’s aims and scope, editorial structure, articles types and the current special issues, followed by an overview of recently published highlights.

An introduction to EPJ Quantum Technology journal.

Zurich Instruments continues to develop instrumentation for quantum technologies with growing demand for high measurement fidelity, control speed, and reliability as well as scalability. In this presentation, I will introduce the latest arbitrary waveform generation (AWG) product SHFSG and quantum analyzer SHFQA that can be seamlessly integrated into a system solution QCCS (Quantum Computing Control System) capable of working with both large and small quantum systems.

Next generation instruments for quantum control and readout from Zurich Instruments

In this talk I will briefly review and discuss a recent no-go theorem on an extended version of the Wigner’s friend paradox [1]. In that work, we showed that a set of metaphysical assumptions we called “Local Friendliness” lead to inequalities that can be in principle violated by quantum correlations. We demonstrated this violation in a “proof of principle” experiment. I will discuss how violation of Local Friendliness puts strictly stronger constraints on physical theories than Bell's theorem. I will then discuss what an ultimate experiment may look like, some implications of this result for quantum foundations, and future research directions.
[1] Bong et al, Nature Physics 16, 1199 (2020)


What’s ahead for Wigner’s friend?

Device-independent quantum key distribution is a secure quantum cryptographic paradigm that allows two honest users to establish a secret key, while putting minimal trust in their devices. Most of the existing protocols have the following structure: first, a bipartite nonlocal quantum state is distributed between the honest users, who perform local projective measurements to establish nonlocal correlations. Then, they announce the implemented measurements and extract a secure key by post-processing their measurement outcomes. We show that no protocol of this form allows for establishing a secret key when implemented on certain entangled nonlocal states, namely on a range of entangled two-qubit Werner states. To prove this result, we introduce a technique for upper-bounding the asymptotic key rate of device-independent quantum key distribution protocols, based on a simple eavesdropping attack. Our results imply that either different tools are needed for device-independent quantum key distribution in the large-noise regime, or Bell nonlocality is not sufficient for this task.

Bell nonlocality is not sufficient for the security of standard device-independent quantum key distribution protocols

Tunnelling is one of the phenomena at the heart of quantum mechanics. Despite its central role in physics, the duration of this process has been debated since the development of quantum theory without a definite answer. We study the tunnelling of a Bose-Einstein condensate in a single collision configuration through a thin optical barrier.  As the atoms tunnel, each of their spins acts as a ‘clock’ to record the time spent in the barrier region. Our experiment achieves a long-sought tunnelling time measurement with a clear interpretation, demonstrating a non-zero tunnelling time.

Measurement of the time spent by a tunnelling atom within the barrier region

Matched filtering is a signal processing technique used to maximize the signal-to-noise ratio by correlating signal templates with detector data. It is central for detecting the extraordinarily weak signals in the field of gravitational wave astronomy. In cases where the signal is too weak and the template bank is too extensive, the matched filtering process can become computationally unfeasible. In this talk I will analyse the potential of Grover's algorithm to speed up match filtering for gravitational wave detection, where even a polynomial speed-up could make previously intractable searches possible. I will present an example of simulated quantum matched filtering applied to the detection of the first gravitational wave signal to demonstrate the algorithm's potential application in data analysis.

Quantum matched filtering for Gravitational Wave Astronomy

The prevalent modus operandi within the framework of quantum resource theories has been to characterize and harness the resources within single objects, in what we can call single-object quantum resource theories. One can wonder, however, whether the resources contained within multiple different types of objects, now in a multiobject quantum resource theory, can simultaneously be exploited for the benefit of an operational task. In this work, we introduce examples of such multiobject operational tasks in the form of subchannel discrimination and subchannel exclusion games, in which the player harnesses the resources contained within the composite object of a state-measurement pair. We prove that for any state-measurement pair in which either of them is resourceful, there exist discrimination and exclusion games for which such a pair outperforms any possible free state-measurement pair. These results hold for arbitrary convex resources of states, and arbitrary convex resources of measurements where the set of free measurements is closed under classical post-processing. Furthermore, we prove that the advantage in these multiobject operational tasks is determined, in a multiplicative manner, by the resource quantifiers of: generalized robustness of resource of both state and measurement for discrimination games and weight of resource of both state and measurement for exclusion games.

Multiobject operational tasks for convex quantum resource theories of state-measurement pairs

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.

Genuine High-Dimensional Quantum Steering

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.

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QuiX Quantum Photonic Processors