The control of discrete quantum states in solids and their use for quantum information processing is complicated by the lack of a detailed understanding of the mechanisms responsible for qubit decoherences [1]. For spin qubits in semiconductor quantum dots, phenomenological models of decoherence currently recognize two basic stages [2-4]; fast ensemble dephasing due to the coherent precession of spin qubits around nearly static but randomly distributed hyperfine fields (∼ 2 ns) [5-8] and a much slower process (> 1 µs) of irreversible relaxation of spin qubit polarization due to dynamics of the nuclear spin bath induced by complex many-body interaction effects [9]. We unambiguosly demonstrate that such a view on decoherence is greatly oversimplified; the relaxation of a spin qubit state is determined by three rather than two basic stages. The additional stage corresponds to the effect of coherent dephasing processes that occur in the nuclear spin bath that manifests itself by a relatively fast but incomplete non-monotonous relaxation of the central spin polarization at intermediate (∼ 750 ns) timescales. This observation changes our understanding of the electron spin qubit decoherence mechanisms in solid state systems.

Pulsed resonant fluorescence is used to probe ultrafast phonon-assisted exciton and biexciton preparation in individual self-assembled InGaAs quantum dots. By driving the system using large area (≥ 10π) near resonant optical pulses, we experimentally demonstrate how phonon mediated dissipation within the manifold of dressed excitonic states can be used to prepare the neutral exciton with a fidelity ≥ 70%. By comparing the phonon-assisted preparation with resonant Rabi oscillations we show that the phonon-mediated process provides the higher fidelity preparation for large pulse areas and is less sensitive to pulse area variations. Moreover, by detuning the laser with respect to the exciton transition we map out the spectral density for exciton coupling to the bulk LA-phonon continuum. Similar phonon mediated processes are shown to facilitate direct biexciton preparation via two photon biexciton absorption, with fidelities > 80%. Our results are found to be in very good quantitative agreement with simulations that model the quantum dot-phonon bath interactions with Bloch-Redfield theory.Due to their discrete electronic structure and strong interaction with light, self-assembled quantum dots (QDs) are often described as artificial atoms in the solid state. Indeed, many quantum optical experiments have recently been performed that exploit these atom-like properties; specific examples including deterministic single [11][12][13]. Moreover, in cavity QED experiments [14] remarkable effects such as photonblockade or photon-tunneling have opened the way to exploit QDs to generate novel quantum states of light [15]. In all these experiments, the solid state environment of the QDs manifests itself primarily in coupling to acoustic phonons -an effect that is unwanted since it results in decoherence of the quantum state, particular examples include incoherent population transfer between a QD and a detuned microcavity mode [16] or intra-molecular tunneling in vertically stacked QD-molecules [17,18]. Moreover, in coherent optical exciton control experiments, coupling to acoustic phonons dominates the damping of Rabi oscillations [19,20], limiting state preparation and control fidelities and the scope for possible applications. However, in other cases the coupling to acoustic phonons was exploited instead, e.g. for the high-fidelity spin initialisation by tunnel ionisation [21] or for achieving population inversion in electrically driven quantum dots [22]. Recently, Glässl et al. [23] proposed that high-fidelity preparation of excitonic states could be achieved by exploiting the coupling of the dressed excitonic states to a quasi continuum of vibrational modes. Their approach involves to combining the relative advantages of both Rabi oscillations and rapid adiabatic passage [24,25] by making use of phonon-mediated relaxation in the presence of a strong, near resonant pulsed optical field.In this paper we investigate the phonon-assisted preparation of neutral exciton (X 0 ) and biexciton (2X) states in QDs using pulsed resonant...

A two-level atom can generate a strong many-body interaction with light under pulsed excitation [1][2][3] . The best known e ect is single-photon generation, where a short Gaussian laser pulse is converted into a Lorentzian single-photon wavepacket 4,5 . However, recent studies suggested that scattering of intense laser fields o a two-level atom may generate oscillations in two-photon emission that come out of phase with the Rabi oscillations, as the power of the pulse increases 6,7 . Here, we provide an intuitive explanation for these oscillations using a quantum trajectory approach 8 and show how they may preferentially result in emission of two-photon pulses. Experimentally, we observe the signatures of these oscillations by measuring the bunching of photon pulses scattered o a two-level quantum system. Our theory and measurements provide insight into the re-excitation process that plagues 5,9 ondemand single-photon sources while suggesting the possibility of producing new multi-photon states.We begin by considering an ideal quantum two-level system that interacts with the outside world only through its electric dipole moment µ (ref. 10). Suppose the system is instantaneously prepared in the superposition of its ground |g and excited |e stateswhere P e is the probability of initializing the system in |e . From this point, spontaneous emission at a rate of Γ governs the remaining system dynamics and a single photon is coherently emitted with probability P e , while no photon is emitted with probability 1 − P e . As detected by an ideal photon counter, this results in the photocount distributionwhere P n is the probability to detect n photons in the emitted pulse. It is on this principle that most indistinguishable single-photon sources based on solid-state quantum emitters operate 4,5 .A popular mechanism for approximately preparing |ψ i is the optically driven Rabi oscillation 4,11 . Here, the system is initialized in its ground state and driven by a short Gaussian pulse from a coherent laser beam (of width τ FWHM ) that is resonant with the |g ↔ |e transition. Short is relative to the lifetime of the excited state τ e = 1/Γ to minimize the number of spontaneous emissions that occur during the system-pulse interaction 5,9 . As a function of the integrated pulse area, that is, A = dtµ · E(t)/ , where E(t) is the pulse's electric field, the system undergoes coherent oscillations between its ground |g and excited |e states. For constant-area pulses of vanishing τ FWHM /τ e , the final state of the system after interaction with the laser field is arbitrarily close to the superpositionwhere φ is a phase set by the laser field. Examining P e (A) (Fig. 1a dotted line), we see Rabi oscillations that are perfectly sinusoidal, with the laser pulse capable of inducing an arbitrary number of rotations between |g and |e . Because |ψ f (A) looks very much like |ψ i for arbitrarily short pulses, it is commonly assumed that the photocount distribution P n always has P 1 P n>1 . However, we will use a quantum trajectory appro...