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Biomedical subjects

A G White

Publications and source records attributed to A G White.

At least 19 recordsLinked to original sources
PubMed2005

Demonstration of a simple entangling optical gate and its use in bell-state analysis.

We demonstrate a new architecture for an optical entangling gate that is significantly simpler than previous realizations, using partially polarizing beam splitters so that only a single optical mode-matching condition is required. We demonstrate operation of a controlled-z gate in both continuous-wave and pulsed regimes of operation, fully characterizing it in each case using quantum process tomography. We also demonstrate a fully resolving, nondeterministic optical Bell-state analyzer based on this controlled-z gate. This new architecture is ideally suited to guided optics implementations of optical gates.

Journal Article↗
PubMed2005

Demonstrating superior discrimination of locally prepared states using nonlocal measurements.

We experimentally demonstrate the superior discrimination of separated, unentangled two-qubit correlated states using nonlocal measurements, when compared with measurements based on local operations and classical communications. When predicted theoretically, this phenomenon was dubbed "quantum nonlocality without entanglement." We characterize the performance of the nonlocal, or joint, measurement with a payoff function, for which we measure 0.72 +/- 0.02, compared with the maximum locally achievable value of 2/3 and the overall optimal value of 0.75.

Journal Article↗
PubMed2005

Measurement of quantum weak values of photon polarization.

We experimentally determine weak values for a single photon's polarization, obtained via a weak measurement that employs a two-photon entangling operation, and postselection. The weak values cannot be explained by a semiclassical wave theory, due to the two-photon entanglement. We observe the variation in the size of the weak value with measurement strength, obtaining an average measurement of the S1 Stokes parameter more than an order of magnitude outside of the operator's spectrum for the smallest measurement strengths.

Journal Article↗
PubMed2004

Laser frequency locking by direct measurement of detuning.

We present a new method of laser frequency locking in which the feedback signal is directly proportional to the detuning from an atomic transition, even at detunings many times the natural linewidth of the transition. Our method is a form of sub-Doppler polarization spectroscopy, based on measuring two Stokes parameters (I2 and I3) of light transmitted through a vapor cell. It extends the linear capture range of the lock loop by as much as an order of magnitude and provides frequency discrimination equivalent to or better than those of other commonly used locking techniques.

Journal Article↗
PubMed2004

Quantum process tomography of a controlled-NOT gate.

We demonstrate complete characterization of a two-qubit entangling process--a linear optics controlled-NOT gate operating with coincident detection--by quantum process tomography. We use a maximum-likelihood estimation to convert the experimental data into a physical process matrix. The process matrix allows an accurate prediction of the operation of the gate for arbitrary input states and a calculation of gate performance measures such as the average gate fidelity, average purity, and entangling capability of our gate, which are 0.90, 0.83, and 0.73, respectively.

Journal Article↗
PubMed2004

Measuring entangled qutrits and their use for quantum bit commitment.

We produce and holographically measure entangled qudits encoded in transverse spatial modes of single photons. With the novel use of a quantum state tomography method that only requires two-state superpositions, we achieve the most complete characterization of entangled qutrits to date. Ideally, entangled qutrits provide better security than qubits in quantum bit commitment: we model the sensitivity of this to mixture and show experimentally and theoretically that qutrits with even a small amount of decoherence cannot offer increased security over qubits.

Journal Article↗
PubMed2004

Measuring a photonic qubit without destroying it.

Measuring the polarization of a single photon typically results in its destruction. We propose, demonstrate, and completely characterize a quantum nondemolition (QND) scheme for realizing such a measurement nondestructively. This scheme uses only linear optics and photodetection of ancillary modes to induce a strong nonlinearity at the single-photon level, nondeterministically. We vary this QND measurement continuously into the weak regime and use it to perform a nondestructive test of complementarity in quantum mechanics. Our scheme realizes the most advanced general measurement of a qubit to date: it is nondestructive, can be made in any basis, and with arbitrary strength.

Journal Article↗
PubMed2003

Demonstration of an all-optical quantum controlled-NOT gate.

The promise of tremendous computational power, coupled with the development of robust error-correcting schemes, has fuelled extensive efforts to build a quantum computer. The requirements for realizing such a device are confounding: scalable quantum bits (two-level quantum systems, or qubits) that can be well isolated from the environment, but also initialized, measured and made to undergo controllable interactions to implement a universal set of quantum logic gates. The usual set consists of single qubit rotations and a controlled-NOT (CNOT) gate, which flips the state of a target qubit conditional on the control qubit being in the state 1. Here we report an unambiguous experimental demonstration and comprehensive characterization of quantum CNOT operation in an optical system. We produce all four entangled Bell states as a function of only the input qubits' logical values, for a single operating condition of the gate. The gate is probabilistic (the qubits are destroyed upon failure), but with the addition of linear optical quantum non-demolition measurements, it is equivalent to the CNOT gate required for scalable all-optical quantum computation.

Journal Article↗
PubMed2003

Ancilla-assisted quantum process tomography.

Complete and precise characterization of a quantum dynamical process can be achieved via the method of quantum process tomography. Using a source of correlated photons, we have implemented several methods, each investigating a wide range of processes, e.g., unitary, decohering, and polarizing. One of these methods, ancilla-assisted process tomography (AAPT), makes use of an additional "ancilla system," and we have theoretically determined the conditions when AAPT is possible. Surprisingly, entanglement is not required. We present data obtained using both separable and entangled input states. The use of entanglement yields superior results, however.

Journal Article↗
PubMed2001

Transforming chaos to periodic oscillations.

We demonstrate that the dynamics of an autonomous chaotic class C laser can be controlled to a periodic state via external modulation of the pump. In the absence of modulation, above the chaos threshold, the laser exhibits Lorenz-like chaotic pulsations. The average amplitude and frequency of these pulsations depend on the pump power. We find that there exist parameter windows where modulation of the pump power extinguishes the chaos in favor of simpler periodic behavior. Moreover we find a number of locking ratios between the pump and laser output follow the Farey sequence.

Journal Article↗
PubMed2000

Experimental verification of decoherence-free subspaces.

Using spontaneous parametric down-conversion, we produce polarization-entangled states of two photons and characterize them using two-photon tomography to measure the density matrix. A controllable decoherence is imposed on the states by passing the photons through thick, adjustable birefringent elements. When the system is subject to collective decoherence, one particular entangled state is seen to be decoherence-free, as predicted by theory. Such decoherence-free systems may have an important role for the future of quantum computation and information processing.

Journal Article↗
PubMed1998

Protection against lethal murine coccidioidomycosis by a soluble vaccine from spherules.

The formaldehyde-killed, whole-spherule vaccine, which is protective against lethal challenge of laboratory animals with Coccidioides immitis, was fractionated. It yielded a soluble, multicomponent, subcellular fraction termed the 27K vaccine. This vaccine, when it was accompanied by adjuvant, protected mice against lethal intranasal and intravenous challenge with C. immitis.

Animals↗
PubMed1997

HLA antigens in Omanis with blinding trachoma: markers for disease susceptibility and resistance.

AIM: To determine the presence of HLA antigens in people with blinding trachoma. METHODS: Fifty Omanis with blinding trachoma were serologically typed for HLA A, B, C, DR, and DQ antigens and DNA typed for class II DR beta and DQ beta alleles and compared with a population of 100 healthy controls. RESULTS: chi 2 analysis of serological reactions did not reveal any significant differences in HLA antigen frequencies after correction of probability, although DR4, DR7, and DR53 were completely absent in the patients and all of the patients were HLA DQ1 positive. In the case of DQ1 the relative risk was 22.6 (95% confidence interval of 20.7-24.7). Class II DNA low resolution DR beta typing showed a significant increase in HLA DR16 (pc = 0.036, relative risk = 3.8) and a significant decrease in HLA DR53 (pc = 0.018, relative risk = 0.05). CONCLUSION: The finding that HLA DR16 (a DR2 subtype) is associated with susceptibility to blinding trachoma, a disease that is caused by an intracellular micro-organism, is consistent with reports of an HLA DR2 association with leprosy and tuberculosis, diseases also caused by an intracellular micro-organism. Similarly, resistance to leprosy is associated with HLA DR53 as is the case with blinding trachoma described here. It is postulated that HLA DR2 or subtypes in association with HLA DQ 1 may enable an intracellular micro-organism to enter the cell or are involved in presentation of peptides derived from intracellular micro-organisms to T lymphocytes initiating a delayed hypersensitivity or autoimmune reaction. These findings are the first report that genetic factors are of major importance in the development and protection against blinding trachoma.

Biomarkers↗
PubMed1996

HLA antigens in an Omani population with dilated cardiomyopathy.

We have investigated the frequency of HLA antigens in 50 Omanis with idiopathic dilated cardiomyopathy to establish whether there are ethnic/racial differences in the reported HLA associations with this disease. There were no statistically significant HLA-A, B, C, DR or DQ antigen frequency differences between the patients and 247 healthy Omanis. THe reported association of HLA-DR4 with idiopathic dilated cardiomyopathy in the Caucasian population does not apply to the Omanis. This confirms the heterogeneity of this disease and points to ethnic/racial origins as important factors when examining the HLA association. This is particularly pertinent as HLD-DR4 has been strongly linked to autoantibody formation in idiopathic dilated cardiomyopathy in Caucasians. The lack of any HLA antigen association in Omanis would argue against the proposed HLA-linked autoimmune pathology of idiopathic dilated cardiomyopathy.

Cardiomyopathy, Dilated↗
PubMed1996

Cloning and expression of the complement fixation antigen-chitinase of Coccidioides immitis.

A chitinase had been isolated from the culture filtrates of Coccidioides immitis endosporulating spherules and from hyphae and shown to be the coccidioidal complement fixation (CF) and immunodiffusion-CF antigen. In the present study, we made use of our previously determined amino-terminal (N-terminal) sequence of the CF-chitinase to design degenerate oligonucleotide primers and to amplify and sequence a PCR product that coded for the N-terminal portion of the CF-chitinase. The PCR product was used as a hybridization probe to screen a developing spherule-(lambda)ZAP cDNA library, and three hybridizing clones were selected. These clones were converted into their pBluescript expression plasmid form in Escherichia coli and induced to express their recombinant proteins. Lysate from only one clone, pCTS 4-2A, yielded an enzymatically functional CF-chitinase and a line of identity with control immunodiffusion-CF-positive antigen. The pCTS 4-2A insert was sequenced and found to contain a deduced open reading frame coding for a 427-amino-acid polypeptide with an approximate molecular weight of 47 kDa. When purified by a chitin adsorption-desorption method, the recombinant protein exhibited virtually identical characteristics to those of the original C. immitis CF-chitinase. Nondenaturing gels of the pCTS 4-2A E. coli lysates and the purified C. immitis and recombinant CF-chitinase revealed proteins that had chitinase activity and similar relative electrophoretic mobilities. The appearance and relative levels of hybridizing RNA from the developing spherules-endospores (SEs) and hyphae correlated with the appearance or presence and level of CF-chitinase enzyme activity found in SEs culture filtrate and in cellular extracts of developing SE and hyphae. Thus, a functional recombinant CF-chitinase antigen was produced in E. coli and was used in serological diagnostic applications. These results also suggest a functional role for this chitinase in SE development and maturation.

Amino Acid Sequence↗
PubMed1995

HLA antigens in Omani patients with vitiligo.

Fifty native Omanis with vitiligo were studied to compare the incidence of HLA ABC and DR antigens with a control population. HLA Bw6 was found in 82% of patients compared with 49% controls (Pc = 0.0009 RR = 4.56) and HLA DR7 occurred in 40% of patients and 9% in controls (Pc = 0.00075 RR = 6.17). HLA DR7 was significantly increased in those patients with acrofacial, compared to focal disease (57% vs. 24% P = 0.038). Sixty-six per cent of the patients in this study had parents who were consanguineous and a positive family history was only found in this group with an incidence of 32%. HLA Bw4 segregated 100% with patients with a positive family history compared with 48% in consanguineous patients without a positive family history (Pc = 0.011 RR = 23). Vitiligo appears to be associated with different HLA antigens in different ethnic groups.

Female↗