United Kingdom

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&#39;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)


Bristol Quantum Information Technologies Workshop

What’s ahead for Wigner’s friend?

**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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The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.


Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. The broad themes covered by Quantum 2020 included:

Quantum communication;
Quantum computing and quantum simulation;
Quantum sensing, metrology and imaging;
Quantum information theory, software and algorithms;
Quantum foundations;
Quantum technology in industry.
The four-day event featured talks from world-renowned scientists, interactive poster sessions and two exciting panel discussions on the themes of Quantum Technology in Industry and Quantum Technology Programmes Around the World.


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If you have a ticket and are experiencing problems in accessing the event, please contact us. 

Quantum 2020

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)


technical paper

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.

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.

What’s ahead for Wigner’s friend?

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