221 research outputs found

### Nanoscale Quantum Solvation of para-H$_2$ around the Linear OCS Molecule inside $^4$He Droplets

We present a microscopic analysis of the quantum solvation structures of
para-H$_2$ around the OCS molecule when embedded in low temperature $^4$He
droplets. The structures of clusters containing M=5 and 6 para-H$_2$ molecules
are compared with corresponding structures for M=1 (OCS-H$_2$ complex) and M=17
(a full solvation shell), as well as with the clusters in the absence of
helium. We find that the helium has negligible effect on the structures for the
small and large OCS(H$_2$)$_M$ clusters, but that it modifies the cluster
structure for M=6. We discuss implications of these results for the onset of
superfluidity in the solvating hydrogen shell and for spectroscopic
measurements.Comment: 4 pages, 2 figures, accepted for publication in J. Low Temp. Phy

### Limits of quantum speedup in photosynthetic light harvesting

It has been suggested that excitation transport in photosynthetic light
harvesting complexes features speedups analogous to those found in quantum
algorithms. Here we compare the dynamics in these light harvesting systems to
the dynamics of quantum walks, in order to elucidate the limits of such quantum
speedups. For the Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria,
we show that while there is indeed speedup at short times, this is short lived
(70 fs) despite longer lived (ps) quantum coherence. Remarkably, this time
scale is independent of the details of the decoherence model. More generally,
we show that the distinguishing features of light-harvesting complexes not only
limit the extent of quantum speedup but also reduce rates of diffusive
transport. These results suggest that quantum coherent effects in biological
systems are optimized for efficiency or robustness rather than the more elusive
goal of quantum speedup.Comment: 9 pages, 6 figures. To appear in New Journal Physics, special issue
on "Quantum Effects and Noise in Biomolecules." Updated to accepted versio

### Microscopic two-fluid theory of rotational constants of the OCS-H$_2$ complex in $^4$He droplets

We present a microscopic quantum analysis for rotational constants of the
OCS-H$_2$ complex in helium droplets using the local two-fluid theory in
conjunction with path integral Monte Carlo simulations. Rotational constants
are derived from effective moments of inertia calculated assuming that motion
of the H$_2$ molecule and the local non-superfluid helium density is rigidly
coupled to the molecular rotation of OCS and employing path integral methods to
sample the corresponding H$_2$ and helium densities. The rigid coupling
assumption for H$_2$-OCS is calibrated by comparison with exact calculations of
the free OCS-H$_2$ complex. The presence of the H$_2$ molecule is found to
induce a small local non-superfluid helium density in the second solvation
shell which makes a non-negligible contribution to the moment of inertia of the
complex in helium. The resulting moments of inertia for the OCS-H$_2$ complex
embedded in a cluster of 63 helium atoms are found to be in good agreement with
experimentally measured values in large helium droplets. Implications for
analysis of rotational constants of larger complexes of OCS with multiple H$_2$
molecules in helium are discussed.Comment: 11 pages, 5 figures, accepted for publication in J. Chem. Phy

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