China

Solid state quantum emitters, like color centers in diamond and systems alike, offer appealing prerequisites for practicable devices that exploit quantum effects. For example, optically addressable quantum sensors, based on the ubiquitous nitrogen-vacancy defect, have been recently commercialized in magnetometers offering unprecedented spatial resolution. Another class of defects in diamond, based on group-IV elements, is currently attracting considerable interest due to potentially further improved properties. Thereby, the vast majority of experimental studies relies on identification, characterization, and manipulation of these quantum emitters by optical means - that is, in practice, by employing suitable laser light sources. Yet, the experimental demands are challenging. In frequency domain experiments, the characterization of single color defects often necessitates quasi-continuous laser frequency tuning over wide spectral ranges, while high finesse spectral resonances require sufficiently narrow linewidths. In turn, experiments that interrogate temporal dynamics, like initialization and read out of spin states, demand fast intensity modulation capability with high on/off ratios. In this context, we present principles, design challenges, and performance highlights of state-of-the-art laser light sources tailored to meet requirements typically encountered in quantum technology research. Throughout the presentation we keep an eye on emerging applications and present recent experimental data in an illustrative manner.

Quantum 2020

Novel continuous-wave lasers tailored for identification, characterization, and manipulation of single color centers in diamond

quantum emitters

continuous wave lasers

optical parametric oscillators

tunable lasers

color centers

**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)

Bristol Quantum Information Technologies Workshop

poster

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.


You will need to [register](https://www.iopconferences.org/iop/frontend/reg/tRegisterEmailNew.csp?pageID=978403&eventID=1552&tempPersonID=536224) to obtain a ticket. . If you have already registered you should have been sent an email from Underline (to the email address that you registered with) with the subject line ‘Welcome to Quantum 2020’. This email includes your ticket for the event. 

If you have a ticket and are experiencing problems in accessing the event, please contact us. 

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. 

Nonlinear optical interaction at a single photon level plays a key role in quantum optical information processing. One of its important applications is optical switching with extremely low power consumption. Although optical switching controlled by a few photons has been demonstrated using strongly coupled quantum dot-photonic crystal cavity systems, the strong coupling regime brings significant fabrication challenges as high cavity quality factor and accurate QD positioning are required. This becomes more problematic for integrated photonic circuits where the cavity Q-factor is strongly degraded due to the waveguide coupling. In this work, we show that low-photon number (~2 photons) optical switching can be realized with a QD weakly coupled to a low-Q (only 600) cavity in the high Purcell factor regime. Our scheme significantly relaxes the fabrication requirement while achieving high switching speed (22.7 ps), high switching contrast (39%), and large tunability. Our work paves the way towards the practical quantum optical switch, especially for on-chip photonic circuits.

Ultrafast few-photon switching with a coherently driven quantum dot in the high Purcell factor regime

Quantum key distribution (QKD) is one of the most practical applications in quantum information processing, which can generate information-theoretical secure keys between remote parties. With the help of the wavelength-division multiplexing technique, QKD has been integrated with the classical optical communication networks. The wavelength-division multiplexing can be further improved by the mode-wavelength dual multiplexing technique with few-mode fiber (FMF), which has additional modal isolation and large effective core area of mode, and particularly is practical in fabrication and splicing technology compared with the multi-core fiber. Here, we present for the first time a QKD implementation coexisting with classical optical communication over weakly-coupled FMF using all-fiber mode-selective couplers. The co-propagation of QKD with one 100 Gbps classical data channel at -2.60 dBm launched power is achieved over 86 km FMF with 1.3 kbps real-time secure key generation. Compared with single-mode fiber using wavelength-division multiplexing, given the same fiber-input power, the Raman noise in FMF using the mode-wavelength dual multiplexing is reduced by 86% in average. Our work implements an important approach to the integration between QKD and classical optical communication and previews the compatibility of quantum communications with the next-generation mode division multiplexing networks.

Long-distance transmission of quantum key distribution coexisting with classical optical communication over a weakly-coupled few-mode fiber

The prospects of laterally-grown quasi-AlGaN/GaN/AlGaN hetero-structure High-Electron-Mobility-Avalanche-Transit-Time (HEMATT) oscillator as high-power THz-signal source is studied through an indigenously developed Quantum-Drift–Diffusion simulator and experimental verification is reported. Active region of the HEMATT oscillator consists of two-Dimensional Electron Gas (2-DEG), trapped within a GaN quantum well that lies in between AlGaN barriers on either side. Both anode and cathode consist of GaN. Under reverse-biased condition, GaN/AlGaN band offset forces minority electrons to enter into the heterostructure active region and remain confined within the GaN quantum well. Carrier current generation in this 2-DEG active region is due to the quantum confinement of minority electrons rather than conventional tunneling phenomenon. Such quantum confinement is effectively functioning as the gate-control terminal as in conventional HEMTs. Introduction of quasi-AlGaN instead of alloy AlGaN, constitutes of alternate fine layers of AlN/GaN, results in carrier mobility doubling (800cm2/V-sec to 1600cm2/V-sec). A subtle interplay between bound and quasi-bound states is observed due to the quasi-AlGaN barrier. The study reveals a spectacular optical sensitivity with designed HEMATT oscillators. This for the first time opens the prospect of newly reported HEMATT oscillators in quantum sensing to cater the needs of ever growing, sophisticated defense and industrial applications.

A 2-terminal quasi-AlGaN/GaN/AlGaN HEMATT oscillator as quanta-image sensor for single-photon detection applications in defense industry

Entanglement plays a central role in quantum information processing and quantum physics. However, few effective ways are known to detect the amount of entanglement of an unknown quantum state. Here, we propose a scheme to estimate the entanglement negativity for any bipartition of a composite system. The proposed scheme is based on the random unitary evolution and local measurements on a single-copy quantum state, which is more practical compared with former methods based on collective measurements on many copies of the identical state. Meanwhile, we generalize the scheme to quantify the total correlation. We demonstrate the efficiency of the scheme with statistical analyses and numerical simulations. Our scheme is quite suitable for state-of-the-art quantum platforms, which can serve as a useful benchmarking tool to advance quantum technologies and a probe to study fundamental quantum physics like entanglement dynamics.

Single-Copies Estimation of Entanglement Negativity

Quantum antiferromagnet is of fundamental interest as it becomes a fertile ground for studying exotic many-body states including spin liquids and high-temperature superconductors. Compared with condensed matter systems, ultracold gases in optical lattices can be microscopically engineered and provide a unique opportunity for exploring bosonic magnetism and spin dynamics. Here, we report on the creation of a one-dimensional Heisenberg antiferromagnet with ultracold bosons. In a two-component Bose-Hubbard system, we switch the sign of the spin-exchange interaction and realize the isotropic antiferromagnetic Heisenberg model in an extended 70-site chain. Starting from a low-entropy N\'eel-ordered state, the bosonic antiferromagnet is approached via an optimized adiabatic passage. We probe the evolution of staggered magnetization and spin correlations of the system, revealing the coherent establishment of the antiferromagnetism. Furthermore, the characteristic spin-rotational symmetry of the state and its robustness against decoherence are verified. Our experiment demonstrates a novel way for generating many-body spin correlations and exploring exotic forms of magnetism.

Realization of a bosonic antiferromagnet

Continuous variable quantum key distribution (CV-QKD) has made great progress recent years. Throughput of error correction is one of the bottlenecks for CV-QKD. For typical distance of CV-QKD system, the signal-to-noise ratio is low, and a large number of iterations is needed for decoder. Since scheduling strategy is an important factor of the decoder’s convergence rate, in this paper, the performance of three different scheduling strategies (flooding scheduling, layered scheduling by row, layered scheduling by column) is compared. The algorithm complexity, adaptation to the parity check matrix, and throughput performance are studied. For the typical parity check matrix for CV-QKD system, the results show that layered scheduling by column is the best method. Throughput of more than 70 Mbits/s is achieved by this method, which surpasses the record in the literature and can improve the real-time processing capacity of CV-QKD system.

Study on error correction decoder scheduling strategies for continuous variable quantum key distribution system

"HqHop: Hybrid Quantum Hopfield Neural Network for Portfolio Selection

RP Mahajan 
 
Faculty at Dept. of Computer Science & Engineering, SIRT, SAGE University, India
E-mail: rpmahajan.sage@gmail.com ; rpmahajan@yahoo.com

 
Abstract: Classical Hopfield neural networks (CHNN) are known for solving portfolio selection problem for asset management. A Hybrid Quantum Hopfield Neural Network (HqHop) is proposed to solve portfolio selection problem based on Markowitz mean variance approach. Due to the large number of portfolios, the CHNN requires huge memory and processing power. The inbuilt parallelism in CHNN makes it more suitable for quantum based system. This paper details an approach, perhaps the first time, for the solution of portfolio selection problem using the involved concept. The developed HqHop can be used for solving other optimization problems as well.

"

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