Macroscopic rotation of photon polarization induced by a single spin (original) (raw)

2015, Nature Communications

Solid-state spins hold many promises for quantum information processing 1,2,3,4,5. Entangling the polarization of a single photon to the state of a single spin would open new paradigms in quantum optics like delayed-photons entanglement 6 , deterministic logic gates 7 or fault-tolerant quantum computing 8. These perspectives rely on the possibility that a single spin induces a macroscopic rotation of a photon polarization 9. Such polarization rotations induced by single spins were recently observed, yet limited to a few 10-3 degrees 10,11,12 due to poor spin-photon coupling. Here we report the amplification by three orders of magnitude of the spin-photon interaction, using a cavity quantum electrodynamics device. A single hole spin trapped in a semiconductor quantum dot is deterministically coupled 13 to a micropillar cavity. The cavity-enhanced coupling 14 between the incoming photons and the solid-state spin results in a polarization rotation by ±6° when the spin is optically initialized in either the up or down state. These results open the way towards a spin-based quantum network. Spin-photon entanglement has recently been demonstrated, between a photon emitted by a quantum emitter and the spin degree of freedom of the same emitter 3,4,5. A more scalable venue to spin-photon interfacing is to make use of the rotation of optical polarization (so-called Faraday or Kerr rotation) induced by a single spin placed at the center of a cavity-quantum electrodynamics (QED) device. This approach allows interfacing the spin with a photon generated by an external source, opening new possibilities in quantum optics 6,7,8,9,15,16,17 such as the engineering of temporally-delayed photon-photon interactions 6. While Faraday or Kerr polarization rotation in a magnetised medium is routinely used for magnetic material characterisation 18 , observations of Kerr rotation induced by a single spin were reported only recently 10,11,12 , with rotation angles in the few 10-3 degrees range. In this work, a resident hole spin in a semiconductor quantum dot-pillar cavity device is initialized 19 and measured using resonant pump and probe beams: a Kerr rotation of several degrees is obtained. Still larger polarization rotations are found to be achievable with realistic cavity-QED devices, allowing the implementation of quantum functionalities with single photons interfaced to stationary spin qubits. We study a single hole spin in a quantum dot (QD) efficiently coupled to the mode of a micropillar cavity (Fig. 1a). The pillar device, deterministically fabricated with the in-situ lithography technique 13 , presents an optimal QD-cavity coupling strength, mostly polarization-degenerate optical modes, and an efficient coupling with external beams 14,20. The quantities describing this device, sketched in Fig. 1b, are the QD-cavity coupling strength g, the QD dephasing rate γ, and the total damping rate κ = κ 1 + κ 2 + κ s (with κ 1, κ 2 and κ s the top mirror, bottom mirror and side leakage rates). These parameters are combined into two independent figures of merit, the top mirror output coupling efficiency κ 1 /κ and the device cooperativity C=g 2 /κ γ .