July 2020

Abstracts of the QSIT Lunch Seminar, Thursday, July 2, 2020

Initialization of Single Spin Dressed States using Shortcuts to Adiabaticity

Johannes Kölbl - Quantum Sensing Lab (Maletinsky group), University of Basel

Quantum technologies building on solid-state spin systems like the Nitrogen-Vacancy (NV) center in diamond usually involve the use of magnetic field-sensitive states. For these, ambient magnetic field fluctuations constitute a serious impediment that shortens the coherence time considerably. Dynamical decoupling is a widely used approach to protect such individual quantum systems from decoherence and thereby to overcome this limitation. In particular, continuous, 'closed-contour' driving of the NV's three-level spin system gives rise to three-level dressed states, which have recently been shown to exhibit efficient coherence protection from external magnetic fluctuations, beyond what their two-level counterparts can offer. However, realizing direct and efficient access to these dressed states challenging. In my talk, I demonstrate the use of state transfer protocols for initialization, readout, and coherent control of the dressed states under closed-contour driving. As a starting point for our studies, we use protocols based on the adiabatic theorem, in which the gradual changes of the controls allow the system to adapt its configuration. However, these processes are intrinsically slow and, therefore, have limited effectiveness. To mitigate this drawback, we take advantage of approaches for speeding up adiabatic protocols, collectively known as `shortcuts to adiabaticity'. These general strategies aim to remedy the limitation of adiabatic approaches by designing fast dynamics that reproduce the results of a slow, adiabatic transition. Using such a technique ultimately yields a transfer fidelity of ~ 99 % while accelerating the transfer speed by a factor of 2.6 compared to the fastest adiabatic protocol with similar fidelity. We further show bidirectionality of the accelerated state transfer, which allows us to directly read out the dressed states after coherently manipulating them. Thus, our results enable direct and efficient access to coherence-protected dressed states of individual spins and thereby offer attractive avenues for applications in quantum information processing or quantum sensing.
 

Accelerating Polaritons using External Electric and Magnetic Fields

Patrick Knüppel – Quantum Photonics Group (Imamoglu group), ETH Zurich

We study a 2D electron system embedded in an optical microcavity. Cavity photons are strongly coupled to Fermi polarons, which leads to the formation of polaron-polaritons [1]. A sample in Hall bar geometry allows us to apply bias voltages and source-drain currents. Applying a large bias voltage at one contact eventually leads to a depletion region in its surroundings. In this regime, we can tune the electron-density gradients across the Hall bar. We explore how this density gradient results in a force on polaritons that resembles an effective electric field for polaritons [2]. When the 2D electron system enters a ferromagnetic state in the quantum Hall regime, the applied bias voltage leads to the formation of electron spin-density gradients. This realizes optical spin-selective forces for polaritons.

[1] S. Ravets et al., Phys. Rev. Lett. 120, 057401 (2018). external page https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.057401
[2] T. Chervy et al., Phys. Rev. X 10 011040 (2020). external page https://journals.aps.org/prx/abstract/10.1103/PhysRevX.10.011040