October 2020
Abstracts of the QSIT Lunch Seminar, Thursday, October 1, 2020
Genuine high-dimensional quantum steering
Sébastien Designolle - Quantum Theory Group (Brunner group), University of Geneva
High-dimensional quantum entanglement can give rise to stronger forms of nonlocal correlations compared to qubit systems. Beyond being of fundamental interest, this offers significant advantages for quantum information processing. The problem of certifying these stronger correlations, however, remains an important challenge, in particular in an experimental setting. Here we theoretically formalise and experimentally demonstrate a notion of genuine high-dimensional quantum nonlocal steering. We show that high-dimensional entanglement combined with judiciously chosen local measurements can lead to a stronger form of steering, provably impossible to obtain via entanglement in lower dimensions. Exploiting the connection between steering and incompatibility of quantum measurements, we derive simple two-setting steering inequalities for certifying the presence of genuine high-dimensional steering. We report the experimental violation of these inequalities using macro-pixel photon-pair entanglement certifying genuine high-dimensional steering in dimensions up to d=15. Our work paves the way for the characterisation and certification of quantum nonlocal correlations in high-dimensional systems.
Light-mediated strong coupling between a mechanical oscillator and atomic spins 1 m apart
Thomas Karg - Quantum Optics Lab (Treutlein group), University of Basel
Strong coupling between quantum systems is an essential ingredient for quantum technology. Typically, it relies on short-range forces or on placing the systems in high-quality electromagnetic resonators. To break the restriction to short distances, we developed a light-mediated coupling scheme that enables long-distance Hamiltonian coupling [1,2].
Our experimental setup consists of a spin-polarized atomic ensemble and a micromechanical membrane oscillator held in independent vacuum systems 1 meter apart. To mediate a bidirectional interaction between the spin and the membrane, we use a free-space laser beam which connects them in a loop geometry. This coherent feedback loop will also allow destructive interference of quantum noise. In this setup, we realize strong Hamiltonian coupling and demonstrate the versatility of light-mediated interactions [3]: With the spin initialized in its ground state we observe normal-mode splitting and coherent energy-exchange oscillations, both hallmarks of strong coupling. Upon inversion of the spin to its highest energy state we observe parametric-gain interactions, resulting in two-mode thermal noise squeezing. Furthermore, by shifting the phase of the coupling beam between the systems we could switch to non-Hamiltonian coupled dynamics, which were marked by the observation of level attraction involving an exceptional point.
Our approach to engineering coherent long-distance interactions with light makes it possible to couple physically different systems in a reconfigurable way and opens up a range of new opportunities for quantum control and quantum networks.
References:
[1] A. F. Kockum, G. Johansson, F. Nori, Phys. Rev. Lett. 120, 140404 (2018).
[2] T. M. Karg, B. Gouraud, P. Treutlein, K. Hammerer, Phys. Rev. A 99, 063829 (2019)
[3] T. M. Karg, B. Gouraud, C. T. Ngai, G.-L. Schmid, K. Hammerer, and P. Treutlein, Science 369, 174 (2020).