We demonstrate optical control of the geometric phase acquired by one of the spin states of an electron confined in a charge-tunable InAs quantum dot via cyclic 2π excitations of an optical transition in the dot. In the presence of a constant in-plane magnetic field, these optically induced geometric phases result in the effective rotation of the spin about the magnetic field axis and manifest as phase shifts in the spin quantum beat signal generated by two time-delayed circularly polarized optical pulses. The geometric phases generated in this manner more generally perform the role of a spin phase gate, proving potentially useful for quantum information applications.PACS numbers: 78.67. Hc, 71.35.Pq, 42.50.Md, 42.50.Hz A single charge confined in an epitaxially grown quantum dot (QD) shows considerable promise as the basic building block in a quantum computing architecture where the spin of the charge serves as the qubit [1][2][3]. Efforts to demonstrate the feasibility of a quantum computer based on such qubits have resulted in a number of achievements towards satisfying the DiVincenzo criteria [4] for quantum computing. Among these achievements are spin readout [5][6][7], the demonstration of long spin coherence times [8][9][10][11], spin initialization [12][13][14][15][16] and the coherent control of electron spins [17][18][19][20][21].Of fundamental importance in executing quantum algorithms is the ability to perform a universal set of unitary operations including arbitrary single qubit operations, one of the requirements that may be met with sequential qubit rotations about two orthogonal axes [22]. Optical approaches to performing these rotations on QD confined spin qubits are attractive as they offer the prospect of ultrafast gates using readily available laser sources [23][24][25][26] and have already demonstrated fast spin rotations about the optical axis [19][20][21].To obtain fast optically driven rotations about a complementary axis, Economou and Reinecke [26] have proposed the use of geometric phases [27] generated by cyclic 2π excitations of the optical transitions of a QD in the presence of an external DC magnetic field applied normal to the optical axis. For a properly tailored pulse, these optically induced geometric phases serve to variably alter the relative phase between the probability amplitudes of the resident spin states. If the resident spin is initially in a coherent superposition of stationary spin states, this change in the relative phase leads to an effective spin rotation about the spin quantization axis, i.e. the axis of the magnetic field. To date, however, such rotations have not been demonstrated, with recent studies [20,21] instead relying upon combinations of pulse driven rotations about the optical axis and spin precession about the magnetic field to vary the rotation axis.In this Report, we demonstrate the use of a narrowbandwidth continuous-wave (CW) optical field (to simulate a long narrow-band pulse) applied between a pair of time-delayed picosecond optical pulses to ...

Although Kramers’ theory for diffusive barrier crossing on a 1D free energy profile plays a central role in landscape theory for complex bio-molecular processes, it has not yet been rigorously tested by experiment. Here we test this 1D diffusion scenario with single molecule fluorescence measurements of DNA hairpin folding. We find an upper bound of 2.5 μs for the average transition path time, consistent with the predictions by theory with parameters determined from optical tweezer measurements.

We demonstrate the all-optical ultrafast manipulation and read-out of optical transitions in a single negatively charged self-assembled InAs quantum dot, an important step towards ultrafast control of the resident spin. Experiments performed at zero magnetic field show the excitation and decay of the trion (negatively charged exciton) as well as Rabi oscillations between the electron and trion states. Application of a DC magnetic field perpendicular to the growth axis of the dot enables observation of a complex quantum beat structure produced by independent precession of the ground state electron and the excited state heavy hole spins. PACS numbers: 78.67.Hc, 71.35.Pq, 42.50.Md, 42.50.Hz Quantum dots (QDs) containing a single spin have been the focus of intensive efforts over the last several years to demonstrate their viability for use in quantum computing schemes [1,2,3,4]. Major advances in this effort include the observation of both a long spin lifetime [5,6,7] and a long spin coherence time [8,9,10]. Crucial to all quantum computing schemes proposing the use of QD-confined spins is the ability not only to carry out arbitrary coherent manipulations of the spin qubit, but to do so on a time-scale much shorter than the spin decoherence time [11]. These capabilities are possible with ultrafast optical excitation of a three level system containing the spin states of an electron confined in a QD.Considerable progress towards optically manipulating QD-confined spins on ultrafast timescales has been made in interface fluctuation dots, where ensemble studies have demonstrated the generation and read-out of electron spin coherence [12] as well as partial rotations of the electron spin [13]. At the single dot level, time-resolved Kerrrotation measurements have recently shown the optical read-out [10] and partial rotation [14] of an electron spin in an interface fluctuation dot integrated in an optical cavity. Despite these successes, the weak lateral confinement in these dots [15] and the inability to grow them in patterned arrays pose substantial challenges for the implementation of interface fluctuation dots in a practical quantum computing architecture.Self-assembled QDs provide an attractive alternative to interface fluctuation dots due to their stronger spatial confinement [16] and their ability to be organized in 2D and 3D lattices during growth [17]. Transient optical studies of these dots, however, are made difficult by their optical dipole moments, which are 1-2 orders of magnitude smaller than those of interface fluctuation dots [18]. This difficulty necessitates extremely low noise levels for pulsed optical measurements. As a result, most transient optical studies of charged self-assembled QDs have mea-sured the optical response of an ensemble of these dots to obtain larger signals [9], or, in the case of single dot studies, the photoluminescence [19] or the photocurrent of a dot embedded in a photodiode structure [20]. Though the last approach has demonstrated ultrafast preparation and read-out of a sing...

We optically generated an electronic state in a single InAs=GaAs self-assembled quantum dot that is a precursor to the deterministic entanglement of the spin of the electron with an emitted photon in the proposal of W. Yao, R.-B. Liu, and L. J. Sham [Phys. Rev. Lett. 95, 030504 (2005).]. A superposition state is prepared by optical pumping to a pure state followed by an initial pulse. By modulating the subsequent pulse arrival times and precisely controlling them using interferometric measurement of path length differences, we are able to implement a coherent control technique to selectively drive exactly one of the two components of the superposition to the ground state. This optical transition contingent on spin was driven with the same broadband pulses that created the superposition through the use of a two pulse coherent control sequence. A final pulse affords measurement of the coherence of this "preentangled" state.