Coherent control to prepare an InAs quantum dot for spin-photon entanglement.

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.

Demonstration of quantum entanglement between a single electron spin confined to an InAs quantum dot and a photon.

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.

Fast spin rotations by optically controlled geometric phases in a charge-tunable InAs quantum dot.

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.

Near-field coherent spectroscopy and microscopy of a quantum dot system.

We combined coherent nonlinear optical spectroscopy with nano-electron volt energy resolution and low-temperature near-field microscopy with subwavelength resolution (

Differences in the pathways for unfolding and hydrogen exchange among mutants of Escherichia coli alkaline phosphatase.

Our initial studies of hydrogen-deuterium (H-D) exchange of tryptophan 109 in Escherichia coli alkaline phosphatase (AP) suggested that significant local unfolding of the protein might occur to allow for the exchange reaction, which is very slow at room temperature (Fischer et al., Biochemistry 39 (2000) 1455-1461). In order to investigate whether the partial unfolding and/or 'breathing' motions leading to H-D exchange were part of the unfolding pathway of the protein we prepared a series of mutants, designed to produce cavities around the exchanging residue, and compared their rates of H-D exchange to their lability (rate of inactivation) in guanidine hydrochloride (Gd:HCl). The complex unfolding kinetics of the mutants in the presence of Gd:HCl showed several components with rates that differed substantially among these proteins, but none of the rates of denaturation induced with Gd:HCl was consistently correlated with the H-D exchange rates. We conclude that the partial opening of the AP structure during the H-D exchange of tryptophan 109, although very slow, is not a rate determining step in the unfolding of this protein.

In vitro renaturation of bovine beta-lactoglobulin A leads to a biologically active but incompletely refolded state.

When bovine beta-lactoglobulin (beta-LG) was refolded after extensive denaturation in 4.8 M guanidine hydrochloride (GuHCl), the functional activity of the protein, retinol binding, as measured by the enhancement of this ligand's fluorescence, was completely recovered. In contrast, the room-temperature tryptophan phosphorescence lifetime of the refolded protein, a local measure of the residue environment, was approximately 10 ms, significantly shorter than the phosphorescence lifetime of the untreated native protein (approximately 20 ms). The lability of the freshly refolded protein, as monitored by following the time course of its unfolding when incubated in 2.5 M GuHCl through the change in fluorescence intensity at 385 nm, was also determined and found to be increased significantly relative to untreated native protein. In contrast to the long term postactivation conformational changes detected previously in Escherichia coli alkaline phosphatase (Subramaniam V, Bergenhem NCH, Gafni A, Steel DG, 1995, Biochemistry 34:1133-1136), we found no changes in either the lability or phosphorescence decays of beta-LG during a period of 24 h. Our results are in agreement with the report by Hattori et al. (1993, J Biol Chem 268:22414-22419), using conformation-specific monoclonal antibodies to recognize native-like structure, that long-term changes occur in the protein conformation, compared with the native structure, on refolding.

Lack of correspondence between the room-temperature phosphorescence decay-components and Trp residues in a series of Trp-->Cys or Trp-->Phe mutants of human carbonic anhydrase II.

The room temperature phosphorescence of native human carbonic anhydrase (CA), and several mutants of this enzyme has been investigated. In these mutants the seven tryptophan residues in the native protein have sequentially been replaced by cysteine or phenylalanine. All of the mutants as well as native CA show room-temperature tryptophan phosphorescence (RTP) spectra. Surprisingly, only small differences in RTP life-times are noticeable among these mutants, indicating that there is more than one tryptophan residue with similar phosphorescence decay kinetics in the protein. The present results illustrate the danger in attributing the room temperature phosphorescence of a multi-tryptophan protein to a particular residue based solely on an analysis of the protein structure.

Time-resolved room temperature tryptophan phosphorescence in proteins.

The application of luminescence, primarily fluorescence, to the study of protein structure and dynamics has been extensively exploited to facilitate the understanding of complex biological problems. The interest in the application of phosphorescence, however, shows that new and complementary information can be had by careful optical studies of the phosphorescence lifetime. As in the early days of fluorescence spectroscopy in proteins, a complete and rigorous interpretation of the room temperature phosphorescence remains to be developed; nevertheless, it is clear that time-resolved phosphorescence yields new information on proteins in solution, for example, the detection of subtle conformational changes during protein folding, which is outside the sensitivity of earlier techniques. In addition, the great sensitivity of the phosphorescence lifetime to structural changes associated with rigidity and of nearby quenchers suggests that detailed structural information can be obtained when this approach is combined with the power of site-directed mutagenesis or other more biophysical techniques such as energy transfer to attached acceptors. We have presented basic aspects of time-resolved room temperature phosphorescence spectroscopy and demonstrated some useful features of the spectroscopic signals as well as the general approach to data analysis. However, it should be understood that extensions of this approach will easily allow faster and improved time resolution with greater sensitivity to highly quenched phosphorescing states. In addition, many extensions of this approach that are common to fluorescence spectroscopy have yet to be developed. For example, combining time-resolved phosphorescence with anaerobic stopped-flow techniques and more rapid data acquisition electronics will enable studies of conformational dynamics with considerably shortened dead times. Other possibilities include extending the preliminary studies of in vivo-based spectroscopy, such as to microscopy. In conclusion, time-resolved phosphorescence presents a new dimension to biophysical methodologies for the study of proteins, and it is likely that this area will continue to grow in capability as the fundamental understanding improves.

Detection of intermediate protein conformations by room temperature tryptophan phosphorescence spectroscopy during denaturation of Escherichia coli alkaline phosphatase.

The reversible denaturation of Escherichia coli alkaline phosphatase (AP) was followed by monitoring changes in enzymatic activity as well as by measurements of the time-resolved room temperature phosphorescence from Trp 109. It is well known that the denaturants, ethylene diamine tetraacetic acid (EDTA), acid and guanidine hydrochloride (GdnHCl) inactive AP by different mechanisms as reflected by differences in the time dependence of inactivation. However, further information about structural changes that result during inactivation is obtained by measurement of the phosphorescence intensity and radiative decay rate. Time-resolved tryptophan phosphorescence is exquisitely sensitive to changes in the local environment of the emitting residue, unlike the steady state phosphorescence intensity which is a composite of both the lifetime and concentration of the emitting protein species. The results show that while inactivation in EDTA proceeds by loss of the zinc ion as expected, denaturation in acid or GdnHCl produces a heterogeneous population of AP molecules, detected by a distribution analysis of the phosphorescence lifetime, which may reflect multiple pathways to the final unfolded state. Time-resolved phosphorescence also demonstrates the existence of an enzymatically active but structurally less rigid intermediate state during unfolding. As the rigidity decreases, the susceptibility to further denaturation decreases at lower pH but increases with GdnHCl concentration. The experiments provide new insight into the mechanism of denaturation of AP and demonstrate the sensitivity of time-resolved room temperature phosphorescence to the structural details of intermediate states produced during unfolding of proteins.

Ultradense reproducible Z-pinch suitable for CO2 laser-pellet simulation experiments.

The design and operating characteristics of a unique reproducible linear, high-density (> 10(19) e(-)/cm(3)), low-temperature (17

Using census data to investigate the causes of the ecological fallacy.

"The authors show how data from the 2% Sample of Anonymised Records (SAR) can be combined with data from the Small Area Statistics (SAS) database to investigate the causes of the ecological fallacy in an Enumeration District (ED) level analysis. A range of census variables are examined in three ¿SAR districts'...in England. Results of comparable analyses from the 1986 Australian census are also given. The ecological fallacy arises when results from an analysis based on area-level aggregate statistics are incorrectly assumed to apply at the individual level.... A methodology is introduced which allows aggregate-level statistics to be adjusted by using individual-level information on those variables that explain much of the within-area homogeneity."

Single charged quantum dot in a strong optical field: absorption, gain, and the ac-Stark effect.

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.

Fast initialization of the spin state of an electron in a quantum dot in the Voigt configuration.

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.

Fast spin state initialization in a singly charged InAs-GaAs quantum dot by optical cooling.

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.

Selective optical control of electron spin coherence in singly charged GaAs-Al0.3Ga0.7As quantum dots.

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.

Density matrix tomography through sequential coherent optical rotations of an exciton qubit in a single quantum dot.

We demonstrate single qubit density matrix tomography in a single semiconductor quantum dot system through consecutive phase sensitive rotations of the qubit via ultrafast coherent optical excitations. The result is important for quantifying gate operations in quantum information processing in the quantum dot systems as well as demonstrating consecutive arbitrary qubit rotations.

Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots.

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.

Narrow nonlinear-optical resonances in CdSSe-doped glass.

We describe frequency-domain spectroscopy studies of glasses doped with CdS(1-x)Se(x) using low-power cw tunable dye lasers. The results show a narrow resonance (4.4 kHz at room temperature) in the backward nearly degenerate four-wave mixing spectrum, which we believe is determined by the phonon-mediated inverse lifetime of a deep level trap involved in the nonlinear response.

Multiline phase conjugation in resonant materials.

We present a direct experimental demonstration of multiwavelength phase conjugation through degenerate four-wave mixing. The experiments were performed using a multiline CO2 TEA laser with SF6 as the nonlinear medium. Using a vidicon to observe the signal in the far field, we show good aberration-correction ability. A qualitative description of the additional cross coupling is also provided.

Reconstruction of spherically symmetric objects from slit-imaged emission: application to spatially resolved spectroscopy.

A simple formula is given for inverting data in the form of measured integrals Phi(x) of a spherically symmetric density distribution f(r) over planes x = constant. The application of the inversion formula is illustrated by analysis of spatially resolved spectroscopy of x-ray emission from a laser-induced microimplosion of a gas-filled glass sphere.