United Kingdom

In this talk I will discuss novel applications of GHZ states in quantum communication and computing.
First, I will present a recent experiment that shows quantum communication in a multipartite network. Here, I show how GHZ states allow achieving security in the communication – and also keeping the identities of the participating parties private.
Second, I will show how GHZ states can act as a resource for computation and that, vice versa, computation can be used as a tool to test quantum correlations.


Bristol Quantum Information Technologies Workshop

Protocols in quantum networks: communication and beyond

**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

Individual quantum systems are extremely sensitive sensors e.g., for magnetic fields. Such quantum sensors harness quantum mechanics for ultimate sensitivity and versatility. Simultaneously, they are atomically small quantum systems and boost spatial resolution in imaging. We employ point defects in diamond crystals, so called nitrogen vacancy (NV) color centers, as nanoscale sensors. Electrons bound to such defects provide highly controllable spins and create bright luminescence that enables us to use even individual color centers as sensors and to read out their spins. Efficient sensor devices rely on sophisticated nanofabrication and a high level of control of the diamond material system. Applications of the sensors include e.g. material science where magnetic nanostructures are characterized to life science were currents in living organisms are detected. We discuss recent advances in sensor devices as well as novel approaches to NV based sensing using near field energy transfer (FRET).

Diamond Nano-Quantum Sensors

We show that the optimal quantum radar has 12 dB better effective signal-to-noise ratio compared with the corresponding optimal classical radar, assuming full polarization antennas with hyperentanglement of photons at low photon flux per mode. The photon states are non-Gaussian, and we hyperentangle M-tuples of photons rather than the boring old pairs of photons. Full polarization means that the radar transmits two orthogonal polarizations simultaneously and also receives two polarizations simultaneously. We use the Belavkin-Zakai equation rather than the boring old Schrödinger equation, because the latter does not model macroscopic noisy measurements of quantum systems. We also compute the minimum cost for the optimal quantum radar at X-Band, which is 20 orders of magnitude more than for the corresponding optimal classical radar today. With the most optimistic assumptions this cost ratio can be reduced to only 15 orders of magnitude in the future. We speculate that further improvements in quantum radar are possible by exploiting non-Gaussian states of photons, hyperentanglement of more than pairs of photons, more advanced protocols for entanglement beyond the so-called “quantum illumination,” more advanced technology, more clever ideas and new physics yet to be discovered using Bayesian quantum field theory.

Bayesian quantum radar with full polarization and hyperentanglement of non-Gaussian photons

The field of Quantum Microscopy (QM) promises highly efficient imaging for low absorption samples and microscopy at the single photon level. The recently developed Skipper-CCD detector technology is able to measure the charge in each pixel, repeatedly, in a non-destructive way. As a result, the readout noise can be reduced as much as desired [Tiffenberg et al, 2017]. It allows the precise counting of the number of electrons in each pixel ranging from empty pixel to more than 1900 electrons [Rodrigues et al. 2021]. In addition, they account for very high quantum efficiency (above 90% in the visible range), extremely low dark current (less than 1 electron per pixel per day), and high resolution (pixel size of 15 microns by 15 microns). The use of Skipper-CCD detectors opens the possibilities for multiple applications within this field. In particular, they are expected to provide comparable resolution and Noise Reduction Factor to state-of-the-art setups for QM, with a dramatically/drastically reduced photon count per pixel. The same advantage can be expressed as an order of magnitude increase in resolution for the same number of photons per pixel. This presents a valuable impact of QM applications in biology and chemistry, when avoiding damage of the sample is required [Taylor et al, 2012]. The status and plans for the implementation of Skipper-CCD in this field will be presented.

[Tiffenberg et al.] Single-Electron and Single-Photon Sensitivity with a Silicon Skipper CCD. Phys. Rev. Lett.,119,13,131802,2017,10.1103/PhysRevLett.119.131802
[Rodrigues et al.] Absolute measurement of the Fano factor using a Skipper-CCD. arXiv:2004.11499v3
[Taylor et al.] Biological measurement beyond the quantum limit. Nature Photon 7, 229–233 (2013). https://doi.org/10.1038/nphoton.2012.346


Impact of Skipper-CCD technology on Quantum Microscopy

Recent breakthroughs in quantum photonics towards the realization of large optical quantum computing (QC) platforms were achieved following both a “full integration” approach [1] or based on an interconnected modular-like platform [2,3]. Either way, each architecture is based on well identified fundamental blocks which independent performances contribute to drive the full platform computation capacity.
These building blocks, forming the hardware side, consist in quantum light sources, active routers, processors/interferometers and detectors; independently of the computational scheme (linear optical QC or measurement-based QC) one can extract the required performance thresholds of each component which allow to scale up, or not, the computing platform.
Besides, these same building blocks are also required in diverse applications, ranging from quantum communication and networks to quantum metrology, encompassing the full spectrum of quantum technologies enabled by light.
At Quandela we work on building these fundamental optoelectronics modules which permit to realize reconfigurable quantum computing platforms in an efficient and scalable way. The modules consist in quantum light sources with emission efficiencies above 30%, interconnected via optical fibers to active demultiplexers (transmission >70% / line), frequency converters and subsequently, to integrated circuits and high-performance detectors. All these components are designed to guarantee plug-and-play operation and reconfigurability, permitting their efficient implementation and adaptability to different quantum technologies and applications.
Website: www.quandela.com
References:
[1] J. M. Arazzola et al., “Quantum circuits with many photons on a programmable nanophotonic chip”, Nature 591, 54–60 (2021) 
[2] H. S. Zhong et al. “Quantum computational advantage using photons”, Science, DOI: 10.1126/ (2020) 
[3] H. Wang et al. “Boson Sampling with 20 Input Photons and a 60-Mode Interferometer in a 10e14 Dimensional Hilbert Space” Phys. Rev. Lett. 123, 250503 (2019)

Modular Optical Quantum Computing

The interface between superconducting qubits with lights can be achieved by an efficient transduction of signals from the microwave to the optical frequency exploiting the optomechanical interaction in nanoscale devices [1][2]. The efficiency is negatively affected by the poor phonons injection efficiency from the transducer and difficulties to maintain single mode operation in the mechanical mode. 
We propose an alternate route for efficient microwave-to-optical transduction at cryogenic temperatures by using HBAR resonator compatible at cryogenic temperature where optical and mechanical field is enhanced by the acousto-optic cavity [2]. The limited efficiency g0 is compensated by the large number of cavity phonon (N) which can be injected into a single acoustic mode, and the efficiency is proportional to g0N, revealing an alternative route to an efficient transduction microwave-to-optical. One of the benefits of HBAR is the heating effect from the laser is negligible at cryogenic temperature, which is a limiting factor in other nanoscale optomechanical devices. 
We report the first acousto-optic transduction at cryogenic temperature (10K) using a HBAR modulator showing the potential of this (bulk) HBAR approach towards quantum transduction. The quantum transduction regime can be obtained by significantly improving the efficiency working on two fronts: improving the optical and mechanical quality factors by novel cavity design and improving g0 by exploiting the elasto-optic effect (Stimulated Brillouin scattering).
[1] K. C. Balram, M. I. Davanço, et al. (2017). ``Acousto-optic modulation and optoacoustic gating in piezo-optomechanical circuits''. Physical review applied, 7(2), 024008.
[2] V.J. Gokhale, B.P. Downey, D.S. Katzer, et al. ``Epitaxial bulk acoustic wave resonators as highly coherent multi-phonon sources for quantum acoustodynamics.'' Nat Commun 11, 2314 (2020).
[3] S. Valle and K. C. Balram, "High-frequency, resonant acousto-optic modulators fabricated in a MEMS foundry platform," Opt. Lett. 44, 3777-3780 (2019)


Cryogenic HBAR: a route toward efficient transduction from microwave to optical frequency

It is possible, in fact. I will present recent experimental results that demonstrate the “non-degenerate HOM effect,” that occurs within the time-envelope of photon pairs that meet at a beam splitter [1]. The coincidence rate shows an oscillation in time at the difference frequencies, both for same-path and different-path output events. This proves to be a useful tool to map the sum and difference frequency spectra in a two-photon state, with frequency resolution impossible to achieve with existing techniques. Hence, it is well suited to characterise narrow-band photon pairs for applications involving quantum light-matter interactions. We also demonstrate the closely-related “time-domain Ghosh-Mandel effect.” For both experiments, we use a cavity enhanced-SPDC source, resonant to the D1 line in cold atomic Rubidium [2]. The source allows for independent control over the central frequency of each photon in the pair and allows to pick and choose which hyperfine transition each photon is resonant to, with MHz resolution. We map the frequency content with the interference effects. 


[1] V. Prakash, A. Sierant, and M.W. Mitchell https://arxiv.org/abs/2101.03144

[2] V. Prakash, L. C. Bianchet, M. T. Cuairan, P. Gomez, N. Bruno, and M. W. Mitchell, Opt. Express 27, 38463 (2019). https://doi.org/10.1364/OE.382474

HOM interference between distinguishable photons? 

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

The goal of demonstrating a quantum advantage for useful applications with currently available technology is an important challenge in quantum information science. In this talk, we discuss examples of such rigorous demonstration of advantage in security and efficiency due to the use of quantum resources for cryptographic and communication complexity protocols. This requires devising novel protocols amenable to experimental implementations, which involve encoding in properties of coherent states of light, linear optic circuits and single-photon detection. We further discuss the extension of our results in quantum communication complexity to the efficient verification of NP-complete problems with limited information.

Demonstrating quantum advantage with practical photonic systems

Efficient conversion between microwave and optical frequency signals plays an important role in classical and quantum information processing. Recent advances in superconducting devices for quantum technologies have put significant constraints on conversion frequency, power and efficiency between phononic and photonic domains. Here, we demonstrate an acousto-optic modulator (AOM) for conversion of microwave signals up to 2.5 GHz to optical signals in the telecom wavelength range. The modulator is fabricated on a suspended Gallium Arsenide (GaAs) integrated photonic circuit platform, where an unsuspended interdigital transducer (IDT) drives surface acoustic waves to modulate the light travelling in a suspended photonic micro-ring resonator.

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