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A huge effort is underway to develop semiconductor nanostructures as low noise hosts for qubits. The main source of dephasing of an electron spin qubit in a GaAs-based system is the nuclear spin bath 1-3 . A hole spin may circumvent the nuclear spin noise 4 . In principle, the nuclear spins can be switched off for a pure heavy-hole spin 4-6 . In practice, it is unknown to what extent this ideal limit can be achieved. A major hindrance is that p-type devices are often far too noisy. We investigate here a single hole spin in an InGaAs quantum dot embedded in a new generation of low-noise p-type device. We measure the hole Zeeman energy in a transverse magnetic field with 10 neV resolution by dark state spectroscopy as we create a large transverse nuclear spin polarization. The hole hyperfine interaction is highly anisotropic: the transverse coupling is < 1% of the longitudinal coupling. For unpolarized, randomly fluctuating nuclei, the ideal heavy-hole limit is achieved down to neV energies; equivalently dephasing times up to a µs. The combination of large T * 2 and strong optical dipole 3,7-11 make the single hole spin in a GaAs-based device an attractive quantum platform.A localized, single spin is a small and fast qubit. The exchange interaction between neighbouring spins facilitates two-qubit operations 12 . Implementation in a semiconductor benefits from advanced semiconductor heterostructures and nano-fabrication; implementation in a GaAs-based heterostructure, the cleanest and most versatile semiconductor system, by trapping the spin to a quantum dot also facilitates the creation of a spin-photon interface. A stumbling block is that the nuclear spins in the quantum dot lead to a rapid loss of electron spin coherence (both T 2 and T * 2 processes). This has motivated interest in isotopically-pure silicon, a nuclear spin-free host 13 . However, the large effective mass of a conduction electron in Si demands much smaller structures and lower operating temperatures; the valley degeneracy is an additional complication 14 ; the strong

A biexciton in a semiconductor quantum dot is a source of polarization-entangled photons with high potential for implementation in scalable systems. Several approaches for nonresonant, resonant, and quasiresonant biexciton preparation exist, but all have their own disadvantages; for instance, low fidelity, timing jitter, incoherence, or sensitivity to experimental parameters. We demonstrate a coherent and robust technique to generate a biexciton in an InGaAs quantum dot with a fidelity close to 1. The main concept is the application of rapid adiabatic passage to the ground-state-exciton-biexciton system. We reinforce our experimental results with simulations which include a microscopic coupling to phonons. DOI: 10.1103/PhysRevB.95.161302 Entangled photon pairs are a powerful resource, especially for quantum teleportation and quantum key distribution protocols. Spontaneous parametric down-conversion in nonlinear optics is a source of entangled photon pairs [1], but success is not guaranteed-the emission is a probabilistic process-and the error rate is high. In contrast, semiconductor quantum dots (QDs) are bright, on-demand sources of both single photons [2] and entangled photon pairs and hence have enormous potential in quantum computing and quantum cryptography [3].A biexciton in a QD is the starting point for a two-photon cascade: when perfectly prepared, biexciton decay leads to the subsequent emission of two photons [ Fig. 1(f)]. In a QD without a significant fine-structure splitting (FSS), the two photons are polarization entangled [4]. The majority of InGaAs QDs show a FSS due to a reduced symmetry [5][6][7]. However, sophisticated techniques were developed to compensate for the FSS with strain [8], electric [9], or magnetic fields [10,11] and with special growth conditions [7].Several approaches for biexciton preparation have been proposed [12][13][14][15] and demonstrated [4,[16][17][18][19][20]. Resonant two-photon schemes involving Rabi rotations [4,17,18] are sensitive to fluctuations in both laser power and QD optical frequency. They are likely to suffer from an imperfect biexciton preparation resulting in undesired exciton photons unrelated to the cascade process.A more robust scheme using phonon-assisted excitation was reported by several groups recently [18][19][20][21][22]. An impressively high biexciton occupation of up to 95% was demonstrated using this quasiresonant scheme [20]. But the strength here is also a weakness. The scheme relies on the coupling to the phonon bath in the semiconductor environment: it is an inherently incoherent process. Also, a dependence on relaxation processes in the state preparation results in a timing jitter. In some cases, charge-carrier relaxation times can reach values of up to a nanosecond [23].We present here a coherent technique to create a biexciton with high probability, low jitter, and weak dependence on the * timo.kaldewey@unibas.ch excitation and system parameters. The technique is based on rapid adiabatic passage (RAP). RAP allows the robust creatio...

Excitation of a semiconductor quantum dot with a chirped laser pulse allows excitons to be created by rapid adiabatic passage. In quantum dots this process can be greatly hindered by the coupling to phonons. Here we add a high chirp rate to ultrashort laser pulses and use these pulses to excite a single quantum dot. We demonstrate that we enter a regime where the exciton-phonon coupling is effective for small pulse areas, while for higher pulse areas a decoupling of the exciton from the phonons occurs. We thus discover a reappearance of rapid adiabatic passage, in analogy to the predicted reappearance of Rabi rotations at high pulse areas. The measured results are in good agreement with theoretical calculations. DOI: 10.1103/PhysRevB.95.241306 In semiconductors, a driven electron is damped by the interaction with phonons. In the context of quantum control, phonons lead to dephasing. The electron-phonon interaction is therefore important in the development of quantum technology with semiconductors. It is a rich and subtle subject.One possible way to suppress electron-phonon damping is to drive the electronic system so quickly that the relatively large inertia of the phonons prevents them from reacting to the driven electron. In the context of Rabi oscillations, the driven oscillations of a two-level system, a "reappearance" has been predicted [1]. As the drive is increased, the Rabi oscillations are initially damped more and more by the phonons, but then the damping decreases and is eventually suppressed. The reappearance regime represents phonon-free quantum control. It has, however, never been observed experimentally. Here, we demonstrate the experimental realization of the reappearance regime. Validation comes from a full microscopic theory.Our quantum system is a single self-assembled quantum dot (QD), an emitter of highly coherent single photons [2,3] and polarization-entangled photon pairs [4,5]. Quantum control of the exciton, an electron-hole pair, proceeds on picosecond timescales well before spontaneous emission takes place (timescale ∼1 ns). Phonons lead to a deterioration of the exciton preparation fidelity for schemes using resonant excitation [1,[6][7][8][9][10]. In fact, the interaction with the phonons is sufficiently strong that an exciton state can be prepared by relying on it (phonon-mediated relaxation following excitation with a detuned pulse) [11][12][13][14][15]. In a Rabi experiment, phonons lead to a clear damping [6,7]. The specific dephasing mechanism was identified as a coupling to longitudinal acoustic (LA) phonons. For higher pulse areas, theory predicts that the electronic oscillations become so fast such that the phonons decouple and the Rabi oscillations recover, the reappearance phenomenon. The existence of a pulse area for which the coupling to the phonons is maximal is a consequence of the nonmonotonic electron-phonon coupling [1,16].For the pulses used so far experimentally (pulses of 1-10 ps duration), the reappearance regime for Rabi oscillations can only be entered at ex...