External seminars

Controling light at the single-photon level with integrated quantum photonic devices

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Light-matter interactions allow adding functionalities to photonic on-chip devices, thus enabling developments in classical and quantum light sources, energy harvesters and sensors. These advances have been facilitated by precise control in growth and fabrication techniques that have opened new pathways to the design and realization of semiconductor devices where light emission, trapping and guidance can be efficiently controlled at the nanoscale. 

In this context, I will show the implementation of semiconductor quantum dots in nano-photonic devices that can create simultaneously bright and pure, triggered single-photon sources [1], critical for quantum information technology.  I will then present photonic geometries for controlling light propagation and brightness in broadband, scalable devices, based on plasmonic nanostructures [2]. 

Hybrid systems can allow overcoming limitations due to specific material properties and I will show how hybrid III-V/Silicon devices can be a platform for low-loss quantum light propagation [3]. I will also present a technique based on the transfer of semiconductor membranes embedding quantum emitters onto different host materials [4], for hybrid quantum photonic applications [5].

Finally, I will discuss photonic designs based on bio-inspired aperiodic, showing efficient light confinement [6] and quantum light emission control [7], and I will present our recent work on quantum biology, focused on the investigation of photosynthetic light harvesters on a chip.


[1] L. Sapienza et al., Nanoscale optical positioning of single quantum dots for bright and pure single-photon emission, Nature Communications 6, 7833 (2015).

[2] O.J. Trojak, S.I. Park, J.D. Song, L. Sapienza, Metallic nanorings for broadband, enhanced extraction of light from solid-state emitters, Applied Physics Letters 111, 021109 (2017).

[3] M. Davanco, J. Liu, L. Sapienza et al., Heterogeneous integration for on-chip quantum photonic circuits with single quantum dot devices, Nature Communications 8, 889 (2017).

[4] C. Haws, B. Guha, E. Perez, M. Davanco, J.D. Song, K. Srinivasan, L. Sapienza, Thermal release tape-assisted semiconductor membrane transfer process for hybrid photonic devices embedding quantum emitters, Materials for Quantum Technology 2, 025003 (2022). 

[5] C. Haws, E. Perez, M. Davanco, J.D. Song, K. Srinivasan, L. Sapienza, Broadband, efficient extraction of quantum light by a photonic device comprised of a metallic nano-ring and a gold back reflector, Applied Physics Letters 120, 081103 (2022). 

[6] O.J. Trojak, S. Gorsky, C. Murray, F. Sgrignuoli, F.A. Pinheiro, L. Dal Negro, L. Sapienza, Cavity-enhanced light–matter interaction in Vogel-spiral devices as a platform for quantum photonics,  Applied Physics Letters 118, 011103 (2021).

[7] O.J. Trojak, S. Gorsky, F. Sgrignuoli, F.A. Pinheiro, S.-I. Park, J.D. Song, L. Dal Negro, L. Sapienza, Cavity quantum electro-dynamics with solid-state emitters in aperiodic nano-photonic spiral devices, Applied Physics Letters 117, 124006 (2020).

Giant Excitonic Faraday Rotation in 2D Semiconductors

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Faraday rotation is a fundamental effect in the magneto-optical response of solids, liquids and gases. Materials with a large Verdet constant find applications in optical modulators, sensors and non-reciprocal devices. 

We measure Faraday rotation spectra around the neutral and charged exciton lines in hBN-encapsulated monolayers of WSe2 and MoSe2, and bilayers of MoS2 under moderate magnetic fields (B<1.4 T) [1] (see Fig. 1). For WSe2 and MoSe2 monolayers, the plane of polarization rotates by several degrees around exciton lines, resulting in a giant Verdet constant of 〖-1.9×10〗^7 deg T^(-1) cm^(-1) and 〖-2.3×10〗^7 deg T^(-1) cm^(-1), respectively. This is the largest measured Verdet constant in the visible/near-infrared regime. The giant Faraday rotation is due to the large oscillator strength and high g-factor of the excitons in monolayers. In comparison to monolayers, the Verdet constant reverses its sign for interlayer excitons in bilayer MoS2 (V_IL~+2×10^5 degT^(-1) cm^(-2)). We deduce the complete in-plane complex dielectric tensors of these materials, which is vital for the prediction of Kerr, Faraday and magneto-circular dichroism spectra of 2D heterostructures. For our measurements, we used a charge-coupled device-based Faraday rotation spectroscopy method for performing temperature-resolved spectroscopy on 2D materials on the microscale [2]. This method is about two-to-three orders of magnitude faster than state-of-the-art modulation magneto-spectroscopy methods, while providing a similar performance.

Finally, our results pose a crucial advance in the potential usage of two-dimensional materials in ultrathin optical polarization devices. 

[1] B. Carey, N. K. Wessling, P. Steeger, R. Schmidt, S. Michaelis de Vasconcellos, R. Bratschitsch, and A. Arora, Nat. Commun. 15, 3082 (2024).

[2] B. Carey, N. K. Wessling, P. Steeger, C. Klusmann, R. Schneider, M. Fix, R. Schmidt, M. Albrecht, S. Michaelis de Vasconcellos, R. Bratschitsch, and A. Arora, Small Methods 6, 2200885 (2022).

Brief biography. Ashish did his PhD from Tata Institute of Fundamental Research, Mumbai (India) in 2014. He was a CNRS postdoctoral researcher in National High Magnetic Field Laboratory (LNCMI-CNRS), Grenoble, France for a year (2014-15). He won the prestigious A. v. Humboldt grant for a postdoc in the University of Muenster, Germany (2015-17). Thereafter, he won the German Research Foundation (DFG) grant and stayed as a junior group leader in the University of Muenster until August 2021. He joined IISER Pune as an Assistant Professor in September 2021. Ashish has won the European Magnetic Field Laboratory EMFL prize 2019 ‘for his ground-breaking discoveries using the excellent infrastructure at the EMFL facilities’, Young Achiever Award 2023 in 67th DAE Solid State Physics Symposium ‘in recognition for his contribution in Condensed Matter Physics’ and Young Physicist Award at the National Physicists’ Conclave 2024 (India).

Integrated Photonics for Quantum Networks

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A key component of quantum communication is the distribution of entanglement through networks, however this comes with several challenges. One of the most significant is that direct distribution between two nodes via a standard fibre link is unfeasible for transmission distances of more than a few hundred kilometers [1]. A solution is the 'quantum repeater' architecture, which distributes entanglement between two nodes via an intermediate repeater node [2].

This talk will showcase our recent work towards entanglement distribution using integrated entangled photon-pair sources, based on silicon nitride micro-ring resonators [3]. Our narrow band photons, compatible with some quantum memories based on rare-earth ion schemes, will have direct application in future quantum repeater schemes which interface such sources with quantum memories.

[1] M. Krenn et al., Quantum Communication with Photons, pp. 455-482, Springer International Publishing (2016)

[2] N. Sangouard & H. Zbinden, J. Mod. Opt., 59, pp. 1458-1464 (2012)

[3] T. Brydges et al., Phys. Rev. A, 107, pp. 052602 (2023)

Optimal control in quantum thermodynamics


Excitation Techniques for Quantum Dots

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This talk will cover coherent and incoherent excitation schemes, including adiabatic rapid passage-enabled resonant excitations, phonon-assisted excitation, and the innovative red-detuned two-color excitation scheme known as 'Swing-UP of Quantum EmitteR' (SUPER).

Optical sensing of strongly correlated electronic phases in atomically-thin materials

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When the Coulomb repulsion between itinerant electrons dominates over their kinetic energy, the electrons in solid state materials start to develop strong correlations. A paradigm state of matter that is expected to emerge in this regime is an electronic Wigner crystal, in which the electrons spontaneously break continuous translation symmetry and arrange themselves into a periodic lattice mimicking that of the real crystals. In this talk, I will review our recent experimental explorations of strongly correlated electronic phases in transition metal dichalcogenides (TMD) and their van der Waals heterostructures. In particular, I will present our novel spectroscopic technique allowing us to detect the Wigner crystal in a TMD monolayer through the periodic potential it generates for the excitons [1]. This potential allows the excitons to undergo a Bragg diffraction, which gives rise to a new, Bragg-umklapp transition in the optical excitation spectrum that heralds the presence of a crystalline electronic order.

In the second part of the talk, I will describe magnetic properties of the electrons forming a charge-ordered Mott insulating state in a MoSe2/WS2 bilayer that hosts strong moire potential. In particular, I will show that when such a Mott state is electron-doped, the system exhibits unusual ferromagnetism [2] that is driven not by Coulomb exchange interactions, but arises due to minimization of kinetic energy of itinerant electrons. This observation provides the first direct evidence for the Nagaoka ferromagnetism in an extended two-dimensional system.

[1] T. Smoleński, P. E. Dolgirev, C. Kuhlenkamp, A. Popert, Y. Shimazaki, P. Back, X. Lu, M. Kroner, K. Watanabe, T. Taniguchi, I. Esterlis, E. Demler, and A. Imamoğlu, Signatures of Wigner crystal of electrons in a monolayer semiconductor. Nature 595, 53-57 (2021).

[2] L. Ciorciaro, T. Smoleński, I. Morera, N. Kiper, S. Hiestand, M. Kroner, Y. Zhang, K. Watanabe, T. Taniguchi, E. Demler, A. Imamoğlu, Kinetic Magnetism in Triangular Moire Materials. arXiv:2305.02150 (2023) (accepted for publication in Nature)

Biography paragraph:

I am a senior postdoctoral researcher in Quantum Photonics Group (QPG) of Prof. Atac Imamoglu at ETH Zurich, where I work on optical spectroscopy of strongly correlated electronic phases in atomically-thin transition metal dichalcogenides (TMDs) and their van der Waals heterostructures. Specifically, my research activity is primarily focused on the development of optical tools enabling detection of elusive collective phases of matter, such as fractional quantum Hall states or spatially-ordered electronic states. Before joining QPG, I received my PhD working on spectroscopy of semiconductor quantum dots containing individual transition metal ions in the group of Prof. Piotr Kossacki at the University of Warsaw. During this time, I was mainly concentrated on developing experimental methods for ultra-fast optical manipulation of the ion spin in such structures as well as on optimizing their performance as single-spin-based magnetic memories.

Living in both worlds: quantum academia and quantum industry

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The fields of quantum technologies and AI evolve rapidly, and offer unique opportunities for doing cutting edge research as a part of industrial job. In my talk I will discuss the transition from academic career, specifically quantum optics research, to the industrial quantum computing. The discussion emphasizes the significance of fostering active collaborations with companies and start-ups to address real-world challenges. By sharing personal experiences and insights, in this presentation I aim to inspire quantum researchers to match their academic pursuits with the dynamic demands of the industry. In particular, I will talk about research projects with HSCB and PASQAL on quantum machine learning, and highlight perks and challenges of consulting and various industrial roles. Everyone is welcome to join this example-driven exploration of the intersection between quantum academia and industry, and discover strategies for a successful career transition.

Efficient learning of t-doped stabilizer states with single-copy measurements

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Abstract: One of the primary objectives in the field of quantum state learning is to develop algorithms that are time-efficient for learning states generated from quantum circuits. Earlier investigations have demonstrated time-efficient algorithms for states generated from Clifford circuits with at most log(n) non-Clifford gates. However, these algorithms necessitate multi-copy measurements, posing implementation challenges in the near term due to the requisite quantum memory. On the contrary, using solely single-qubit measurements in the computational basis is insufficient in learning even the output distribution of a Clifford circuit with one additional T gate under reasonable post-quantum cryptographic assumptions. In this work, we introduce an efficient quantum algorithm that employs only nonadaptive single-copy measurement to learn states produced by Clifford circuits with a maximum of O(log n) non-Clifford gates, filling a gap between the previous positive and negative results.

Spectrally selective metasurfaces: a versatile platform for enhanced light-matter coupling

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Metasurfaces composed of sub-wavelength dielectric resonators are a powerful platform for controlling light on the nanoscale, providing significant advantages over their plasmonic counterparts. Combining them with the emerging physical concept of bound states in the continuum (BICs) has enabled the realization of metasurfaces with ultra-narrow line widths and therefore unprecedented spectral control over the nanophotonic enhancement. In my talk, I will introduce several examples of such spectrally selective metasurfaces and show how they can be used to open new perspectives both for fundamental light-matter coupling physics and for practical meta-device applications ranging from biodetection to van der Waals materials. Specifically, I will focus on the use of BIC-driven systems for the development of compact biospectroscopy devices with a vision towards point-of-care diagnostics and new concepts for strong light-matter coupling in polaritonic systems. 

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).

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