China

Structured photons have attracted a lot of attention in quantum photonics ranging from fundamental studies to quantum information applications. In the latter, they often serve as powerful tools to realize high-dimensional quantum states beneficial to quantum information tasks such as cryptography or quantum simulations. While mature techniques exist for the generation and detection of structured photons, many applications are suffering from the absence of methods for performing complex unitary operations between transverse structured photons. We demonstrate that using recent advances in multi-plane mode-conversion techniques, it is possible to perform any unitary operation between different sets of transverse structures in a versatile, simple, and efficient manner. More importantly, we study different two-photon interferences such as coalesce and anti-coalescence between various orthogonal transverse structures. In contrast to previous studies, where two photons bunched into different paths depending on the spatial structures, the interference that we observed occurs along a single beamline. Finally, we show that the ability to perform bi-photon interferences using custom-tailored spatial structures enables the generation of spatial-mode NOON-states, which offer angular super-sensitivity.

Quantum 2020

Single-Path Two-photon Interferences Between Spatial Structures

high-dimensional quantum states

twisted photons

orbital angular momentum

structured photons

quantum computation

quantum sensing

**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. 

While taking the complementary position of the well-known qubit architecture, Continuous Variable (CV) architecture offers an alternative perspective of quantum computing, which encodes quantum information in a continuous physical observable. Initial works have shown that it allows constructing a neural network similar to the structure of a classical fully connected layer by decomposing it into a combination of CV gates. Using this CV neural network (CVNN), we built a simple binary classifier to discriminate classical data encoded in quantum states. Especially, its performance has been evaluated through an application in High Energy Physics (HEP) to distinguish real data, which are the calorimeter outputs interpreted as pixelated images, from random fake data. Following the successful results obtained in the CV classifier test with reduced problem size, we introduce and analyze a new prototype of quantum Generative Adversarial Networks (qGAN) employed in CV quantum computing. Two CV qGAN models with a quantum and a classical discriminator have been tested to reproduce the same training set used in the previous classifier case, and their limitations are discussed.

Quantum Generative Adversarial Networks in Continuous Variable Architecture to Simulate High Energy Physics Detectors

Noise in quantum metrology reduces the sensitivity to which one can determine an unknown parameter in the evolution of a quantum state, such as time. Here, we consider a noiseless quantum system (the probe, or the clock) prepared in a pure state that encodes some time t, on which we apply an arbitrary noise channel. We show an uncertainty relation stating that the noisy probe’s sensitivity to time trades off exactly with the environment’s sensitivity to the energy of the noiseless probe. We obtain necessary and sufficient conditions for when zero sensitivity is lost after application of the noise channel. These conditions are analogous to the Knill-Laflamme quantum error correction conditions but they are easier to satisfy. I will mention applications to many-body quantum metrology, such as computing the sensitivity of many-body interacting probe states exposed to local amplitude-damping noise.

Bipartite energy-time uncertainty relation  and  quantum error correction for metrology with noise

Any realistic physical system is unavoidably coupled to some external degrees of freedom and should then be treated as an open system. That is why the open, reduced quantum dynamics plays an important role also in quantum technology applications. 
In the study of the dynamics of an open quantum system it is well known that alternative approximated descriptions are often useful. Here we show that also equivalent exact evolution equations can be introduced, investigating a newly introduced link between time local and time non-local master equations [1, 2]. Though these equations are in principle equivalent as their solutions are the same reduced density operator, the knowledge of both can be beneficial. Our approach, based on the damping basis representation, give some insights into the occurrence of different dephasing channels in time local and time non-local master equations and the non-Markovianity property of the exact and the approximated reduced dynamics.



[1] - N. Megier, A. Smirne, B. Vacchini, New J. Phys. 22: 083011, 2020

[2] - N. Megier, A. Smirne, B. Vacchini, Entropy 22(7): 796, 2020

Different descriptions of open quantum system dynamics - master equations

The standard quantum optics treatment of the squeezed light generation the material susceptibility is taken as a constant prefactor. This treatment is justified when the fields are far off-resonant with respect to matter. Resonant nonlinear spectroscopy investigates material properties, which requires an explicit treatment of the matter response via nonlinear susceptibilities.

We propose a set of nonlinear spectroscopic measurements with quantum light [1]. It involves a strong pump and a weak probe interacting with a solid state target to generate a conjugated beam via nondegenerate four-wave mixing. The third order nonlinear susceptibility can be measured by the noise spectra of the intensity difference of the squeezed beams.

We further discuss three spectroscopic
setups based on squeezed light: first, single four-wave mixing in a single crystal; second, cascading scheme involving two crystals; third, an SU(1, 1) interferometer based on two separate four-wave mixing processes. We further investigate the microscopic noise and optical losses in all three setups. Simulations are presented for silicon-vacancy color centers in diamond.

References:

Z. Yang, P. Saurabh, F. Schlawin, S. Mukamel, and K. E. Dorfman, Appl. Phys. Lett., 116, 244001 (2020).

Multidimensional spectroscopy with squeezed light

Error correcting codes with a universal set of transversal gates are the desiderata of realising quantum computing. Such codes, however, are ruled out by the Eastin-Knill theorem. Moreover, it also rules out codes which are covariant with respect to the action of transversal unitary operations forming continuous symmetries. In this work, starting from an arbitrary code, we construct approximate codes which are covariant with respect to local SU(d) symmetries using quantum reference frames. We show that our codes are capable of efficiently correcting different types of erasure errors. When only a small fraction of the n qudits upon which the code is built are erased, our covariant code has an error that scales as 1/n^2, which is reminiscent of the Heisenberg limit of quantum metrology. When every qudit has a chance of being erased, our covariant code has an error that scales as 1/n. We show that the error scaling is optimal in both cases. Our approach has implications for fault-tolerant quantum computing, reference frame error correction, and the AdS-CFT duality.

Covariant Quantum Error Correcting Codes via Reference Frames

Quantum light mediated matter-matter interactions is a topic of interest. Started with the seminal works of Dicke or Hepp & Lieb nowadays nice physics and applications are found. Some examples are controlling superconductivity, chemical reactivity or novel phases of matter. In quantum technologies these light mediated interactions are also used to generate squeezing, q-gate construction or for building quantum simulators. 



In this talk, I will review what I call magnetic cavity QED where spins (rather than dipoles) are magnetically (rather than electrically) coupled to superconducting cavities, resonators or waveguides. Magnetic coupling, being weaker than electric, has been somehow ignored but, recently, several tricks to enlarge it has been proposed and experimentally reported [1]. This opens the possibility of using magnetic molecules as qubits and quantum light as mediators. During the talk, I will emphasize the differences and equivalences between electrical and magnetic systems and the experimental state of the art. Besides, within the theory and experimental situation I will explain some applications in squeezing generation[3], magnetic phases detection [4] and the realization of superradiance equilibrium phase transition within magnetic cavity QED[5].

[1] Jenkins...Dalton Trans.45,16682(2016). 



[2] Gimeno...ACS Nano,14,8707(2020)



[3] Macri...PRA101,053818(2020)



[4] Roca...(in preparation)



[5] Román-Roche...(in preparation)

Quantum magnetism driven by vacuum fields

Quantum sensors made of NV centers in diamond have generated tremendous interest over the past decade, owing to a unique combination of sensitivity and atomic-scale sampling volumes. Significant progress has been made in a growing number of applications ranging from material science, to navigation, to detection of single proteins. The technology is now at a tipping point where commercial instruments start to be available. One particular field that benefits from nano-scale images with high sensitivity is that of electronics and magnetic memories.

In this work we show imaging of antiferromagnetic spin textures with state-of-the-art accuracy using a commercial platform. We present two modes of operation allowing rapid convergence towards quantitative understanding of the magnetic textures at the surface of an antiferromagnet. We use BiFeO3 as a prototype example. Remarkably, the data reveal a “zigzag” shaped modulation on the periodic stray field, which are assigned to changing cycloid wave-vector directions due to the presence of ferroelectric domains. The used platform for real-space, nanoscale magnetic imaging offers access to non-trivial spin textures which would be largely inaccessible otherwise. The ability to perform quantitative and non-perturbative analysis of such materials is key to the development of advanced materials in the field of spintronics.

Using a commercial scanning single spin microscope to diagnose magnetic materials for next generation magnetic memories

If nature allowed nonlocal correlations other than those predicted by quantum mechanics, would that contradict some physical principle? Various approaches have been put forward in the past two decades in an attempt to single out quantum nonlocality. However, none of them can explain the set of quantum correlations arising in the simplest scenarios. In [Sci. Adv. 5, eaav8370] we have shown that generalized uncertainty relations, as well as a specific notion of locality, give rise to both familiar and new characterizations of quantum correlations. In particular, we have identified a condition, relativistic independence, which states that uncertainty relations are local in the sense that they cannot be influenced by other experimenters’ choices of measuring instruments. We proved that theories with nonlocal correlations stronger than the quantum ones do not satisfy this notion of locality, and therefore, they either violate the underlying generalized uncertainty relations or allow experimenters to nonlocally tamper with the uncertainty relations of their peers. In this talk I will discuss several implications of this work to multiplicative Bell inequalities, causality and networks. Finally, I will support the claim that locally consistent indeterminism should be understood as the enabler of most quantum phenomena (and possibly all of them).

Relativistic independence and the immense explanatory power of quantum uncertainty

Randomized benchmarking (RB) protocols represent an efficient tool to compare different qubit technologies and architectures. Despite traditional methods of characterizing gate fidelity that involve quantum process tomography, RB possesses some advantages in terms of the robustness against the state preparation and measurement (SPAM) errors, beyond the fact that it scales efficiently with the system size, requiring fewer resources. RB experiments have been performed in many different physical contexts for quantum technologies, ranging from superconducting qubits, trapped ions or semiconducting qubits, i.e. nitrogen-vacancy centers in diamond, quantum dots and donors (e.g. phosphorous) atoms in silicon. In this study, we simulated randomized benchmarking protocols in a semiconducting all-electrical three-electron double-quantum dot qubit, i.e. hybrid qubit, under different error models, that include quasi-static Gaussian and the more realistic 1/f noise model, for the input controls. The average error of specific quantum computational gates is extracted through interleaved randomized benchmarking obtained including Clifford gates between the gate of interest. Quantitative results are reported that on one hand allow to predict the fidelity when specific error parameters are included, on the other provide a complete picture of the hybrid qubit operations in view of realization of more complex circuits, such as quantum error correction circuits.

Randomized benchmarking protocols in the all-electrical quantum dot qubit with 1/f noise

The concept of topological phases is a powerful framework to characterize many-body systems beyond the paradigm of symmetry breaking. In addition to studying topological phases in condensed-matter systems, a large effort is underway to realize topological matter in quantum simulators. Various platforms have already succeeded in designing non-trivial topological single-particle modes [Ozawa2018,Cooper2018]; however, these modes have only been populated with non-interacting particles, where properties of the many-body states are solely determined by the single-particle modes. A current challenge is to introduce strongly interacting particles in these systems and search for emergent properties of the many-body states. 

Here, we use a quantum simulator based on atoms trapped in optical tweezers and excited to Rydberg levels [Browaeys2020] to realize a symmetry-protected topological (SPT) phase of interacting bosons in a 1D-lattice hosting non-trivial topological modes [deLéséleuc2019]. I will present signatures of the single-particle modes and of the many-body SPT phase obtained by half-filling the lattice with hard-core bosons. Especially, I will show that some perturbations affect single-particle modes, but not the many-body phase: in striking contrast from what occurs in non-interacting topological phases.

[Ozawa2018,Cooper2018] RMP 91, 015005 and 015006 (2018)
[Browaeys2020] Nat. Phys. 16, 132 (2020)
[deLéséleuc2019] Science 365, 775 (2019)


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