Biomedical subjects

K Birgitta Whaley

Publications and source records attributed to K Birgitta Whaley.

6 recordsLinked to original sources

PubMed2006

Hyperfine coupling constants of the azafullerenes C19N, C59N, C69N, and C75N.

The isomers of the nitrogen-substituted fullerenes (azafullerenes) C19N, C59N, C69N, and C75N are examined using all-electron Gaussian atomic orbital basis density functional theory, to determine the doublet radical geometries and hyperfine coupling constants. We find that the inaccuracy of previously calculated hyperfine coupling constants of C59N resulted from a poor treatment of the geometry optimization. We find that UB3LYP minimization of the radical geometry in the 6-31G basis, followed by single-point evaluation of the hyperfine constants in which an expanded basis is used on the atomic sites of interest, forms an efficient compromise between computational cost and accuracy with respect to experimental hyperfine constants. Using this approach, we assign the hyperfine signals observed in experiments on the C69N radical by calculating the hyperfine coupling constants for all five of the isomers and examine the electron spin density distribution. Finally, we present predicted hyperfine coupling constants for the isomers of C19N and C75N for use in the interpretation of future experiments.

Carbon Isotopes↗

PubMed2005

Ohmic and step noise from a single trapping center hybridized with a Fermi sea.

We show that single electron tunneling devices such as the Cooper-pair box or double quantum dot can be sensitive to the zero-point fluctuation of a single trapping center hybridized with a Fermi sea. If the trap energy level is close to the Fermi sea and has linewidth gamma > k(B)T, its noise spectrum has an Ohmic Johnson-Nyquist form, whereas for gamma < k(B)T the noise has a Lorentzian form expected from the semiclassical limit. Trap levels above the Fermi level are shown to lead to steps in the noise spectrum that can be used to probe their energetics, allowing the identification of individual trapping centers coupled to the device.

Journal Article↗

PubMed2005

Multiple solvation configurations around phthalocyanine in helium droplets.

Recent measurements of the emission spectrum of phthalocyanine solvated in superfluid helium nanodroplets exhibit a constant 10.3 cm(-1) splitting of each emission line relative to the absorption spectrum. This splitting has been attributed to two distinct helium environments near the surface of the phthalocyanine molecule. Rigid-body path-integral Monte Carlo provides a means of investigating the origin of the splitting on a detailed microscopic level. Path-integral Monte Carlo simulations of 4He(N)-phthalocyanine at 0.625 K with N ranging from 24 to 150 show two distinct helium configurations. One configuration is commensurate with the molecular substrate and the other is a triangular lattice. We investigate the energetics of these two configurations and use a method for calculating electronic spectral shifts for aromatic molecule-rare-gas clusters due to dispersive interactions to estimate the spectral splitting that would arise from the two helium configurations seen for N=150. The results are in reasonable agreement with the experimentally measured splitting, supporting the existence of two distinct local helium environments near the surface of the molecule in the nanodroplets.

Journal Article↗

PubMed2004

Minimum construction of two-qubit quantum operations.

Optimal construction of quantum operations is a fundamental problem in the realization of quantum computation. We here introduce a newly discovered quantum gate, B, that can implement any arbitrary two-qubit quantum operation with minimal number of both two- and single-qubit gates. We show this by giving an analytic circuit that implements a generic nonlocal two-qubit operation from just two applications of the B gate. Realization of the B gate is illustrated with an example of charge-coupled superconducting qubits for which the B gate is seen to be generated in shorter time than the CNOT gate.

Journal Article↗

PubMed2003

Exact two-qubit universal quantum circuit.

We provide an analytic way to implement any arbitrary two-qubit unitary operation, given an entangling two-qubit gate together with local gates. This is shown to provide explicit construction of a universal quantum circuit that exactly simulates arbitrary two-qubit operations in SU(4). Each block in this circuit is given in a closed form solution. We also provide a uniform upper bound of the applications of the given entangling gates, and find that exactly half of all the controlled-unitary gates satisfy the same upper bound as the CNOT gate. These results allow for the efficient implementation of operations in SU(4) required for both quantum computation and quantum simulation.

Journal Article↗

PubMed2002

Nanoscale molecular superfluidity of hydrogen.

We present a microscopic quantum theoretical analysis of the nanoscale superfluid properties of solvating clusters of para-H2 around the linear OCS molecule. Path-integral calculations with N=17 para-H2 molecules, constituting a full solvation shell, show the appearance of a significant superfluid response to rotation around the molecular axis at T=0.15 K. This low-temperature superfluid response is highly anisotropic and drops sharply as the temperature increases to T approximately 0.3 K. These calculations provide definitive theoretical evidence that an anisotropic superfluid state exists for molecular hydrogen in this microscopic solvation layer.

Journal Article↗