China

We present an imaging system that uses a photon’s position and time-of-flight information to image an object. Since the position of the photons is correlated with their momentum, it is possible to identify the pairs by checking the photons’ timestamp information and localize one of the pair for Spatio-temporal ghost imaging thanks to the momentum conservation of down conversion event. While the timing information of signal (idler) is recorded as reference the idler (signal) is measured after being back-scattered from the object being imaged. With a real-time cross-correlation check of signal and idler photons, the pairs can be identified. The results exhibit multifold quantum enhancement over classical detection in an extremely noisy background. The two-dimensional imaging is performed by mapping the position of signal (idler) photons and register the corresponding coincidence events. The rate of coincidences for a certain position is used to create the image of the object. We also analyze the parameters affecting the image resolution in two dimensions. The method is implemented on imaging of microscopic objects and it was shown that the polarization entanglement is also well preserved during the scattering from the object being imaged.

Quantum 2020

Noise Tolerant Spatio-temporal Ghost Imaging with an Entangled Source

quantum time correlation

quantum imaging

down-conversion process

**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. 

The design of quantum experiments can be challenging for humans. Computational and artificial intelligence methods have therefore been introduced to discover tailored quantum experiments with particular desired functionalities. However, these algorithms are slow, require large amounts of data or work for specific platforms that are difficult to generalize. 

Here we present Theseus, an efficient algorithm for the design of quantum experiments. The algorithm' core is a physics-inspired, graph-theoretical representation of quantum states, which makes it significantly faster than previous comparable approaches. Topological optimization then uncovers the solution's conceptual cores which human scientists can understand and generalize without performing any further calculations. We use Theseus to solve several open questions in experimental quantum optics. The solutions are not only within reach of modern laboratories but also provide new generalizable ideas, for example, a method to exploit destructive interference of vacuum.

We anticipate that Theseus will become an essential tool in quantum optics for studies of fundamental questions and developments of practical applications. Furthermore, the unique ability to systematically inspire new understanding goes beyond standard optimization and enters realm of providing scientific understanding, an essential aim of science.

Conceptual understanding through efficient inverse-design of quantum optical experiments

In this work, we answer the question: how sudden or adiabatic is a change in the frequency of a quantum harmonic oscillator (HO)? 
With this purpose, we study and characterise the squeezing parameter (SP) and the variance of the so-called instantaneous quadrature operator when performing a frequency transition in the HO for different transition speeds interpolating the sudden and adiabatic limits. Then we characterise adiabaticity concerning to squeezing. Finally, we obtain analytic expressions enabling us to fit the SP and the variance of the instantaneous quadrature operator as a function of the parameters of the system and the transition speed. 


Time-dependent quantum harmonic oscillator: a continuous route from adiabatic to sudden changes

Semiconductor nanocrystals (NCs), thanks to tunability and control afforded by quantum confinement effects with unique properties, have been broadly applied in light emitting diodes displays, chiral sensing and asymmetric catalysis, etc. However, a systematic control of these NCs was extremely difficult not to say impossible. Thus, it is still mandatory and relevant to find out an effective synthetic approach to achieve semiconductor NCs with tunable properties. In this work, we studied the precise control of quantum rods synthesis, and used them for chirality and laser-induced deposition purposes.
The anisotropic factor of circularly polarized luminescence (CPL, glum) can reach 8.5×10-4, and the the raito of single-tipped HNPs could be improved significantly from 42% to 81% in Au-CdSe/CdS HNPs .

Improved semiconductor nanocrystal: a platform for CPL and controlled laser photodeposition of single metal nanodot

In this study, we showed that it is possible to explain the quantum measurement process within the framework of quantum mechanics without any additional postulate. We examined a Stern—Gerlach experiment to explain how the uncertainty relation between position and momentum introduces decoherence prior to measurement. The EPR—Bohm experiment was considered to conclude that the correlation of the EPR pair is not a consequence of instantaneous propagation of the collapse of the wave function. We also referred to its relation to the counterfactual non-definiteness of quantum mechanics. Finally, we showed that our theory can be applied to experiments with continuous eigenvalues, such as the double-slit experiment with electrons. This also illuminates how pure quantum mechanics describes the fact that electrons behave as interfering particles, I.e., particles whose detection rate is consistent with interference.

Decoherence as an inherent characteristic of quantum mechanics

We delve into the problem of the choice of the interaction Hamiltonian in quantum transitions by using the gauge invariant formalism. We demonstrate that the gauge invariant transition amplitudes for the bare-bare transitions, which is the case in the majority of experiments, are identical to the conventional results with the velocity form interaction Hamiltonian. Under the electric dipole approximation, the gauge invariant formalism derives the modified length form interaction Hamiltonian in any gauge. We propose a simple experiment that one can perform to verify our prediction. On the other hand, the gauge invariant transition amplitudes for the dressed-dressed transitions, which is the case in the Lamb shift experiment, are identical to the conventional results with the length form interaction Hamiltonian. There is no preferred gauge in the calculations of transition amplitudes. The present investigation resolves the long-standing versus controversy and clarifies the prevailing misconception in the literature.

Reference
(1) Hsiang-Shun Chou, J. Phys. Commn. 3, 115014 (2019)


Gauge invariant resolution of the A.p versus r.E controversy

The awareness of the relevance of the projectile’s beam coherence effects in ion-atom and ion-molecule collisions has motivated an extensive study on this subject in recent years. The traditional way to compute the fully differential cross section (FDCS) of a reaction describes the incoming beam of projectiles as a purely coherent quantum system, i.e. a plane-wave. Nevertheless, this assumption seems not justified enough when we look at this postulate under the light of the density matrix theory. We describe how the time propagation of an incoherent mixture of an ensemble of massive particles, described by identical wave packets located at different positions upon a much larger region, can eventually develop coherence at a given distance L, and its implications for scattering theory. Furthermore, we show the results of these concepts applied to specific experimental results.

The role of classical uncertainties on the coherence properties of collisions processes

We present optimal schemes, based on photon number measurements, for Gaussian state tomography and for Gaussian process tomography. An n-mode Gaussian state is completely specified by 2n^2 +3n parameters. Our scheme requires exactly 2n^2 +3n distinct photon number measurements to tomograph the state and is therefore optimal. Further, we describe an optimal scheme to characterize Gaussian processes by using coherent state probes and photon number measurements.

Optimal characterization of Gaussian states and Gaussian channels using photon-number- resolving detectors

Quantum key distribution is a pioneering scheme in the quantum cryptography. On the other hand, quantum direct secure communication scheme shows the possibility of transferring the secret information without the need for a secure key. It is known that bell states are extensively employed to achieve bidirectional quantum communication. While most of the quantum communication protocols revolves around the controller, who prepares and shares the bell states with the communicants, recently controller independent bidirectional quantum communication has been proposed [1]. In the proposed scheme, the sender and the receiver become empowered to generate bell states and hence the role of controller becomes insignificant. Further, the protocol is modified so as to use non-maximally entangled states [2]. The protocol is shown to be secure against intercept and resend attack. As the non-maximally entangled states are relatively easy to realize in comparison with maximally entangled states, controller independent protocol becomes physically feasible. 

(1) Mohapatra, A.K., Balakrishnan, S.: Controller-independent bidirectional quantum direct communication. Quantum Inf. Process. 16, 147 (2017)
(2) Akaash, S., Balakrishnan, S.: Controller-independent quantum bidirectional communication using non-maximally entangled states. Quantum Inf. Process. 19, 133 (2020)

The poster is essentially a summary of the results presented in the aforementioned papers. More technical details can be found in the papers.


Insignificant controller in the quantum communication protocols

Understanding what happens to a qubit thrown into a black hole is not an easy task; is the information lost to any outside observer or can it be retrieved by local measurements? This problem boils down to studying the scrambling of information over the event horizon, which draws attention to a general class of problems: scrambling of quantum information. In our work, we study the scrambling properties of quantum systems through the mutual information (MI). To this end, we lower bound the MI by the Out-of- Time-Order Correlators (OTOC) and determine its dynamical properties, relating the MI to thermodynamic quantities such as irreversible entropy production. We then illustrate, through examples, the thermodynamic significance of the MI for quantum information scrambling. Thus, as a main result, we establish a clear link between the MI and the OTOC, which enables us to probe the thermodynamics of quantum information scrambling and opens the door for further studies of the thermodynamics of quantum chaotic systems.

Quantum scrambling and the growth of mutual information

In this work, we analyze the concept of quantum bridge to achieve Quantum Teleportation (QT) and propose two schemes for QT using an Non-Maximally Entangled (NME) channel as a bridge between the sender and the receiver. In the rst scheme, QT is achieved by the application of unitary operator. Here the probability of success does not change even if we interchange the Degree of Entanglement (DoE) of the quantum channels. In the second scheme, generalized Bell basis measurements are utilized to achieve successful QT. Here, successful QT is achieved under two situations: (i) the DoE of channels and measurements are the same, pairwise and (ii) the DoE of the channels are the same, and the DoE of the measurements are the same. We show that quantum channels and the measurements are equivalent in achieving the QT only when the DoE is the maximum.

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Conceptual understanding through efficient inverse-design of quantum optical experiments

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