## INAQT seminar series

Seminars take place monthly via zoom on Mondays at 2pm UK time, unless otherwise stated.

Organisers: Michael Antesberger, Otavio Dantas Molitor, Hector Spencer-Wood, Sonja Franke-Arnold and Lee Rozema

## Upcoming talks

Our online seminars have restarted for 2024. Our next seminar is:

- Quantum technology certification through quantum causal inference, Otavio A.D. Molitor, 26/03/2024 10:00 GMT / 11:00 CET.

Check back for updates, follow us on twitter, or contact us to join our mailing list and receive seminar announcements.

**Speaker:** Otavio A.D. Molitor

**Title:** Quantum switch as a thermodynamic resource in the context of passive states

**Abstract:** In the context of indefinite causal order, the main implementation paradigm in the literature so far is the quantum switch (QS). Here we explore advantages of the use of the QS in quantum thermodynamics, specifically the case of activation of passive states: is the QS capable of state activation alone or with extra resources (active control state) and/or operations (measurement of the control system)? We move forward to show that the former is false and the latter is true. To illustrate our results we apply the framework to specific examples: a qubit system subject to rotations around the x and y axes in the Bloch sphere, as well as general unitaries from the U(2) group; and the system as a quantum harmonic oscillator with displacement operators, and with a combination of displacement and squeeze operators.

## Previous talks

- Quantum technology certification through quantum causal inference, Iris Agresti, 11/12/2023 14:00 GMT
- Entanglement assisted classical communication simulates "classical communication'' without causal order, Mio Murao, 13/11/2023 10:00 GMT
- Indefinite causal key distribution, Hector Spencer-Wood, 16/10/2023 14:00 BST
- Activation of wavelike behaviour and work extraction from the quantum switch, Pedro Dieguez, 18/09/2023 14:00 BST
- Superposing two time directions using single photons, Teodor Strömberg, 08/12/22 9:00 GMT
- Quantum photonics in microstructured optical fibre, Peter Mosley, 01/12/22 9:00 GMT
- Indefinite causality and its advantages for thermodynamics, Kyrylo Simonov, 10/11/22 14:00 GMT
- Magnetic field shaping for portable quantum devices, Peter Hobson, 27/10/22 9:00 BST
- How to find a metrological advantage using indefinite causal order, Aaron Goldberg, 13/10/22 13:00 BST
- Spin-orbit photonics for optical simulations of quantum walks, Filippo Cardano, 21/07/22 9:00 BST
- On the Computational Advantage from a Quantum Superposition of Gate Orders, Martin J. Renner, 07/07/22 9:00 BST

**Speaker:** Iris Agresti

**Title:** Quantum technology certification through quantum causal inference

**Abstract:** The development of quantum technologies that has been witnessed in recent years promises to bring significant advances in a wide range of applications, e.g. truly random number generation, secure communication protocols and so on. However, these advances are unlocked by the presence of non-classical behaviors and, as such, it is crucial to develop techniques to detect and characterize them.

The "device-independent" approach allows for certifying non-classicality by examining input/output statistics without requiring knowledge of the adopted device. In this framework, the majority of protocols relies on causal inference, which exploit the cause/effect relationships among the observed physical quantities to retrieve mathematical constraints. However, such constraints often change when non-classical phenomena come into play and induce a discrepancy between the predictions of classical and quantum causal inference. Hence, a violation of the classical causal constraints becomes a valuable tool to detect the presence of non-classical behaviors in a device-independent way. The best known example of is constituted by Bell tests.

In this talk, I will introduce the simplest process that can be exploited to detect and quantify non-classicality in a device-independent way, i.e. the so-called instrumental scenario, along with our experimental demonstration on a photonic platform. Then, I will show how the above findings can find practical applications, by illustrating an experimental certified random number generator.

Recording, passcode: ihNu$4iW

**Speaker:** Mio Murao

**Title:** Entanglement assisted classical communication simulates "classical communication'' without causal order

**Abstract:** Phenomena induced by the existence of entanglement, such as nonlocal correlations, exhibit characteristic properties of quantum mechanics distinguishing from classical theories. When entanglement is accompanied by classical communication, it enhances the power of quantum operations jointly performed by two spatially separated parties. Such a power has been analyzed by the gap between the performances of joint quantum operations implementable by local operations at each party connected by classical communication with and without the assistance of entanglement. In this work, we present a formulation for joint quantum operations connected by classical communication beyond special relativistic causal order but without entanglement and still within quantum mechanics. Using the formulation, we show that entanglement assisting classical communication necessary for implementing a class of joint quantum operations called separable maps can be interpreted to simulate "classical communication'' not respecting causal order. Our results reveal a new counter-intuitive aspect of entanglement related to spacetime.**References:**

[1] S. Akibue, M. Owari, G. Kato and M. Murao, Phys. Rev. A 96, 062331 (2017)

[2] S. Akibue (Ph.D. Thesis, The University of Tokyo, 2016), arXiv:2202.06518

Recording, passcode: .pw1ne9k

**Speaker:** Hector Spencer-Wood

**Title:** Indefinite causal key distribution

**Abstract:** We propose a quantum key distribution (QKD) protocol that is carried out in an indefinite causal order (ICO). In QKD, one considers a setup in which two parties, Alice and Bob, share a key with one another in such a way that they can detect whether an eavesdropper, Eve, has learnt anything about the key. To our knowledge, in all QKD protocols proposed until now, Eve is detected by publicly comparing a subset of Alice and Bob's key and checking for errors. If operations can be applied in an indefinite causal order, we show that, interestingly, the presence of Eve can be detected by Alice alone, without publicly comparing any information about the key with Bob. Indeed, it turns out that both correlated and uncorrelated eavesdroppers cannot extract any useful information about the shared key without inducing a nonzero probability of being detected.

**Speaker:** Pedro Dieguez

**Title:** Activation of wavelike behaviour and work extraction from the quantum switch

**Abstract:** In this seminar, we will explore effects that emerge due to the indefinite causal order inherent in quantum switch operations in two distinct contexts. Firstly, we will investigate two quantum delayed-choice experiments, each with an opposite causal order of operations, to assess the robustness of Bohr's complementarity principle and wave-particle duality in such scenarios. Subsequently, we will discuss a simple model wherein wavelike behaviour can be induced by exploiting the absence of a well-defined causal order. In the second part, we will introduce thermodynamic cycles based on generalized measurements and explore how the quantum switch can be utilized to enable work extraction in heat engines. This will be particularly relevant in situations where work extraction would be otherwise unattainable if we were to consider only operations with a definite causal order.

Recording, passcode: $3FRHw0c

**Speaker:** Teodor Strömberg

**Title:** Superposing two time directions using single photons

**Abstract:** Most quantum information processing tasks are described using the quantum circuit model, in which states evolve through a a fixed sequence of gates. In the last decade, the study of processes with an indefinite causal order has revealed that this is not the most general model of computation allowed admitted by quantum mechanics. Recently Chiribella and Liu [1] proposed an even more general process, in which not only the causal order is indefinite, but also the time direction of the operators in the circuit. This process has been named the quantum time flip. In analogy to how processes with indefinite causal order can outperform casually ordered circuits for certain problems, there are tasks for which quantum time flip outperforms any process with a definite time direction.

In this talk I will present our recent experimental implementation of the quantum time flip [2]. I will show how a superposition of time directions is naturally generated for a polarization qubit by exploiting two propagation directions through a set of linearly birefrigent elements, and discuss the relation of our work to no-go theorems forbidding the quantum time flip.

**References:**

[1] Commun Phys 5, 190 (2022)

[2] arXiv:2211.01283

**Speaker:** Pete Mosley

**Title:** Quantum photonics in microstructured optical fibre

**Abstract:** Photonic crystal fibre (PCF), formed of a matrix of air holes running along the length of a strand of silica glass, has for the last 20 years enabled unprecedented control over the propagation of light. In PCF with a solid core, the size and distribution of micron-scale holes in the cladding controls the fibre dispersion, allowing precision engineering of nonlinear processes across brightness scales from photon-pair generation to supercontinuum sources. On the other hand, silica glass walls only hundreds of nanometres thick but many of metres long can confine light to a hollow core in which dispersion and nonlinearity are minimized to yield high-fidelity transport of both intense ultrashort laser pulses and quantum states of light.

In this talk, I will present developments in the application of PCF to quantum technologies, where its unique capabilities have potential in all-photonic as well as photonic-enabled architectures for computation, communication, and sensing. Our recent work has spanned photon-pair sources, wavelength conversion for universal quantum interfaces, and topological effects in PCF, as well as engineering hollow-core fibre for noise reduction in pulsed laser systems, for quantum memories in atomic vapour, and for enhancing light-matter interaction with nanoparticles.

Recording, passcode: y3.0s4?j

**Speaker:** Kyrylo Simonov

**Title:** Indefinite causality and its advantages for thermodynamics

**Abstract:** The nature of causality remains one of the key puzzles in science. In quantum theory, the causal structure is not subject to quantum uncertainty and plays rather a background role. The process matrix formalism (PMF) has introduced indefinite causal structures by assuming validity of quantum theory in local laboratories while relaxing the global definite causal order between them. Looking for the possible applications of the PMF has been the subject of growing interest in the scientific community as the PMF could provide a communication and computational resource not realizable via standard quantum theory. Recent findings on thermal machines boosted by the quantum switch suggest that indefinite causality could also be a valuable thermodynamic resource and have triggered an interest in thermodynamic protocols boosted by it. In my talk, I cover the recent progress in thermodynamic applications of indefinite causality and discuss its future perspectives.

**Speaker:** Peter Hobson

**Title:** Magnetic field shaping for portable quantum devices

**Abstract:** Advances in the understanding and control of atomic systems have enabled atoms to become integral components of many devices, from quantum computers [1] and clocks [2] to miniaturised atomic magnetometers for healthcare [3] and atom interferometers for gravity sensing [4]. These devices operate with unprecedented accuracy, speed, and precision, but their real-world performance may be limited by magnetic field noise. Traditionally, this is mitigated by enclosing magnetically-sensitive components with sheets of passive magnetic shielding. However, this shielding adds weight, size, and cost, may magnetise under applied fields, and distorts the fields generated by internal active current-carrying structures which are required to confine atoms and generate a quantisation axis.

To overcome this, here we present new theoretical techniques [5–7] to shape magnetic fields specifically for quantum devices. We include the electromagnetic coupling to passive shields directly into the design of active current-carrying networks, creating hybrid shields which have improved performance with fewer, smaller passive shielding layers. By adopting new manufacturing methods like 3D-printing and flexible printed circuits, we design, build, and demonstrate smaller hybrid shields for benchmarking atomic magnetometers [8], minimising the quadratic Zeeman effect in atom interferometers [9], and for housing superconducting qubits. We utilise this technology in a commercially-available, laboratory bench-sized, hybrid shield which actively nulls the geomagnetic field by a factor of 200000 along its axis. Finally, we provide theoretical perspectives on future magnetic field shaping systems, which will have evermore stringent size, weight, power, cost, and durability requirements but also must be operated in less magnetically ideal settings.

**References:**

[1] P. Wang, C.-Y. Luan, M. Qiao, et al., “Single ion qubit with estimated coherence time exceeding one hour”, Nature Communications 12 (2021).

[2] L. Liu, D.-S. Lü, W.-B. Chen, et al., “In-orbit operation of an atomic clock based on laser-cooled 87rb atoms”, Nature Communications 9 (2018).

[3] E. Boto, N. Holmes, J. Leggett, et al., “Moving magnetoencephalography towards real-world applications with a wearable system”, Nature 555 (2018).

[4] B. Stray, A. Lamb, A. Kaushik, et al., “Quantum sensing for gravity cartography”, Nature 602, 590–594 (2022).

[5] M. Packer, P. J. Hobson, N. Holmes, et al., “Optimal inverse design of magnetic field profiles in a magnetically shielded cylinder”, Physical Review Applied 14, 054004 (2020).

[6] M. Packer, P. J. Hobson, N. Holmes, et al., “Planar coil optimization in a magnetically shielded cylinder”, Physical Review Applied 15, 064006 (2021).

[7] M. Packer, P. J. Hobson, A. Davis, et al., “Magnetic field design in a cylindrical high-permeability shield: the combination of simple building blocks and a genetic algorithm”, Journal of Applied Physics 131, 093902 (2022).

[8] P. J. Hobson, M. Packer, N. Holmes, et al., “Optimised hybrid shielding and magnetic field control for emerging quantum technologies”, SPIE Quantum Technology: Driving Commercialisation of an Enabling Science II, 111–120 (2021).

[9] P. J. Hobson, J. Vovrosh, B. Stray, et al., “Bespoke magnetic field design for a magnetically shielded cold atom interferometer”, Scientific Reports 12 (2022).

Recording, passcode: EyL%tn@1

**Speaker:** Aaron Goldberg

**Title:** How to find a metrological advantage using indefinite causal order

**Abstract:** Indefinite causal order (ICO) promises and has delivered super-quantum advantages broadly distributed among quantum information tasks. As with most quantum advantages, the problem sets are discrete; i.e., only applicable to particular problems. Is there a systematic method for finding more problems that are amenable to quantum advantages? I will discuss steps for finding and expanding quantum advantages stemming from ICO to metrology, including the impact of this perspective on procedures with definite causal order and examples of new results in multiparameter estimation using ICO.

Recording, passcode: D#Xs7y^f

**Speaker:** Filippo Cardano

**Title:** Spin-orbit photonics for optical simulations of quantum walks

**Abstract:** Engineering synthetic quantum evolutions in artificial and controllable systems has proved a powerful resource in various applications. An interesting example is provided by quantum walks (QWs), introduced back in the 90’s to describe a peculiar discrete-time motion of quantum particles on a lattice. Inspired by a qualitative analogy with random walks, periodically each particle (“walker”) takes a step in a direction that is dictated by the instantaneous state of an internal degree of freedom, dubbed as coin. Several variants of QWs have been widely used for quantum simulation/computation, to model transport phenomena and to engineer topological phases of matter. They have been successfully engineered in diverse quantum simulators, like those based on trapped ions or atoms, superconductive circuits, and photonic systems. Here I will report on our approach to the experimental implementation of QWs based on spin-orbit photonics. After associating walker positions with optical modes carrying quantized transverse momentum, we emulate the unitary QW evolution by coupling these via the diffractive action of periodic spin-orbit metasurfaces. These are made of a thin layer of a liquid-crystal material, engineered to have a space-dependent orientation of the local optic axis. After illustrating the working principle of this platform, I will present the results of a recent experiments on the implementation of QWs in their long-time limit. Eventually, I will discuss some prospects of these research activities.

**References:**

[1] A. D’Errico et al., Two-dimensional topological quantum walks in the momentum space of structured light, Optica 7, 108-114 (2020).

[2] C. Esposito et al., Quantum walks of two correlated photons in a 2D synthetic lattice, npj Quantum Information 8, 34 (2022).

[3] F. Di Colandrea et al., Ultra-long photonic quantum walks via spin-orbit metasurfaces, arXiv:2203.15051 (2022).

**Speaker:** Martin J. Renner

**Title:** On the Computational Advantage from a Quantum Superposition of Gate Orders

**Abstract:** In conventional quantum algorithms, the gates are applied in a fixed order on the systems. The introduction of indefinite causal structures allows to relax this constraint and control the order of the gates with an additional quantum state. It is known that this quantum-controlled ordering of gates can reduce the query complexity for certain computational tasks. For instance, these structures can decide more efficiently than any conventional quantum algorithm whether two gates commute or anticommute. In this talk, we analyze two classes of problems that generalize this effect to an arbitrary number of gates. For the first class, we improve known causally ordered quantum algorithms and conclude that the advantage of using indefinite causal structures is smaller than previously expected. For the second class of problems, we find tasks for any number of gates and show that they offer an advantage of using a quantum-controlled gate ordering. Furthermore, problems of the second class require only qubit gates and are therefore suitable to demonstrate this advantage experimentally.