Spin-resolved quantum-dot resonance fluorescence

Confined spins in self-assembled semiconductor quantum dots promise to serve both as probes for studying mesoscopic physics in the solid state and as stationary qubits for quantum-information science1–7. Moreover, the excitations of self-assembled quantum dots can interact with nearinfrared photons, providing an interface between stationary and ‘flying’ qubits. Here, we report the observation of spinselective photon emission from a resonantly driven quantumdot transition. The Mollow triplet8 in the scattered photon spectrum—the hallmark of resonance fluorescence when an optical transition is driven resonantly—is presented as a natural way to spectrally isolate the photons of interest from the original driving field. We also demonstrate that the relative frequencies of the two spin-tagged photon states can be tuned independent of an applied magnetic field through the spin-selective dynamic Stark effect, induced by the same driving laser. This demonstration should be a step towards the realization of challenging tasks such as electron-spin readout, heralded single-photon generation for linear-optics quantum computing and spin–photon entanglement. In the realm of solid-state emitters generating flying qubits, milestone achievements so far include single-photon antibunching9,10, entangled photon-pair generation11 and cavity quantum electrodynamics in the strong coupling regime12–14. A common feature in all of these studies is the incoherent pumping of the quantum-dot transitions through carrier generation in either the host matrix such as GaAs or the quasi-continuum states above the higher-lying confined states of the quantum dot. This excitation method leads to photon-emission-time jitter and spectral wandering of the quantum-dot transition larger than the transition’s linewidth. Both effects reduce the usefulness of non-resonantly generated single photons in linear-optics quantum computing algorithms15, even if the quantum dot is coupled to a cavity. In an attempt to both address this previous shortcoming and provide spectrally selective access to the quantum-dot electronic transitions, increasing attention has turned to resonant optical excitation. So far, resonant optical addressing of quantum dots has relied predominantly on a remarkably powerful laser-based spectroscopy technique: differential transmission16. Although differential transmission has enabled progress in spin-selective excitation of quantum dots, access to the scattered photons correlated with the quantum-dot spin has proven elusive. Recently, differential transmission and, independently, cavity-assisted temporal correlation measurements have shown clear signatures of dressed-state formation under strong laser light excitation on neutral quantum dots17–21. Noting all successful quantum-information science (QIS) implementations on well-developed qubit candidates, such as trapped ions, have relied on spin-selective resonance scattering22, it is clear that an immediate goal for quantum dots is the observation of