Preparation of a specific quantum state is a required step for a variety of proposed practical uses of quantum dynamics. We report an experimental demonstration of optical quantum state preparation in a semiconductor quantum dot with electrical readout, which contrasts with earlier work based on Rabi flopping in that the method is robust with respect to variation in the optical coupling. We use adiabatic rapid passage, which is capable of inverting single dots to a specified upper level. We demonstrate that when the pulse power exceeds a threshold for inversion, the final state is independent of power. This provides a new tool for preparing quantum states in semiconductor dots and has a wide range of potential uses.Preparation of a specific quantum state in a semiconductor quantum system is a required step for quantum computation 1,2 , generation of single photons 3 and entangled photon pairs 4 , and studies of Bose-Einstein condensation 5 . A two-level quantum system, such as that of an exciton in a single quantum dot, can be driven into a specified state by use of a coherent interaction between the system and a tuned optical field. Previously, the interaction used to invert a two-level system in semiconductor quantum dots has driven the system with a resonant transform-limited light field. In this case, in the Bloch sphere representation the Bloch vector precesses about a field vector which lies in the equatorial plane, and so the optical pulse rotates the Bloch vector from its initial position at the south pole (ground state) through an angle θ = π to the north pole (inversion). The angle θ = (µ·E) h dt is defined as the pulse area in a Rabi rotation where µ is the dipole moment describing the twolevel system and E(t) is the envelope of the optical field. Coherent resonant interaction has been shown to be capable of generating several such Rabi cycles, and permits readout of the state of the system optically 6-8 , or electrically by ionisation of the optical excitation and extraction of a current 9 . The Rabi approach requires precise control over the integrated pulse area (determined by the temporal field profile and the dipole coupling strength) to achieve an inversion angle of π as shown schematically in Fig. 1a.Here we show experimentally that state preparation is also possible by adiabatic rapid passage (ARP), which has the advantage that it is largely unaffected by variation in the dipole coupling, which is a normal feature of dot systems, and likewise insensitive to variation in the optical field which typically arises from laser fluctuation or positional variation in arrays of dots 10 . Several theoretical proposals have recognized the potential of ARP excitation to create entanglement between locally separated electron spins for robust two-qubit quan-FIG. 1: Schematic representation of the dynamics of the twolevel quantum system in time in the (a) Rabi excitation regime with a transform-limited pulse and (b) the ARP regime with a chirped pulse. The curves are the eigenenergies versus time of the two ...

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We review the practical conditions required to achieve a non-equilibrium BEC driven by quantum dynamics in a system comprising a microcavity field mode and a distribution of localised two-level systems driven to a step-like population inversion profile. A candidate system based on eight 3.8nm layers of In0.23Ga0.77As in GaAs shows promising characteristics with regard to the total dipole strength which can be coupled to the field mode.

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