Below you will find a list of seminars organised by ICTQT.
(click on Abstract to expand the text)
Speaker: Ekta Panwar (UG/ICTQT)
Abstract
The predictions of quantum theory are incompatible with local-causal explanations. This phenomenon is called Bell non-locality and is witnessed by the violation of Bell-inequalities. The maximal violation of certain Bell-inequalities can only be attained in an essentially unique manner. This feature is referred to as self-testing and constitutes the most accurate form of certification of quantum devices. While self-testing in bipartite Bell scenarios has been thoroughly studied, self-testing in the more complex multipartite Bell scenarios remains largely unexplored. This work presents a simple and broadly applicable self-testing argument for N-partite correlation Bell inequalities with two binary outcome observables per party. Our proof technique forms a generalization of the Mayer-Yao formulation and is not restricted to linear Bell-inequalities, unlike the usual sum of squares method. To showcase the versatility of our proof technique, we obtain self-testing statements for N party Mermin-Ardehali-Belinskii-Klyshko (MABK) and Werner-Wolf-Weinfurter-Zukowski-Brukner (WWWZB) family of linear Bell inequalities, Uffink’s family of N party quadratic Bell-inequalities, and the novel Uffink’s complex-valued N partite Bell expressions.Speaker: Rafał Demkowicz Dobrzański (University of Warsaw)
Speaker: Jan Kolodynski (Center of New Technologies, University of Warsaw)
Abstract
Device-independent quantum key distribution (DIQKD) constitutes the most pragmatic approach to quantum cryptography that does not put any trust in the inner workings of the devices. This is possible by constructing security proofs at the level of correlations being shared by the end-users, leveraging from the phenomenon of Bell nonlocality. In particular, quantum nonlocality allows one then to lower-bound the asymptotically achievable key rates, even in the presence of the most general eavesdropping attacks. However, only recently first proof-of-principle implementations of DIQKD have been demonstrated, as the device-independent framework imposes very stringent requirements on the noise tolerance, even in the absence of any eavesdroppers. In our works, we follow a complementary approach in which we propose an easy-to-optimise attack on any DIQKD protocol, with help of which we construct upper bounds on the asymptotic key. On one hand, it allows us to disprove a long-standing conjecture that any form of Bell nonlocality is sufficient for distributing secret keys in a device-independent manner. On the other, it allows us to verify that current state-of-the-art implementations already operate very close to the ultimate noise thresholds, and cannot be thus improved by resorting to more profound security-proof techniques.Speaker: Adam Miranowicz, Adam Mickiewicz University (Poznań) and RIKEN (Wako)
Abstract
Experimental demonstrations and control of strong coupling of light and matter has lead to various applications for lasers, quantum sensing, and quantum information processing since 1980s. In my talk, I will review [1] recent theoretical and experimental progress in the ultrastrong coupling (USC) and deep-strong coupling (DSC) regimes of light and matter, which are characterized by the coupling strengths comparable to their transition frequencies. In the last few years, the USC regime has been experimentally achieved in a wide range of different systems with very different spectral ranges. These systems include: superconducting quantum circuits, intersubband polaritons, Landau polaritons, organic molecules, magnetic systems, nano-plasmonics, and optomechanical systems. Emerging applications of the USC and DSC regimes are focused on quantum technologies and quantum information processing. The ground state of light-matter systems in the DSC regime is a Schroedinger-cat state, where virtual photons are entangled with virtual excitations of the matter. A recent experimental demonstration of the cat state can be considered as the discovery of a new stable molecular state in which light and matter are hybridized [2]. The USC and DSC regimes can be effectively reached by squeezing a cavity field as proposed in [3] and experimentally demonstrated in [4]. This type of light squeezing can also be used to increase spin squeezing [5], which is of paramount importance for quantum metrology. The USC and DSC regimes enable also generating giant Schrodinger cat states of real photons and atomic real excitations by applying squeezing [6,7]. The USC methods have inspired developing a promising technique to beat the 3 dB limit for intracavity squeezing and, thus, to effectively apply it for nondemolition qubit experiments [8]. Co-authors: Franco Nori, Wei Qin, Ye-Hong Chen, Anton Kockum, Simone De Liberato, Salvatore Savasta [1] A. F. Kockum, A. Miranowicz, S. De Liberato, S. Savasta, and F. Nori: Ultrastrong coupling between light and matter, Nat. Rev. Phys. 1, 19 (2019).[2] F. Yoshihara, T. Fuse, S. Ashhab, K. Kakuyanagi, S. Saito, and K. Semba, Superconducting qubit-oscillator circuit beyond the ultrastrong-coupling regime, Nat. Phys. 13, 44 (2017).
[3] W. Qin, A. Miranowicz, P.-B. Li, X.-Y. Lu, J.-Q. You, and F. Nori: Exponentially Enhanced Light-Matter Interaction, Cooperativities, and Steady-State Entanglement Using Parametric Amplification, Phys. Rev. Lett. 120, 093601 (2018).
[4] S. C. Burd, R. Srinivas, H. M. Knaack, W. Ge, A. C. Wilson, D. J. Wineland, D. Leibfried, J. J. Bollinger, D. T. C. Allcock, and D. H. Slichter, Quantum amplification of boson-mediated interactions, Nat. Phys. 17, 898 (2021).
[5] W. Qin, Y.-H. Chen, X. Wang, A. Miranowicz, and F. Nori: Strong Spin Squeezing Induced by Weak Squeezing of Light inside a Cavity, Nanophotonics 9, 4853 (2020).
[6] W. Qin, A. Miranowicz, H. Jing, and F. Nori: Generating long-lived macroscopically distinct superposition states in atomic ensembles, Phys. Rev. Lett. 127, 093602 (2021).
[7] Y.-H. Chen, W. Qin, X. Wang, A. Miranowicz, F. Nori: Shortcuts to Adiabaticity for the Quantum Rabi Model: Efficient Generation of Giant Entangled Cat States via Parametric Amplification, Phys. Rev. Lett. 126, 023602 (2021).
[8] W. Qin, A. Miranowicz, and F. Nori: Beating the 3 dB Limit for Intracavity Squeezing and Its Application to Nondemolition Qubit Readout, Phys. Rev. Lett. 129, 123602 (2022).