China

An excited electron can relax by emitting a photon. In a radiative Auger process, this optical decay leads to the simultaneous excitation of another particle (shake-up). On a single quantum emitter, radiative Auger has not been observed. Here, we report radiative Auger for trions in individual quantum dots. For the trion, just one electron is left after the optical recombination. The radiative Auger process promotes this Auger electron to a higher shell of the quantum dot, and the emitted photon is red-shifted. We show that the energy splitting between this red-shifted photon and the resonance fluorescence directly measures the single-particle splittings of the quantum dot [1], which is otherwise difficult to acquire. We prove the radiative Auger mechanism by measuring the photon statistics and the magnetic field dispersion of the emission. Going beyond the original work in the X-ray spectrum of atoms, we apply quantum optics techniques to the radiative Auger photons. We show how quantum optics gives access to the single-electron dynamics, notably the relaxation and tunneling rates. All these properties of radiative Auger can be exploited on other semiconductor nanostructures and quantum emitters in the solid-state.

[1] M. C. Löbl et al., Nat. Nanotechnol. 15, 558–562 (2020)

Quantum 2020

Radiative Auger process in the single-photon limit

trion

shake-up

radiative auger

quantum dot

**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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Bristol Quantum Information Technologies Workshop

technical paper

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. 

We propose an efficient, scalable and deterministic scheme[1] to generate up to hundreds of indistinguishable photons over multiple channels, on demand. Our design relies on multiple single-photon sources, each coupled to a waveguide, and all of them interacting with a common cavity mode. The cavity synchronizes and triggers the simultaneous emission of one photon by each source, which are collected by the waveguides. For a state-of-the-art circuit QED implementation, this scheme supports the creation of single photons with purity, indistinguishability, and efficiency of $99\%$ at rates of $\sim $MHz. We also discuss conditions to create a device to produce 30-photon simultaneously with efficiency above $70\%$ at a rate of hundreds of kHz. This is several orders of magnitude more efficient than previous demultiplexed sources for boson sampling, and enables the realization of deterministic multi-photon sources and scalable quantum information processing with photons.

[1] Li, M., et al., arXiv:2009.02382 (2020).

Synchronization of single-photon sources via a cavity

The nonclassical properties of quantum states are of tremendous interest due to their potential applications in future technologies. It has recently been realized that the concept of a resource theory is a powerful approach to quantifying and understanding nonclassicality. To realize the potential of this approach, one must first find resource theoretic measures of nonclassicality that are operational, meaning that they also quantify the ability of quantum states to provide enhanced performance for specific tasks, such as precision sensing.

In this talk, I will discuss some interesting results we have achieved in this endeavor by presenting the first operational resource theoretic measure [1]. In addition to satisfying the requirements of a resource measure, I will show that our measure has the closest possible relationship to the quantum enhancement provided by a nonclassical state for measuring phase-space displacement: It is equal to this enhancement for pure states and has a tight upper bound on it for mixed states. I will further show that a lower bound on this measure can be obtained experimentally using a simple Mach-Zehnder interferometer.

[1] W. Ge, K. Jacobs, S. Asiri, M. Foss-Feig, and M. Suhail Zubairy Phys. Rev. Research 2, 023400 (2020)

Quantifying Nonclassicality via the Precision in Quantum Metrology

Quantum metrology is the interdisciplinary of investigating how to utilize the principles of quantum mechanics to perform parameter estimation and improve the measurement precision by quantum effects. Attributed to well-developed techniques of quantum control, one can prepare several exotic non-Gaussian multi-particle entangled states in the ensembles of ultracold atoms. Based on many-body quantum interferometry, and using these non-Gaussian entangled states as probe, the high-precision measurement beyond the standard quantum limit can be realized . Utilizing suitable operations of quantum control, the non-Gaussian entangled states can be used for realizing Heisenberg-limited sensors, such as Sagnac interferometers, magnetometers, and many-body lock-in amplifiers. In this presentation, we will introduce the backgrounds and advances of this field.

Quantum metrology with non-Gaussian entangled states

The existence of fundamentally identical particles represents a foundational distinction between classical and quantum mechanics. Due to their exchange symmetry, identical particles can appear to be entangled -- another uniquely quantum phenomenon with far-reaching practical implications. However, a long-standing debate has questioned whether identical particle entanglement is physical or merely a mathematical artefact. In this work, we provide such particle entanglement with a consistent theoretical description as a quantum resource in processes frequently encountered in optical and cold atomic systems. This leads to a plethora of applications of immediate practical impact. On one hand, we show that the metrological advantage for estimating phase shifts in systems of identical bosons amounts to a measure of their particle entanglement, with a clearcut operational meaning. On the other hand, we demonstrate in general terms that particle entanglement is the property resulting in directly usable mode entanglement when distributed to separated parties, with particle conservation laws in play. Application of our tools to an experimental implementation with Bose-Einstein condensates leads to the first quantitative estimation of identical particle entanglement. Further connections are revealed between particle entanglement and other resources such as optical nonclassicality and quantum coherence. Overall, this work marks a resolutive step in the ongoing debate by delivering a unifying conceptual and practical understanding of entanglement between identical particles.

Entanglement between identical particles is a useful and consistent resource

The classical Gibbs paradox concerns the change in entropy upon mixing two gases. Whether or not an observer assigns an entropy increase to the process depends on their ability to distinguish the gases. A resolution is provided by realising that an ``ignorant'' observer, who cannot distinguish the gases with devices in their lab, has no way of extracting work by mixing them. Moving the thought experiment into the quantum realm, we reveal new and surprising behaviour. We show that the ignorant observer can in fact extract work from mixing different gases, even if the gases could not be directly distinguished. Moreover, in a macroscopic limit that classically recovers the ideal gas from statistical mechanics, there is a marked divergence in the quantum case: as much work can be extracted as if the gases had been fully distinguishable. Our analysis reveals that the ignorant observer assigns more microstates to the system than are found by naive state-counting in semiclassical statistical mechanics. This effect demonstrates the importance of carefully accounting for the level of knowledge of an observer, and its implications for genuinely quantum modifications to thermodynamics.

Extracting work from mixing indistinguishable systems: A quantum Gibbs "paradox''

CCP-QC is an EPSRC-funded network linking computational scientists with quantum computing scientists and engineers, to develop some of the first useful applications of quantum computers.
An important set of computational tasks in materials, chemistry,physics, 
biology, and engineering is developed by communities supported by 
collaborative computational projects (CCPs).
CCP-QC will network across these CCPs and the quantum computing 
community, to enable the CCP communities to enhance their computations 
by using quantum computers. It will do this by organising joint meetings, holding training days to teach computational scientists about quantum computing, supporting small projects to develop proof-of principle code and demonstrations on early quantum computing hardware, and providing an online information resource on early quantum computing applications. This contribution introduces CCPs in general, CCP-QC in particular, and explains how to get involved.

Collaborative Computational Project - Quantum Computing

The development of quantum technologies requires reliable processing of quantum information on large quantum systems. This is highly challenging as quantum systems are more sensitive to perturbations with increasing system size. In the last years, out-of-time-order correlations (OTOC) have gained special interest for measuring sensitivity to perturbations of many-body systems. These OTOCs can be experimentally measured with time reversion of quantum evolutions, on which the reversion is considered perfect. However, in practice the time reversion always contains imperfections. We study the spreading of information from a local state by monitoring OTOC dynamics. Using quantum simulations with solid-state Nuclear Magnetic Resonance, we engineer a ½-spin system Hamiltonian and we add a variable perturbation. When the perturbation interaction Hamiltonian is not reverted in time, we observe a localized spreading of information by the OTOC dynamic. The OTOC measures the effective cluster size of spins that were correlated by the ideal -control- Hamiltonian. If the perturbation is included in the time reversion, the localization effects are not manifested, and the information spreads over the system indefinitely. These results provide a new tool to determine the maximum system size that can be controlled in realistic experimental setups with unavoidable imperfect control operations.

Quantum information spreading measured by out-of-time-order correlations with magnetic resonance

How to characterize the interacting topological states is of particular importance in understanding the interplay between topology and interaction [1]. The main obstacle is that single-particle Bloch states are destroyed by interactions and fail to define topological invariant. Here, we use multi-particle Bloch states due to co-translational symmetry to define topological invariant for the first time [2, 3, 4]. Owing to the multi-particle topological band theory, in spin systems we have revealed topological bound edge states [2], and predicted interaction-induced topological phase transitions [4]; In bosonic systems we prove that Chern number of the multi-particle Bloch band is related to the shift of multi-particle Wannier state [3]; In fermionic systems we show interaction-induced topological bound states and Thouless pumping [5], where interaction plays an essential role. Our work creates a paradigm for multi-particle topological effects and provides a way to detect topological states in interacting multi-particle systems.

[1] X.-G. Wen, Rev. Mod. Phys. 89, 041004 (2017)
[2] X. Qin, et. al., Phys. Rev. B 96, 195134 (2017)
[3] Y Ke, et. al., Phys. Rev. A 95, 063630 (2017)
[4] X. Qin, et. al., New J. Phys. 20, 013003 (2018)
[5] L. Lin, et. al., Phys. Rev. A 101, 023620 (2020)

Theory of topological multi-particle bands

Photon correlations are central to quantum applications with light. We show that the photon-statistics of the light emitted by optical systems under coherent and resonant excitation can be modulated from Sub- to Super-Poissonian due to a self-homodyne interference. This scheme can be further controlled and/or optimized at the N-photon level by purposely tuning the coherent excitation. As one application of our theory, we design a perfectly antibunched and subnatural-linewidth (monochromatic) source, based on resonance fluorescence (a qubit weakly excited). Furthermore, the study highlights the close relation between two relevant quantum attributes of light, photon antibunching and quadrature squeezing, which were so far studied separately. This paradigm is not only interesting for single-mode correlations but can also be extended to multi-frequency spectroscopy. When the laser-qubit detuning is large, a perfect circle of antibunching between photons with different frequencies arises due to quantum interferences, revealing exotic multi-mode squeezing properties of great potential for applications.

Bringing together squeezing and antibunching of light

All-optical quantum teleportation and entanglement swapping are essential to the operation of quantum networks. Both rely on properties of entangled states of light that, in the prospect of real-life applications, should be encoded on photon pairs on demand [1]. Despite recent advances [2], however, the goal has proved elusive. In this work, we show that entangled photon pairs deterministically generated by a single quantum dot can successfully be used to perform multi-photon experiments. 
We demonstrate photonic entanglement swapping relying on a quantum emitter [3], as well as quantum teleportation with fidelity violating the classical limit for any arbitrary input state [4]. The experimental results are quantitatively predicted based on entanglement fidelity and photon indistinguishability. The role of the Bell state measurement implementation and spectral filtering are investigated within the same theoretical framework. Moving from these considerations, we envision the next steps for quantum dots towards implementing fundamental quantum protocols in practical quantum networks.
[1] D. Huber, et al., Phys. Rev. Lett. 121, 33902 (2018).
[2] J. Nilsson, et al., Nat. Phot. 7, 311 (2013).
[3] F. Basso Basset, M. Rota, C. Schimpf, D. Tedeschi, et al., Phys. Rev. Lett. 123, 160501 (2019).
[4] F. Basso Basset., et al., arXiv:2006.02733 (2020).

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Synchronization of single-photon sources via a cavity

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