United Kingdom

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


Bristol Quantum Information Technologies Workshop

Impact of Skipper-CCD technology on Quantum Microscopy

shot-noise limit

quantum microscopy

skipper ccd

sub-shot-noise imaging

**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

Dynamic quantum photonics Dynamic quantum circuits—those featuring operations conditional on fed-forward measurement results—are key to scalable quantum information processing techniques such as error-correction and teleportation. Dynamic circuits are especially vital to quantum photonic implementations, as they can be used to overcome the platform's exponential sensintivity to loss, via multiplexing and fixed-optical-depth quantum computation. Meanwhile, thin-film lithium niobate photonic circuits have emerged as a disruptive platform form classical and quantum optics. Modulation by single-figure voltages can provide bandwidths nearing 100 GHz, while propagation losses as low as 0.027 dB/cm have been demonstrated. Combining low-loss circuitry with 10^7 Hz clock speeds and high-speed logic will enable a new set of capabilities in quantum photonics, such as large-scale temporally encoded states, and multiplexing with efficient on-chip delay lines—both for photon generation and the creation of entanglement. 

In this poster we explore thin-film lithium niobate's potential to scale photonic quantum information processing. We model a variety of devices for efficient photon generation and entangled state creation, and share preliminary results on a FPGA-controlled multiplexed single photon source. We also discuss today's technical limitations, and the path to overcome them both, for the next generation of quantum photonic capability.

Dynamic quantum photonics

Photonics is the platform of choice to build a modular, easy-to-network quantum computer operating at room temperature. However, no concrete architecture has been presented so far that exploits both the advantages of qubits encoded into states of light and the modern tools for their generation. Here we propose such a design for a scalable fault-tolerant photonic quantum computer informed by the latest developments in theory and technology. Central to our architecture is the generation and manipulation of three-dimensional resource states comprising both bosonic qubits and squeezed vacuum states. The proposal exploits state-of-the-art procedures for the non-deterministic generation of bosonic qubits combined with the strengths of continuous-variable quantum computation, namely the implementation of Clifford gates using easy-to-generate squeezed states. Moreover, the architecture is based on two-dimensional integrated photonic chips used to produce a qubit cluster state in one temporal and two spatial dimensions. By reducing the experimental challenges as compared to existing architectures and by enabling room-temperature quantum computation, our design opens the door to scalable fabrication and operation, which may allow photonics to leap-frog other platforms on the path to a quantum computer with millions of qubits.

Blueprint for a Scalable Photonic Fault-Tolerant Quantum Computer

A quantum internet aims at harnessing networked quantum technologies, namely by distributing bipartite entanglement between distant nodes. However, multipartite entanglement between the nodes may empower the quantum internet for additional or better applications for communications, sensing, and computation. In this work, we present an algorithm for generating multipartite entanglement between different nodes of a quantum network with noisy quantum repeaters and imperfect quantum memories, where the links are entangled pairs. Our algorithm is optimal for GHZ states with 3 qubits, maximising simultaneously the final state fidelity and the rate of entanglement distribution. Furthermore, we determine the conditions yielding this simultaneous optimality for GHZ states with a higher number of qubits, and for other types of multipartite entanglement. Our algorithm is general also in the sense that it can optimise simultaneously arbitrary parameters. This work opens the way to optimally generate multipartite quantum correlations over noisy quantum networks, an important resource for distributed quantum technologies.

Distributing Multipartite Entanglement over Noisy Quantum Networks

We demonstrate a method to create maximal entanglement between two qubits encoded in two spectrally distinct solid-state emitters embedded in a waveguide interferometer. The qubits are repeatedly probed using readily accessible squeezed light, generated by parametric down-conversion, and the photon scattering process results in a build-up of entanglement between the two qubits. The method accommodates spectral variations that arise due to the fabrication process and therefore relaxes the requirement of perfectly identical emitters.

Maximal entanglement of spectrally distinct solid-state qubits by iteration

Small, controllable quantum systems, known as quantum probes, have been proposed to estimate various parameters characterizing complex systems such as the environments of quantum systems. These probes, prepared in some initial state, are allowed to interact with their environment, and subsequent measurements reveal information about different quantities characterizing the environment such as the system-environment coupling strength, the cutoff frequency, and the temperature. These estimates have generally been made by considering only the way that the probe undergoes decoherence. However, we show that information about the environment is also imprinted on the probe via the probe and environment correlations that exist before the probe state preparation. This information can then be used to improve our estimates for any environment. We apply this general result to the particular case of a two-level system probe undergoing pure dephasing, due to a harmonic oscillator environment, to show that a drastic increase in the quantum Fisher information, and hence the precision of our estimates, can indeed be obtained. We also consider applying periodic control pulses to the probe to show that with a combination of the two - the effect of the control pulses as well as the initial correlations - the quantum Fisher information can be increased by orders of magnitude.

Improving the estimation of environment parameters via initial probe-environment correlations

Multilayer polymer photonic crystals, with a bandgap in the near-infrared (~1.3um), can be easily fabricated using two-photon lithography [1]. To achieve shorter period structures, we explore thermal decomposition to eliminate unpolymerized material and reduce the oxygen content of exposed polymer structures using heat [2]. This method has been shown to shrink polymer structures by a factor of 4 [2,3]. The current challenge is about conserving symmetry and geometry. Those two factors play a significant role in the resulting optical properties. The material strongly adheres to the substrate and requires a deformable/elastic anchoring to enable isotropic (shape-conserving) shrinking of the main photonic crystal structure. After shrinkage, the relative rod diameter appears larger, and we are investigating isotropic oxygen plasma etching to remove some material [2]. Finally, we use Fourier-Image spectroscopy to characterize the bandgaps of fabricated structures (before & after thermal post-processing), comparing results with various simulation tools (Plane-Wave Expansion and finite-difference time-domain). We plan to extend this technique to 3D photonic crystal defect cavities [4,5], in order to facilitate the creation of future quantum devices using quantum emitters in the visible wavelength region.


Shrinking of direct laser written polymeric structures by thermal annealing to move the bandgap into the visible range

We have developed two quantum classifier models for the $t\bar{t}H$ classification problem, both of which fall into the category of hybrid quantum-classical algorithms for Noisy Intermediate Scale Quantum devices (NISQ). Our results, along with other studies, serve as a proof of concept that Quantum Machine Learning (QML) methods can have similar or better performance, in specific cases of low number of training samples, with respect to conventional ML methods even with a limited number of qubits available in current hardware. To utilise algorithms with a low number of qubits --- to accommodate for limitations in both simulation hardware and real quantum hardware --- we investigated different feature reduction methods. Their impact on the performance of both the classical and quantum models was assessed. We addressed different implementations of two QML models, representative of the two main approaches to supervised quantum machine learning today: a Quantum Support Vector Machine (QSVM), a kernel-based method, and a Variational Quantum Circuit (VQC), a variational approach.

Higgs analysis with quantum support vector machines and variational quantum classifiers

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.

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Impact of Skipper-CCD technology on Quantum Microscopy

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