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We demonstrate the extension of coherence between all four two-electron spin ground states of an InAs quantum dot molecule (QDM) via nonlocal suppression of nuclear spin fluctuations in two vertically stacked quantum dots (QDs), while optically addressing only the top QD transitions. Long coherence times are revealed through dark-state spectroscopy as resulting from nuclear spin locking mediated by the exchange interaction between the QDs. Line shape analysis provides the first measurement of the quieting of the Overhauser field distribution correlating with reduced nuclear spin fluctuations.

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.

The electron spin state of a singly charged semiconductor quantum dot has been shown to form a suitable single qubit for quantum computing architectures with fast gate times. A key challenge in realizing a useful quantum dot quantum computing architecture lies in demonstrating the ability to scale the system to many qubits. In this Letter, we report an all optical experimental demonstration of quantum entanglement between a single electron spin confined to a single charged semiconductor quantum dot and the polarization state of a photon spontaneously emitted from the quantum dot's excited state. We obtain a lower bound on the fidelity of entanglement of 0.59±0.04, which is 84% of the maximum achievable given the timing resolution of available single photon detectors. In future applications, such as measurement-based spin-spin entanglement which does not require sub-nanosecond timing resolution, we estimate that this system would enable near ideal performance. The inferred (usable) entanglement generation rate is 3×10(3) s(-1). This spin-photon entanglement is the first step to a scalable quantum dot quantum computing architecture relying on photon (flying) qubits to mediate entanglement between distant nodes of a quantum dot network.

We describe a protocol to entangle two qubits at a distance by using resonance fluorescence. The scheme makes use of the postselection of large and distinguishable fluorescence signals corresponding to entangled and unentangled qubit states and has the merits of both high success probability and high entanglement fidelity owing to the multiphoton nature. Our result shows that the entanglement generation is robust against photon fluctuations in the fluorescence signals for a wide range of driving fields. We also demonstrate that this new protocol has an average entanglement duration within the decoherence time of corresponding qubit systems, based on current experimental photon efficiency.

We demonstrate the suppression of nuclear-spin fluctuations in an InAs quantum dot and measure the timescales of the spin narrowing effect. By initializing for tens of milliseconds with two continuous wave diode lasers, fluctuations of the nuclear spins are suppressed via the hole-assisted dynamic nuclear polarization feedback mechanism. The fluctuation narrowed state persists in the dark (absent light illumination) for well over 1 s even in the presence of a varying electron charge and spin polarization. Enhancement of the electron spin coherence time (T2*) is directly measured using coherent dark state spectroscopy. By separating the calming of the nuclear spins in time from the spin qubit operations, this method is much simpler than the spin echo coherence recovery or dynamic decoupling schemes.

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 2pi 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.

A single electron or hole spin trapped inside a semiconductor quantum dot forms the foundation for many proposed quantum logic devices. In group III-V materials, the resonance and coherence between two ground states of the single spin are inevitably affected by the lattice nuclear spins through the hyperfine interaction, while the dynamics of the single spin also influence the nuclear environment. Recent efforts have been made to protect the coherence of spins in quantum dots by suppressing the nuclear spin fluctuations. However, coherent control of a single spin in a single dot with simultaneous suppression of the nuclear fluctuations has yet to be achieved. Here we report the suppression of nuclear field fluctuations in a singly charged quantum dot to well below the thermal value, as shown by an enhancement of the single electron spin dephasing time T(2)*, which we measure using coherent dark-state spectroscopy. The suppression of nuclear fluctuations is found to result from a hole-spin assisted dynamic nuclear spin polarization feedback process, where the stable value of the nuclear field is determined only by the laser frequencies at fixed laser powers. This nuclear field locking is further demonstrated in a three-laser measurement, indicating a possible enhancement of the electron spin T(2)* by a factor of several hundred. This is a simple and powerful method of enhancing the electron spin coherence time without use of 'spin echo'-type techniques. We expect that our results will enable the reproducible preparation of the nuclear spin environment for repetitive control and measurement of a single spin with minimal statistical broadening.

We present calculations for the oscillator strength of the recombination of excitons bound to phosphorus donors in silicon. We show that the direct recombination of the bound exciton cannot account for the experimentally measured oscillator strength of the no-phonon line. Instead, the recombination process is assisted by an Umklapp process of the donor electron state. We make use of the empirical pseudopotential method to evaluate the Umklapp-assisted recombination matrix element in second-order perturbation theory. Our result is in good agreement with experiment. Being potentially useful for quantum computing, the process of Umklapp-assisted recombination can be used to detect optically the spin state of the nucleus of a phosphorus donor, which requires that the energy levels of the nuclear spin are optically resolvable. We therefore present two methods to improve the optical resolution of the optical detection of the spin state of a single nucleus in Si:P.

We investigate a singly charged quantum dot under a strong optical driving field by probing the system with a weak optical field. We observe all critical features predicted by Mollow for a strongly driven two-level atomic system in this solid state nanostructure, such as absorption, the ac-Stark effect, and optical gain. Our results demonstrate that even at high optical field strengths the electron in a single quantum dot with its dressed ground state and trion state behaves as a well-isolated two-level quantum system.

Quantum computation requires a continuous supply of rapidly initialized qubits for quantum error correction. Here, we demonstrate fast spin state initialization with near unity efficiency in a singly charged quantum dot by optically cooling an electron spin. The electron spin is successfully cooled from 5 to 0.06 K at a magnetic field of 0.88 T applied in Voigt geometry. The spin cooling rate is of order 10(9) s-1, which is set by the spontaneous decay rate of the excited state.

Coherent transient excitation of the spin ground states in singly charged quantum dots creates optically coupled and decoupled states of the electron spin. We demonstrate selective excitation from the spin ground states to the trion state through phase sensitive control of the spin coherence via these three states, leading to partial rotations of the spin vector. This progress lays the ground work for achieving complete ultrafast spin rotations.

Quantum dots are typically formed from large groupings of atoms and thus may be expected to have appreciable many-body behavior under intense optical excitation. Nonetheless, they are known to exhibit discrete energy levels due to quantum confinement effects. We show that, like single-atom or single-molecule two- and three-level quantum systems, single semiconductor quantum dots can also exhibit interference phenomena when driven simultaneously by two optical fields. Probe absorption spectra are obtained that exhibit Autler-Townes splitting when the optical fields drive coupled transitions and complex Mollow-related structure, including gain without population inversion, when they drive the same transition. Our results open the way for the demonstration of numerous quantum level-based applications, such as quantum dot lasers, optical modulators, and quantum logic devices.

Research in semiconductor spintronics aims to extend the scope of conventional electronics by using the spin degree of freedom of an electron in addition to its charge. Significant scientific advances in this area have been reported, such as the development of diluted ferromagnetic semiconductors, spin injection into semiconductors from ferromagnetic metals and discoveries of new physical phenomena involving electron spin. Yet no viable means of developing spintronics in semiconductors has been presented. Here we report a theoretical design that is a conceptual step forward-spin accumulation is used as the basis of a semiconductor computer circuit. Although the giant magnetoresistance effect in metals has already been commercially exploited, it does not extend to semiconductor/ferromagnet systems, because the effect is too weak for logic operations. We overcome this obstacle by using spin accumulation rather than spin flow. The basic element in our design is a logic gate that consists of a semiconductor structure with multiple magnetic contacts; this serves to perform fast and reprogrammable logic operations in a noisy, room-temperature environment. We then introduce a method to interconnect a large number of these gates to form a 'spin computer'. As the shrinking of conventional complementary metal-oxide-semiconductor (CMOS) transistors reaches its intrinsic limit, greater computational capability will mean an increase in both circuit area and power dissipation. Our spin-based approach may provide wide margins for further scaling and also greater computational capability per gate.

For a two-state quantum object interacting with a slow mesoscopic interacting spin bath, we show that a many-body solution of the bath dynamics conditioned on the quantum-object state leads to an efficient control scheme to recover the lost quantum-object coherence through disentanglement. We demonstrate the theory with the realistic problem of one electron spin in a bath of many interacting nuclear spins in a semiconductor quantum dot. The spin language can be easily generalized to a quantum object in contact with a bath of interacting multilevel quantum units with the caveat that the bath is mesoscopic and its dynamics is slow compared with the quantum object.

We consider the initialization of the spin state of a single electron trapped in a self-assembled quantum dot via optical pumping of a trion level. We show that with a magnetic field applied perpendicular to the growth direction of the dot, a near-unity fidelity can be obtained in a time equal to a few times the inverse of the spin-conserving trion relaxation rate. This method is several orders of magnitude faster than with the field aligned parallel, since this configuration must rely on a slow hole spin-flip mechanism. This increase in speed does result in a limit on the maximum obtainable fidelity, but we show that for InAs dots, the error is very small.

Extraction of electrons from a semiconductor to a ferromagnet as well as the case of injection in the reverse direction may be formulated as a scattering theory. However, the presence of bound states at the interface arising out of doping on the semiconductor side must be taken into account in the scattering theory. Inclusion of the interface states yields an explanation of a recent result of spin-imaging measurement which contradicts the current understanding of spin extraction. The importance of an extraction theory to spintronics is illustrated by an application to a spin switch.

Time-resolved magneto-optical Kerr spectroscopy of ferromagnetic InMnAs reveals two distinct demagnetization processes--fast (<1 ps) and slow (approximately 100 ps). Both components diminish with increasing temperature and are absent above the Curie temperature. The fast component rapidly grows with pump power and saturates at high fluences (>10 mJ/cm(2)); the saturation value indicates a complete quenching of ferromagnetism on a subpicosecond time scale. We attribute this fast dynamics to spin heating through p-d exchange interaction between photocarriers and Mn ions, while the approximately 100 ps component is interpreted as spin-lattice relaxation.

A cavity coupling, a charged nanodot, and a fiber can act as a quantum interface, through which a stationary spin qubit and a flying photon qubit can be interconverted via a cavity-assisted Raman process. This Raman process can be made to generate or annihilate an arbitrarily shaped single-photon wave packet by pulse shaping the controlling laser field. This quantum interface forms the basis for many essential functions of a quantum network, including sending, receiving, transferring, swapping, and entangling qubits at distributed quantum nodes as well as a deterministic source and an efficient detector of a single-photon wave packet with arbitrarily specified shape and average photon number. Numerical study of errors from noise and system parameters on the operations shows high fidelity and robust tolerance.

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.

From the solution of a two-band model, we predict that the thermal and electrical transport across the junction of a semimetal and an excitonic insulator will exhibit high resistance behavior and low entropy production at low temperatures, distinct from a junction of a semimetal and a normal semiconductor. This phenomenon, ascribed to the dissipationless exciton flow which dominates over the charge transport, is based on the much longer length scale of the change of the effective interface potential for electron scattering due to the coherence of the condensate than in the normal state.

Ferromagnetism is predicted in undoped diluted magnetic semiconductors illuminated by intense sub-band-gap laser radiation. The mechanism for photoinduced ferromagnetism is coherence between conduction and valence bands induced by the light which leads to an optical exchange interaction. The ferromagnetic critical temperature T(C) depends both on the properties of the material and on the frequency and intensity of the laser and could be above 1K.

We describe a new means for all-electrical generation of spin polarization in semiconductors. In contrast with spin injection of electrons by tunneling through a reverse-biased Schottky barrier, we observe accumulation at the metal-semiconductor interface of forward-biased ferromagnetic Schottky diodes, which is consistent with a theory of spin-dependent reflection off the interface. Spatiotemporal Kerr microscopy is used to image the electron spin and the resulting dynamic nuclear polarization that arises from the nonequilibrium carrier polarization.

Quantum electrodynamics of excitons in a cavity is shown to be relevant to quantum operations. We present a theory of an integrable solid-state quantum controlled-phase gate for generating entanglement of two photons using a coupled nanodot-microcavity-fiber structure. A conditional phase shift of O(pi/10) is calculated to be the consequence of the giant optical nonlinearity keyed by the excitons in the cavities. Structural design and active control, such as electromagnetically induced transparency and pulse shaping, optimize the quantum efficiency of the gate operation.

We report coherent optical control of a biexciton (two electron-hole pairs), confined in a single quantum dot, that shows coherent oscillations similar to the excited-state Rabi flopping in an isolated atom. The pulse control of the biexciton dynamics, combined with previously demonstrated control of the single-exciton Rabi rotation, serves as the physical basis for a two-bit conditional quantum logic gate. The truth table of the gate shows the features of an all-optical quantum gate with interacting yet distinguishable excitons as qubits. Evaluation of the fidelity yields a value of 0.7 for the gate operation. Such experimental capability is essential to a scheme for scalable quantum computation by means of the optical control of spin qubits in dots.

We show how a spin interaction between electrons localized in neighboring quantum dots can be induced and controlled optically. The coupling is generated via virtual excitation of delocalized excitons and provides an efficient coherent control of the spins. This quantum manipulation can be realized in the adiabatic limit and is robust against decoherence by spontaneous emission. Applications to the realization of quantum gates, scalable quantum computers, and to the control of magnetization in an array of charged dots are proposed.

We present a theory of generation or alteration of the electron spin coherence and population in an n-doped semiconductor by reflection at the interface with a ferromagnet. The dependence of the spin reflection on the Schottky barrier height and the doping concentration in the semiconductor was computed for a generic model. The theory provides an explanation for the spontaneous electron spin coherence and nuclear polarization in the semiconductor interfaced with a ferromagnet and associated phenomena recently observed by time-resolved Faraday rotation experiments. The study also points to an alternative approach to spintronics different from spin injection.

Nondegenerate (two-wavelength) two-photon absorption using coherent optical fields is used to show that there are two different quantum mechanical pathways leading to formation of the biexciton in a single quantum dot. Of specific importance to quantum information applications is the resulting coherent dynamics between the ground state and the biexciton from the pathway involving only optically induced exciton/biexciton quantum coherence. The data provide a direct measure of the biexciton decoherence rate which is equivalent to the decoherence of the Bell state in this system, as well as other critical optical parameters.

Transient nonlinear optical spectroscopy, performed on excitons confined to single GaAs quantum dots, shows oscillations that are analogous to Rabi oscillations in two-level atomic systems. This demonstration corresponds to a one-qubit rotation in a single quantum dot which is important for proposals using quantum dot excitons for quantum computing. The dipole moment inferred from the data is consistent with that directly obtained from linear absorption studies. The measurement extends the artificial atom model of quantum dot excitonic transitions into the strong-field limit, and makes possible full coherent optical control of the quantum state of single excitons using optical pi pulses.

We present a theory to further a new perspective of proactive control of exciton dynamics in the quantum limit. Circularly polarized optical pulses in a semiconductor nanodot are used to control the dynamics of two interacting excitons of opposite polarizations. Shaping of femtosecond laser pulses keeps the quantum operation within the decoherence time. Computation of the fidelity of the operations and application to the complete solution of a minimal quantum computing algorithm demonstrate in theory the feasibility of quantum control.

The influence of four-particle correlations on the nonlinear optics of a semiconductor microcavity is determined by a pump-and-probe investigation. Experiments are performed on a nonmonolithic microcavity which contains a ZnSe quantum well. In this system the biexciton binding energy exceeds both the normal-mode splitting between exciton and cavity mode and all damping constants. Oscillatory spectral features below the excitonic resonance are observed in the response for counterpolarized beams. Comparison with model calculations shows that in this case the coherent nonlinearity is dominated by biexciton-exciton interactions beyond the Hartree-Fock approximation.