QuSCo on air allows all the members of the consortium to take part to network’s wide seminars, which are planned on regular basis. The seminars aims at consolidating the scientific exchange between the partners while serving also as outreach channel. The webinars are, in fact, recorded and published on our youtube channel. This page will also be updated after each new installment has been made public.

The webinar are organised and moderated by the ESRs

**Abstract:**

The design of efficient and robust pulse sequences is a fundamental requirement in quantum control and quantum technologies. Numerical methods can be used for this purpose, but with relatively little insight into the control mechanism or into the fundamental limits of the pulse [1]. In this talk, we show that the rotation of a classical system plays a fundamental role in the control of two-level quantum systems, or qubits. In the first part, we focus on the tennis racket effect, a geometric phenomenon which occurs in the free rotation of a rigid body. We prove that a perfect twist of the racket is achieved in the limit of an ideal asymmetric object [2]. A similar approach describes the Dzhanibekov effect in which a wing nut, spinning around its central axis, suddenly makes a half-turn flip and the monster flip, an almost impossible skateboard trick. Using a mapping between the rotation of a rigid body and the dynamics of a qubit, we derive for the qubit a family of control fields from the tennis racket effect. This family depends on two free parameters, which allow us to adjust the efficiency, the time and the robustness of the control process [3]. A quantum analog of the tennis racket eﬀect is proposed and experimental results illustrate this theoretical study. Finally, we discuss other applications of this classical effect in the rotation of asymmetric molecules [4].

[1]- Training Schrodinger’s cat: quantum optimal control

- J. Glaser et al.

Eur. Phys. J. D 69, 79 (2015)

[2]- Geometric origin of the Tennis Racket Effect

- Mardesic, G. J. Gutierrez Guillen, L. Van Damme and D. Sugny

Phys. Rev. Lett. 125, 064301 (2020)

[3]- The quantum tennis racket effect: Linking the rotation of a rigid body to the Schrodinger equation

- Van Damme, D. Leiner, P. Mardesic, S. J. Glaser and D. Sugny

Sci. Rep. 7, 3998 (2017)

[4]- Quantum control of molecular rotation

- P. Koch, M. Lemeshko and and D. Sugny

Rev. Mod. Phys. 91, 035005 (2019)

**Abstract:**

The problem of quantum measurements stands as an independent fundamental problem since the Gedanken Stern-Gerlach experiment. The collapse of the wave function was the main triggering source of numerous scientific debates. Different tractations were adopted, and eventually, up to now the concept of generalized quantum measurements is accepted which be the central line in our discussion. In our lecture, we will consider a simplified model of a quantum spin bath, probed with a quantum sensor – *ancilla*.

The possible scenarios of measurements would be discussed, including generalized weak measurements, and quantum phase estimation. By the end of the lecture, you will learn the main concepts of generalized measurements, measurement induced quantum back-action and its influence on applications as well as the ways to control it.

**Abstract:**

Qubit reset is a basic prerequisite for operating quantum devices, requiring the export of entropy. The fastest and most accurate way to reset a qubit is obtained by coupling the qubit to an ancilla on demand. In my talk I will derive fundamental bounds on qubit reset in terms of maximum fidelity and minimum time, assuming control over the qubit and no control over the ancilla. By using the Cartan decomposition of the Lie algebra of qubit plus two-level ancilla I will show that it is possible to identify those types of interaction and controls for which the qubit can be purified at all. For those configurations, a time-optimal protocol consists of purity exchange between qubit and ancilla brought into resonance, where the maximum fidelity is identical for all cases but the minimum time strongly depends on the type of interaction and control. Furthermore, I will show that the maximally achievable fidelity increases and the minimal reset time decreases once we switch from uncorrelated to correlated initial states of qubit and ancilla.

**Abstract:**

Much research has dealt with the sensitivity of measurement schemes based on the preparation, evolution, and final detection of non-classical states of a quantum system. In this talk, I shall discuss the analysis of continuous precision probing, i.e., the situation where a fluctuating signal is retrieved continuously in time from a single quantum system (the emitter). This is the typical situation, e.g., in spectroscopy, where we can use a master equation to predict the mean optical signal properties as a function of any system parameter, and thus we can infer its value from experimental data.

In a real experiment, the signal record fluctuates with time, and while this imposes measurement uncertainty, it also imposes a stochastic measurement back action: the system that emits the signal is constantly “quenched”. Analyses that takes this “permanently transient” dynamics explicitly into account yield much more information than the ones based on only mean signal values. It is (surprisingly) easy to derive the limit of sensitivity of any hypothetical measurement scheme on an emitted light signal, and it is (perhaps) surprising that this limit is actually reached by some of our standard detection methods in quantum optics.

In this session of QuSCo on-air, our guest, Dr Bartolo Albanese, from CEA, gives a talk about adiative cooling of a spin ensemble.

**Abstract:**

Spins in solids interact only weakly with their electromagnetic environment and in usual situations, they reach thermal equilibrium by exchanging energy with their host lattice. However, recent experiments have demonstrated that radiative emission can become the fastest energy relaxation channel for the electron spins if the sample is inserted in a resonant microwave cavity of small mode volume and low loss rate, as predicted by Purcell. In this regime spins are then expected to thermalize to the cavity mode regardless of the lattice temperature. We demonstrate this by showing that spins can be radiatively cooled below the sample temperature by coupling the cavity to a cold thermal radiation source. The experiment is realized with an ensemble of electron spins consisting of bismuth donors in silicon coupled to a micron-size superconducting resonator and the spin temperature is inferred by measuring the spin polarization with pulsed electron spin resonance techniques. A more than twofold increase of polarization is observed when the resonator input is connected to a cold resistive load, proving that spins are radiatively cooled with respect to their host lattice. The demonstrated technique represents a new and universal method to enhance electron spin polarization beyond thermal equilibrium, with potential applications in electron spin resonance spectroscopy.

The extreme sensitivity of Rydberg levels to their environment makes them particularly appealing for emerging quantum sensing. The lifetime of low-angular-momentum laser-accessible levels is however limited to a few 100μs by optical transitions and microwave blackbody radiation (BBR) induced transfers at room temperature. An improvement of more than two orders of magnitude would be obtained with circular Rydberg levels in a cryogenic environment, extending the capabilities of Rydberg atoms platforms for quantum technologies.

In the talk, Dr Favier focuses on his group’s experimental preparation and manipulation of laser-cooled circular Rydberg atoms in an optical-access 4K-cryostat. Lifetime measurements reveal a 9K microwave blackbody temperature and a corresponding 5ms lifetime of the circular Rydberg states. Ramsey interferometry shows coherence times solely limited by magnetic field noise. He also presents his group’s latest results on the first laser-trapping of circular Rydberg atoms, a crucial tool for all cold-atoms-based quantum technologies. They have demonstrated 2D laser-trapping of the long-lived circular Rydberg states for up to 10 ms in hollow Laguerre-Gauss laser trap generated with a spatial light modulator. they have characterized the trapping potential (including a measurement of the trapping frequency for the Rydberg atoms) and verified that it doesn’t affect the circular levels coherence properties.

This work is a key milestone towards new developments in quantum simulation, quantum metrology and quantum information processing with Rydberg atoms platforms.

In this session of QuSCo on-air, our guest, Matthias Müller, from Forschungszentrum Jülich, gives a talk about Optimal Control in the Chopped Random Basis, explaining also what RedCrab is. Abstract: We are at the verge of the second quantum revolution where quantum technology leaves the lab and enters industrial products. Fragile quantum systems with their unique features like superposition and entanglement can offer new perspectives in computation, communication and sensing/metrology. However, they need sophisticated mechanisms of control to perform the desired tasks. Quantum Optimal Control has proven to be a powerful tool to accomplish this task [1]. I will report on the optimization in the dressed chopped random basis (dCRAB) [2], a versatile and robust approach to Quantum Optimal Control, that allows both closed-loop and open-loop optimization with limited pulse bandwidth and guaranteed convergence in a broad range of typical applications. The interplay of constraints, control resources and noise [3] is crucial for the overall performance of the controlled operation. I will also present the software package RedCRAB that comes with a very user-friendly interface that allows the connection with any existing simulation [4] or experiment [5].

[1] C. Brif et al., New J. Phys. 12, 075008 (2010), S. Glaser et al., Phys. J. D 69, 279 (2015), P. Rembold et al., AVS Quantum Sci. 2, 024701 (2020)

[2] P. Doria et al., PRL 106, 190501 (2011), N. Rach et al., PRA 92, 052343 (2015) (ES)

[3] S. Lloyd et al., PRL 113, 010502 (2014), M. Müller et al., arxiv:2006.16113 (2020)

[4] A. Omran et al., Science 365, 570 (2019)

[5] F. Frank et al., npj Quantum Information 3, 48 (2017)

Dr David Bruschi (USAAR) joined our QuSCo on air to discuss the finite decoupling of Hamiltonians. The recording is available to the students of the consortium only.

*Time evolution is ubiquitous in all studies of quantum systems.*

In order to be able to gain ultimate control over a physical system, such as molecules interacting with light, modes of the electromagnetic field in a superconducting circuit, or atoms trapped in a lattice, one aims at being able to obtain the explicit expression of the time evolution of the system at any point in time.

Typically, obtaining an exact expression of the state, or any useful quantity, as the systems evolves is an overwhelmingly difficult task. Any technique or method that allows to remove the hurdles along the way and overcome the computational difficulties can provide huge advantages in our studies of quantum systems, and the development of more advanced applications.

In this talk we present techniques developed to decouple the time evolution operator exactly as a sequence of elementary operations. These operations act in a simple way on creation and annihilation operators, therefore providing closed formulas for the expectation values of important quantities, and more manageable expressions for the quantum state at any point in time. We present examples where these techniques have been successful and we conclude by stating a result recently proven that provides the only classes of Hamiltonians for which a finite decoupling is possible.

In this talk, Dr Luca Pezzè (LENS) gives a pedagogical introduction to parameter estimation, focusing on key concepts such as quantum and classical Cramer-Rao bounds, Fisher information and statistical distinguishability.

He focuses, in particular, on the role played by multipartite entanglement for enhancing the estimation sensitivity over the limit achievable with separable states: he introduces the key notion of useful entanglement in quantum metrology. The theory is illustrated with relevant examples and experimental achievements. Finally, Ihe discusses the current research trends: the quantum Fisher information as a witness of entanglement in complex many-body systems and the simultaneous estimation of multiple parameters.

Dr Jonathan Zopes, from Prof Degen group (ETH Zurich) , will present the talk “”Three-dimensional localization spectroscopy of individual nuclear spins in diamond”.

In the first part of the talk a protocol to detect time-dependent magnetic field waveforms using a single NV center will be discussed [1]. This spin-echos based method enables to achieve a time-resolution of ∼ 20 ns and a sensitivity of ∼ 4 μT/.

In the second part of the talk a method to precisely localize in space the 13C nuclear spin surrounding and interacting with an NV center will be discussed. The idea and the methods of the experiments will be explained in order to achieve a better understanding of the results.

A particular attention will be kept on the experimental methods and on the pulses sequences used during the experiments.

[1] **Reconstruction-Free Quantum Sensing of Arbitrary Waveforms**, J. Zopes and C.L. Degen, Phys. Rev. Applied 12, 054028

[2] **Three-dimensional localization spectroscopy of individual nuclear spins with sub-angstrom **resolution, Zopes, J. et al. Nat. Commun. 9, 4678 (2018).18.

[3] **Three-dimensional nuclear spin positioning using coherent radio-frequency control**, Zopes, J., Herb, K., Cujia, K. S. & Degen, C. L. Phys. Rev. Lett. 121, 170801 (2018)

**Upon request of the speaker, the webinar is only available to the members of the consortium**

In this special session of our QuSCo on air webinar we are joined by Prof. Tommaso Calarco (FZJ), chair of the Quantum Community Network, who has several years of coordinating successful projects such as AQUTE, SIQS and RySQ. The topic is “Preparing your first successful project application for a European call”

We encourage to visit the website: qt.eu and to take part to the consultation at this page https://ec.europa.eu/eusurvey/runner/HorizonEurope_Codesign_2021-2024

What does Quantum Zeno Dynamics have to do with the Doctor Who’s weeping angel?

Come and find out in our third QuSCo on air installment, led by Sabrina Patsch (UNIKASSEL)

The slides connected to this presentation can be found here

1] A. Signoles, A. Facon, D. Grosso, I. Dotsenko, S. Haroche, J.-M. Raimond, M. Brune, and S. Gleyzes, Confined quantum Zeno dynamics of a watched atomic arrow, Nat. Phys. 10, 715 (2014).

[2] F. Schäfer, I. Herrera, S. Cherukattil, C. Lovecchio, F. S. Cataliotti, F. Caruso, and A. Smerzi, Experimental realization of quantum zeno dynamics, Nat. Commun. 5, 1 (2014).

[3] P. Facchi and S. Pascazio, Quantum Zeno dynamics: mathematical and physical aspects, J. Phys. A 41, 493001 (2008).

[4] S. Patsch, S. Maniscalco, C. P. Koch, Quantum simulation of non-Markovianity using the quantum Zeno effect, arXiv:1906.11492

Prof. Steffen Glaser of Munich Technical University presents an in-depth lecture about Cooperative pulses for roubust quantum optimal control

The slides connected to this presentation can be found here

In this first installment, Dr. Shai Machnes (University of Saarland) gives a general introduction on the topic of **Quantum Optimal Control**.

The slides connected to this presentation can be found here