A basic requirement for quantum information processing systems is the ability to completely control the state of a single qubit. For qubits based on electron spin, a universal single-qubit gate is realized by a rotation of the spin by any angle about an arbitrary axis. Driven, coherent Rabi oscillations between two spin states can be used to demonstrate control of the rotation angle. Ramsey interference, produced by two coherent spin rotations separated by a variable time delay, demonstrates control over the axis of rotation. Full quantum control of an electron spin in a quantum dot has previously been demonstrated using resonant radio-frequency pulses that require many spin precession periods. However, optical manipulation of the spin allows quantum control on a picosecond or femtosecond timescale, permitting an arbitrary rotation to be completed within one spin precession period. Recent work in optical single-spin control has demonstrated the initialization of a spin state in a quantum dot, as well as the ultrafast manipulation of coherence in a largely unpolarized single-spin state. Here we demonstrate complete coherent control over an initialized electron spin state in a quantum dot using picosecond optical pulses. First we vary the intensity of a single optical pulse to observe over six Rabi oscillations between the two spin states; then we apply two sequential pulses to observe high-contrast Ramsey interference. Such a two-pulse sequence realizes an arbitrary single-qubit gate completed on a picosecond timescale. Along with the spin initialization and final projective measurement of the spin state, these results demonstrate a complete set of all-optical single-qubit operations.

No abstract

We observe antibunching in the photons emitted from a strongly-coupled single quantum dot and pillar microcavity in resonance. When the quantum dot was spectrally detuned from the cavity mode, the cavity emission remained antibunched, and also anticorrelated from the quantum dot emission. Resonant pumping of the selected quantum dot via an excited state enabled these observations by eliminating the background emitters that are usually coupled to the cavity. This device demonstrates an on-demand single photon source operating in the strong coupling regime, with a Purcell factor of 61 ± 7 and quantum efficiency of 97%.PACS numbers: 78.67. Hc, 78.55.Cr, 78.90.+t Cavity quantum electrodynamics (CQED), addressing the interaction between a quantum emitter and a cavity, has been a central topic in atomic physics for decades [1,2,3,4] and has recently come to the forefront of semiconductor physics [5,6,7,8]. If the coupling between the single quantum emitter and cavity mode is strong compared to their decay rates, the emitter and cavity coherently exchange energy back and forth leading to Rabi oscillations. This strong coupling (SC) regime is of great interest for a variety of quantum information applications, especially with a solid-state implementation. A SC QD-microcavity system could lead to a nearly ideal single photon source (SPS) for quantum information processing, with extremely high efficiency and photon indistinguishability [9]. The same technology could be applied as an interface between a spin qubit and single photon qubit in a quantum network [10].SC between a single atom and a cavity was first achieved more than a decade ago [4]. An analogous system in the solid-state is the excitonic transition of a semiconductor quantum dot (QD) together with a semiconductor microcavity. Several groups have recently reported SC between a single (In,Ga)As QD and either micropillar [5], photonic crystal [6], or microdisk [7] resonators. SC can also occur between a single cavity mode and a collection of degenerate emitters, such as an ensemble of atoms or a quantum well [11]. However, in the latter case the behavior is classical: adding or removing one emitter or one photon from the system has little effect.In previous studies of QD-cavity SC [5,6,7] it was argued that the spectral density of QDs was sufficiently low that it is unlikely that several degenerate emitters contributed to the anticrossing. However, it was not verified that the system had one and only one emitter. There was a surprisingly large amount of emission from the cavity mode when the QD was far detuned. It was unclear whether this emission originated from the particular single QD or from many background emitters. An important step to establish SC in solid-state CQED is verification that the double-peaked spectrum originates from a single quantum emitter, not a collection of emitters, interacting with the cavity mode.In this Letter we present proof that the emission from a strongly-coupled QD-microcavity system is dominated by a single quantum emitter....

Future communication and computation technologies that exploit quantum information require robust and well-isolated qubits. Electron spins in III-V semiconductor quantum dots, while promising candidates, see their dynamics limited by undesirable hysteresis and decohering effects of the nuclear spin bath. Replacing electrons with holes should suppress the hyperfine interaction and consequently eliminate strong nuclear effects. Using picosecond optical pulses, we demonstrate coherent control of a single hole qubit and examine both free-induction and spin-echo decay. In moving from electrons to holes, we observe significantly reduced hyperfine interactions, evidenced by the reemergence of hysteresis-free dynamics, while obtaining similar coherence times, limited by non-nuclear mechanisms. These results demonstrate the potential of optically controlled, quantum dot hole qubits. arXiv:1106.5676v1 [quant-ph]

We study the momentum distribution and relaxation dynamics of semiconductor microcavity polaritons by angle-resolved and time-resolved spectroscopy. Above a critical pump level, the thermalization time of polaritons at positive detunings becomes shorter than their lifetime, and the polaritons form a quantum degenerate Bose-Einstein distribution in thermal equilibrium with the lattice. DOI: PACS numbers: 71.36.+c, 42.50.ÿp, 78.47.+p, 78.67.ÿn Bose-Einstein condensation (BEC) has been of intense interest to the physics community for decades [1][2][3][4][5]. While atom BEC has been demonstrated since 1995 in various species of atomic gases, no analogue has been established in solid state systems. The outstanding problem for solid state BEC is to have an equilibrium system with high enough density. Earlier works in Cu 2 O excitons showed thermal equilibrium but no quantum degeneracy [6,7]. More recent works on quantum-well excitons showed indirect evidence of quantum degeneracy, but thermal equilibrium could not be inferred [8][9][10][11]. Solid state polariton systems are interesting because they have an effective mass 8 orders of magnitude smaller than the hydrogen atom mass, and 4 orders of magnitude smaller than the exciton mass. Thus the critical temperature of polariton phase transitions range from 1 K to above room temperature. Quantum degeneracy has been demonstrated by many groups in recent years [12 -16], yet thermal equilibrium is never established. In the current work, we obtained for the first time clear and direct evidence of simultaneous thermal equilibrium and quantum degeneracy.In a semiconductor microcavity with embedded quantum wells (QWs), when the confined cavity photon modes strongly couple to the QW excitons, new eigenmodes are formed called the polaritons [17]. As quasiparticles in semiconductors, polaritons have relatively short lifetimes, thus it is generally difficult to cool hot polaritons to the lattice temperature before they decay. On the lower energy branch, polaritons change from excitonlike lower polaritons (ELPs) at large in-plane wave number k to halfexciton half-photon lower polaritons (LPs) at k 0. Correspondingly, their lifetime decreases by 2 orders of magnitude and their energy density of states decreases by 4 orders of magnitude. Hence an energy relaxation bottleneck is commonly observed [18][19][20] at low densities where spontaneous linear phonon-LP scattering is the dominant, yet insufficient, cooling mechanism. At higher densities, however, when the quantum degeneracy condition of N LP 1 is fulfilled, bosonic final state stimulation greatly enhances both the nonlinear LP-LP scattering and the linear LP-phonon scattering [21][22][23]. Recently, a degenerate Bose-Einstein distribution (BED) of LPs has been observed [24], but the LP temperature was T LP 100 K, much higher than the lattice temperature T lat 4:2 K. This suggests that although LP-LP scattering establishes quasiequilibrium among LPs, cooling by the phonon bath is still slower than the decay of the LPs.Fortu...