A spin-photon interface: quantum entanglement and quantum measurements – SPIQE

The main goal of the present project is to investigate both experimentally and theoretically the potential of a semiconductor spin-photon interface for quantum information processing. Our objectives range from fundamental studies in quantum physics, to demonstrations of first functionalities for quantum information. To implement the spin-photon interface we will make use of the Faraday/Kerr rotation effect: when a single photon interacts with a spin, its polarization is rotated depending on the spin state. We will take advantage of the recent breakthrough experimentally demonstrated in LPN: by embedding a single quantum dot spin in a pillar cavity-QED device, a giant amplification of the Faraday/Kerr effect is obtained, by three orders of magnitude. Moreover, thanks to the very high coupling between the pillar cavity and the external environment, it becomes possible to address the device with single incident photons.

The spin-photon interface opens major perspectives for quantum physics and quantum information processing, such as the Quantum Non Demolition measurement of the spin qubit using a single photon, and the possible engineering of spin-photon entanglement. This novel form of entanglement, where a stationary spin qubit is entangled with a single incident photon generated by an external source, has never been achieved before. It constitutes a crucial milestone towards the connection of separate quantum systems via flying qubits.

On the way to the demonstration of a truly ideal spin-photon interface, important breakthroughs are expected:
- We will enhance the spin-photon interaction, such that fast QND readout of the spin becomes possible, allowing direct monitoring of the spin dynamics in real time.
- To engineer a coherent superposition of the spin states, Rabi oscillations will be induced and observed using either weak or strong Quantum Non Demolition measurements.
- A spin-photon entangled state will be produced and observed, using correlation measurements in order to deduce the entanglement fidelity.
- The spin-photon interface will be modeled, to simulate its use in the above-mentioned experiments, and to guide the optimization of the devices and experimental setups.
- The interaction between the spin qubit and the measured environment will be modeled, to describe the measurement-induced back action on the spin system.
- Finally, the potential of quantum measurements on this new platform will be investigated in the framework of emerging topics in fundamental physics.