External seminars

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Quantum Optics with Rydberg excitons in cuprous oxide

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The observation of excitons with internal principal quantum numbers up to n=25 in cuprous oxide created a fascinating cross-over between atomic physics and semiconductor quantum optics. The open problems encompass both fundamental questions such as the validity of atomic theories of exciton behaviour and the role of defects, as well as applications in quantum light sources and sensors. In this talk I will provide an atomic physics perspective on Rydberg excitons, focussing in particular on the driving of microwave intra-band transitions between Rydberg states of opposite parity. I will show that is possible to reach the ultra-strong driving regime where the microwave coupling strength becomes the dominant energy scale, and discuss applications to microwave-optical conversion at ultra-cold temperatures.

How to use quantum simulation for partial differential equations?

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Abstract: When Feynman introduced quantum simulation, it was purpose-built for the efficient simulation of problems obeying Schrodinger’s equations. However, for systems whose dynamics do not have the form of Schrodinger’s equation, it is unclear if and how the power of quantum simulation can be best utilized for general partial differential equations. Yet the simulation of more general partial differential equations is of paramount importance in most areas of physics.

I’ll introduce a simple new way–called Schrodingerisation– to simulate general linear partial differential equations via quantum simulation. Using a simple new transform and introducing one extra dimension, any linear partial differential equation can be recast into a system of Schrodinger’s equations – in real time — in a straightforward way. This approach is not only applicable to PDEs for classical problems but also those for quantum problems – like the preparation of quantum ground states, Gibbs states and the simulation of quantum states in random media in the semiclassical limit.

Furthermore, we will see ways in which a quantum device can be used to efficiently solve not just linear PDEs but also certain classes of nonlinear PDEs, like nonlinear Hamilton-Jacobi equations and scalar hyperbolic equations, which have important applications many areas. I’ll also explain methods on how partial differential equations with uncertainty can be tackled with a quantum device with quantum advantage. 

Spin physics in lead halide perovskite crystals and nanocrystals

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A survey of our studies on spin-dependent phenomena in lead halide perovskite semiconductors (bulk crystals, nanocrystals and 2D materials) will be given. Several experimental techniques are used: optical spin orientation, pump-probe Faraday rotation with picosecond time resolution, spin-flip Raman scattering, and optically-detected magnetic resonance. Measurements were performed at low temperatures and in strong magnetic fields up to 10 T (dc fields) and 60 T (pulsed fields). The perovskite semiconductors are very friendly to these techniques, which allows us to use experimental and model approaches developed for conventional III-V and II-VI semiconductors. We evaluate the exciton and carrier g-factors, characteristic spin relaxation times and examine interaction of carrier spins with nuclear spin system. The received values are comparable with the parameters of conventional semiconductors. Inverted band structure of the perovskites offers interesting spin properties, which make them promising model materials for spin physics and spintronics.

[1] V. V. Belykh, D. R. Yakovlev, et al., Coherent spin dynamics of electrons and holes in CsPbBr3 perovskite crystals, Nature Commun. 10, 673 (2019).

[2] E. Kirstein, D. R. Yakovlev, et al., Lead-dominated hyperfine interaction impacting the carrier spin dynamics in halide perovskites, Advanced Materials 34, 2105263 (2022).

[3] E. Kirstein, D. R. Yakovlev, et al., The Lande factors of electrons and holes in lead halide perovskites: universal dependence on the band gap, Nature Commun. 13, 3062 (2022).

[4] P. S. Grigoryev, V. V. Belykh, D. R. Yakovlev, et al., Coherent spin dynamics of electrons and holes in CsPbBr 3 colloidal nanocrystals, Nano Letters 21, 8481 (2021).

[5] E. Kirstein, N. E. Kopteva, D. R. Yakovlev, et al., Mode locking of hole spin coherences in CsPb(Cl,Br) 3 perovskite nanocrystals, Nature Commun. 14, 699 (2023).

[6] E. Kirstein, E. A. Zhukov, D. R. Yakovlev, et al., Coherent spin dynamics of electrons in two dimensional (PEA) 2 PbI 4 perovskites, Nano Letters 23, 205 (2023).


Localized spins in semiconductors under pulsed excitation with GHz repetition

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A laser with a GHz repetition will be applied to drive the coherent electron spin dynamics of an ensemble of singly charged (In,Ga)As/GaAs quantum dots. In my talk, I will cover several effects that become available under the high repetition period of laser excitation. So, using the optically induced Stark field, one can generate a directed dynamic nuclear polarization that can be used as a universal tool for strongly reducing the nuclear spin fluctuations. Furthermore, we will discuss the realization of the resonant spin amplification (RSA) effect in Faraday geometry, where a magnetic field is applied parallel to the optically induced spin polarization so that no RSA is expected. Finally, I will present an experimental observation on the effects of the quantum back action under pulsed optical measurements and demonstrate that the nuclei-induced spin relaxation can be influenced. I will show that the fast measurements freeze the spin dynamics and increase the effective spin relaxation time, the so-called quantum Zeno effect. Additionally, we demonstrate that if the measurement rate is comparable with the spin precession frequency in the effective magnetic field, the spin relaxation rate increases and becomes faster than in the absence of the measurements, an effect known as the quantum anti-Zeno effect.

Estimating concentratable entanglement with projective measurements on copy states

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The detection and characterisation of multipartite entanglement is a rich and convoluted field, with many proposed methods and measures. The recently introduced family of measures 'concentratable entanglements' [1] encompasses many well-known multipartite entanglement measures and can be directly and efficiently estimated via parallelised controlled SWAP test [2] or parallelised Bell basis measurements [3]. I will introduce these two-copy tests and present an error treatment scheme based on the underlying structure of the tests' outputs [4].

[1] Beckey et al. Phys. Rev. Lett. 127, 140501 (2021)

[2] Foulds et al. QST 6, 035002 (2021)

[3] Beckey et al. arXiv:2210.02575 (2022)

[4] Prove et al. arXiv:2112.04333 (2022)

Robust Quantum Information Processing using Quantum Spin Networks

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"Having the ability to transfer quantum states or to generate entangled states with high fidelity is essential for Quantum Information Processing (QIP). Spin chain systems have been shown as a promising candidate for QIP, as they can be used to achieve quantum state transfer with high fidelity and to generate entanglement [1,2,3]. An extension to 1D spin chain systems is 2D Spin Network (SN) systems, which involves more topology and therefore more interesting phenomena can be achieved [4]. In this talk, I will first go through background and current state-of-art of spin chains. Then I will introduce our SN system and illustrate how it is designed. Then, I will demonstrate the performance of our SN for delivering various phenomena, such as routing, entanglement generation and phase sensing, in the presence of different types of disorder [4]. Finally, the scalability of our SN for long-range QIP as well as the realisation of different types of entanglement will also be shown [5]."

[1] Bose, Sougato. "Quantum communication through an unmodulated spin chain." Physical review letters 91.20 (2003): 207901.

[2] Estarellas, Marta P., Irene D'Amico, and Timothy P. Spiller. "Robust quantum entanglement generation and generation-plus-storage protocols with spin chains." Physical Review A 95.4 (2017): 042335.

[3] Apollaro, Tony JG, Guilherme MA Almeida, Salvatore Lorenzo, Alessandro Ferraro, and Simone Paganelli. "Spin chains for two-qubit teleportation." Physical Review A 100, no. 5 (2019): 052308.

[4] Alsulami, Abdulsalam H., Irene D'Amico, Marta P. Estarellas, and Timothy P. Spiller. "Unitary Design of Quantum Spin Networks for Robust Routing, Entanglement Generation, and Phase Sensing." Advanced Quantum Technologies 5, no. 8 (2022): 2200013.

[5] Alsulami, Abdulsalam H., Irene D'Amico, Marta P. Estarellas, and Timothy P. Spiller. " Scalable Quantum Spin Networks for Robust Quantum Information Processing.” To be published.

Optically trapped exciton-polariton condensates in magnetic fields 

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Nonequilibrium polariton condensates, or quantum fluids of light, are created with various methods. One of them stands out: nonresonantly pumped non-Hermitian annular traps simultaneously feed and confine polariton condensates. Moreover, they provide many handles overcondensate degrees of freedom: polarization and angular momentum. Some of these will be discussed with the focus on emergent effective and real externally applied magnetic fields.

Transverse magnetic routing of light emission and plasmon-to-exciton spin conversion in semiconductor-metal hybrid nanostructures

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Tailoring the optical and magnetic properties of semiconductor structures by bringing them together with plasmonic and ferromagnetic materials can be used to establish new functionalities in spintronic and photonic devices. The talk is focused on manifestation of novel magneto-optical phenomena in hybrid structures, which combine semiconductors quantum wells (QW) with excellent optical properties and metalic (plasmonic) nanostructures (gratings) with strong localization of electromagnetic field and tailored polarization properties.

First, a new class of emission phenomena where directionality is established perpendicular to the externally applied magnetic field for light sources located in the vicinity of a surface is presented. Here, we revealed transverse magnetic routing of light emission for excitons in a diluted-magnetic semiconductor QW [1]. When the distance between the excitons and the surface is large, the transverse spin of the emitted light is caused by the far-field interference effect and the routing is weak. The strongest directionality is achieved for a QW located several tens of nm apart from a metal-semiconductor interface. At such distance the QW is coupled to surface plasmon polaritons, that carry large transverse spin (spin flux) and are efficiently controlled by the magnetic field direction. In hybrid plasmonic semiconductor structures the directionality can reach up to 60%, which means that the ratio of light intensities for opposite emission angles is equal to 4.

Second, we demonstrate optical orientation of electron spins by linearly polarized light via plasmon-to-exciton spin conversion in the same type of hybrid plasmonic structures [2]. The metallic grating is exploited for the generation of plasmonic spin fluxes on ultimately short timescales. Using a pump-probe Kerr rotation experiment with 30 fs optical pulses we resolve in time the THz Larmor precession of photoexcited electron spins in external magnetic field, which is applied in-plane of the QW structure along the SPP propagation direction. It is demonstrated that the pump induced orientation of the photoexcited electron spins is locked to the propagation direction of the optically excited SPPs in hybrid nanostructure. Next, we show that using the polarization of the incident light as an additional degree of freedom, one can adjust not only the longitudinal, but also the transverse electron spin components normal to the light propagation at will.


[1] F. Spitzer, A.N. Poddubny, I.A. Akimov, et al., "Routing the emission of a near-surface light source by a magnetic field", Nature Physics 14, 1043 (2018).

[2] I. A. Akimov, A. N. Poddubny, J. Vondran, et al., Plasmon-to-exciton spin conversion in semiconductor-metal hybrid nanostructures, Phys. Rev. B 103, 085425 (2021).