410 research outputs found
Coherent Control of Quantum Dynamics with Sequences of Unitary Phase-Kick Pulses
Coherent optical control schemes exploit the coherence of laser pulses to
change the phases of interfering dynamical pathways in order to manipulate
dynamical processes. These active control methods are closely related to
dynamical decoupling techniques, popularized in the field of Quantum
Information. Inspired by Nuclear Magnetic Resonance (NMR) spectroscopy,
dynamical decoupling methods apply sequences of unitary operations to modify
the interference phenomena responsible for the system dynamics thus also
belonging to the general class of coherent control techniques. Here we review
related developments in the fields of coherent optical control and dynamical
decoupling, with emphasis on control of tunneling and decoherence in general
model systems. Considering recent experimental breakthroughs in the
demonstration of active control of a variety of systems, we anticipate that the
reviewed coherent control scenarios and dynamical decoupling methods should
raise significant experimental interest.Comment: 52 pages, 7 figure
An innovative quality improvement curriculum for third-year medical students
Background: Competence in quality improvement (QI) is a priority for medical students. We describe a self-directed QI skills curriculum for medical students in a 1-year longitudinal integrated third-year clerkship: an ideal context to learn and practice QI. Methods: Two groups of four students identified a quality gap, described existing efforts to address the gap, made quantifying measures, and proposed a QI intervention. The program was assessed with knowledge and attitude surveys and a validated tool for rating trainee QI proposals. Reaction to the curriculum was assessed by survey and focus group. Results: Knowledge of QI concepts did not improve (mean knowledge score±SD): pre: 5.9±1.5 vs. post: 6.6±1.3, p=0.20. There were significant improvements in attitudes (mean topic attitude score±SD) toward the value of QI (pre: 9.9±1.8 vs. post: 12.6±1.9, p=0.03) and confidence in QI skills (pre: 13.4±2.8 vs. post: 16.1±3.0, p=0.05). Proposals lacked sufficient analysis of interventions and evaluation plans. Reaction was mixed, including appreciation for the experience and frustration with finding appropriate mentorship. Conclusion: Clinical-year students were able to conduct a self-directed QI project. Lack of improvement in QI knowledge suggests that self-directed learning in this domain may be insufficient without targeted didactics. Higher order skills such as developing measurement plans would benefit from explicit instruction and mentorship. Lessons from this experience will allow educators to better target QI curricula to medical students in the clinical years
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Dynamic H Imaging of Hyperpolarized [1-C]Lactate In Vivo Using a Reverse INEPT Experiment
Dynamic magnetic resonance spectroscopic imaging of hyperpolarized C-labeled cell substrates has enabled the investigation of tissue metabolism in vivo. Currently observation of these hyperpolarized substrates is limited mainly to C detection. We describe here an imaging pulse sequence that enables proton observation by using polarization transfer from the hyperpolarized C nucleus to spin-coupled protons.
The pulse sequence transfers C hyperpolarization to H using a modified reverse insensitive nuclei enhanced by polarization transfer (INEPT) sequence that acquires a fully refocused echo. The resulting hyperpolarized H signal is acquired using a 2D echo-planar trajectory. The efficiency of polarization transfer was investigated using simulations with and without T and T relaxation of both the H and C nuclei.
Simulations showed that H detection of the hyperpolarized C nucleus in lactate should increase significantly the signal-to-noise ratio when compared with direct C detection at 3T. However the advantage of H detection is expected to disappear at higher fields. Dynamic H images of hyperpolarized [1-C]lactate, with a spatial resolution of 1.25 × 1.25 mm, were acquired from a phantom injected with hyperpolarized [1-C]lactate and from tumors in vivo following injection of hyperpolarized [1-C]pyruvate.
The sequence allows H imaging of hyperpolarized C-labeled substrates in vivo.Grant sponsor: Cancer Research UK Programme; Grant number: 17242; Grant sponsor: CRUK-EPSRC Imaging Centre in Cambridge and Manchester; Grant number: 16465; Grant sponsor: Danish Strategic Research Council (LIFE-DNP: Hyperpolarized magnetic resonance for in vivo quantification of lipid, sugar and amino acid metabolism in lifestyle related diseases); Grant sponsor: Marie Curie ITN studentship (EUROPOL); Grant number: 642773
Composite-pulse magnetometry with a solid-state quantum sensor
The sensitivity of quantum magnetometers is challenged by control errors and,
especially in the solid-state, by their short coherence times. Refocusing
techniques can overcome these limitations and improve the sensitivity to
periodic fields, but they come at the cost of reduced bandwidth and cannot be
applied to sense static (DC) or aperiodic fields. Here we experimentally
demonstrate that continuous driving of the sensor spin by a composite pulse
known as rotary-echo (RE) yields a flexible magnetometry scheme, mitigating
both driving power imperfections and decoherence. A suitable choice of RE
parameters compensates for different scenarios of noise strength and origin.
The method can be applied to nanoscale sensing in variable environments or to
realize noise spectroscopy. In a room-temperature implementation based on a
single electronic spin in diamond, composite-pulse magnetometry provides a
tunable trade-off between sensitivities in the microT/sqrt(Hz) range,
comparable to those obtained with Ramsey spectroscopy, and coherence times
approaching T1
A large geometric distortion in the first photointermediate of rhodopsin, determined by double-quantum solid-state NMR
Double-quantum magic-angle-spinning NMR experiments were performed on 11,12-C-13(2)-retinylidene-rhodopsin under illumination at low temperature, in order to characterize torsional angle changes at the C11-C12 photoisomerization site. The sample was illuminated in the NMR rotor at low temperature (similar to 120 K) in order to trap the primary photointermediate, bathorhodopsin. The NMR data are consistent with a strong torsional twist of the HCCH moiety at the isomerization site. Although the HCCH torsional twist was determined to be at least 40A degrees, it was not possible to quantify it more closely. The presence of a strong twist is in agreement with previous Raman observations. The energetic implications of this geometric distortion are discussed
A Quantum Scattering Interferometer
The collision of two ultra-cold atoms results in a quantum-mechanical
superposition of two outcomes: each atom continues without scattering and each
atom scatters as a spherically outgoing wave with an s-wave phase shift. The
magnitude of the s-wave phase shift depends very sensitively on the interaction
between the atoms. Quantum scattering and the underlying phase shifts are
vitally important in many areas of contemporary atomic physics, including
Bose-Einstein condensates, degenerate Fermi gases, frequency shifts in atomic
clocks, and magnetically-tuned Feshbach resonances. Precise measurements of
quantum scattering phase shifts have not been possible until now because, in
scattering experiments, the number of scattered atoms depends on the s-wave
phase shifts as well as the atomic density, which cannot be measured precisely.
Here we demonstrate a fundamentally new type of scattering experiment that
interferometrically detects the quantum scattering phase shifts of individual
atoms. By performing an atomic clock measurement using only the scattered part
of each atom, we directly and precisely measure the difference of the s-wave
phase shifts for the two clock states in a density independent manner. Our
method will give the most direct and precise measurements of ultracold
atom-atom interactions and will place stringent limits on the time variations
of fundamental constants.Comment: Corrected formatting and typo
Introduction to magnetic resonance methods in photosynthesis
Electron paramagnetic resonance (EPR) and, more recently, solid-state nuclear magnetic resonance (NMR) have been employed to study photosynthetic processes, primarily related to the light-induced charge separation. Information obtained on the electronic structure, the relative orientation of the cofactors, and the changes in structure during these reactions should help to understand the efficiency of light-induced charge separation. A short introduction to the observables derived from magnetic resonance experiments is given. The relation of these observables to the electronic structure is sketched using the nitroxide group of spin labels as a simple example
Bang-bang control of fullerene qubits using ultra-fast phase gates
Quantum mechanics permits an entity, such as an atom, to exist in a
superposition of multiple states simultaneously. Quantum information processing
(QIP) harnesses this profound phenomenon to manipulate information in radically
new ways. A fundamental challenge in all QIP technologies is the corruption of
superposition in a quantum bit (qubit) through interaction with its
environment. Quantum bang-bang control provides a solution by repeatedly
applying `kicks' to a qubit, thus disrupting an environmental interaction.
However, the speed and precision required for the kick operations has presented
an obstacle to experimental realization. Here we demonstrate a phase gate of
unprecedented speed on a nuclear spin qubit in a fullerene molecule (N@C60),
and use it to bang-bang decouple the qubit from a strong environmental
interaction. We can thus trap the qubit in closed cycles on the Bloch sphere,
or lock it in a given state for an arbitrary period. Our procedure uses
operations on a second qubit, an electron spin, in order to generate an
arbitrary phase on the nuclear qubit. We anticipate the approach will be vital
for QIP technologies, especially at the molecular scale where other strategies,
such as electrode switching, are unfeasible
High-fidelity quantum driving
The ability to accurately control a quantum system is a fundamental
requirement in many areas of modern science such as quantum information
processing and the coherent manipulation of molecular systems. It is usually
necessary to realize these quantum manipulations in the shortest possible time
in order to minimize decoherence, and with a large stability against
fluctuations of the control parameters. While optimizing a protocol for speed
leads to a natural lower bound in the form of the quantum speed limit rooted in
the Heisenberg uncertainty principle, stability against parameter variations
typically requires adiabatic following of the system. The ultimate goal in
quantum control is to prepare a desired state with 100% fidelity. Here we
experimentally implement optimal control schemes that achieve nearly perfect
fidelity for a two-level quantum system realized with Bose-Einstein condensates
in optical lattices. By suitably tailoring the time-dependence of the system's
parameters, we transform an initial quantum state into a desired final state
through a short-cut protocol reaching the maximum speed compatible with the
laws of quantum mechanics. In the opposite limit we implement the recently
proposed transitionless superadiabatic protocols, in which the system perfectly
follows the instantaneous adiabatic ground state. We demonstrate that
superadiabatic protocols are extremely robust against parameter variations,
making them useful for practical applications.Comment: 17 pages, 4 figure
Terahertz underdamped vibrational motion governs protein-ligand binding in solution
Low-frequency collective vibrational modes in proteins have been proposed as being responsible for efficiently directing biochemical reactions and biological energy transport. However, evidence of the existence of delocalized vibrational modes is scarce and proof of their involvement in biological function absent. Here we apply extremely sensitive femtosecond optical Kerr-effect spectroscopy to study the depolarized Raman spectra of lysozyme and its complex with the inhibitor triacetylchitotriose in solution. Underdamped delocalized vibrational modes in the terahertz frequency domain are identified and shown to blue-shift and strengthen upon inhibitor binding. This demonstrates that the ligand-binding coordinate in proteins is underdamped and not simply solvent-controlled as previously assumed. The presence of such underdamped delocalized modes in proteins may have significant implications for the understanding of the efficiency of ligand binding and protein–molecule interactions, and has wider implications for biochemical reactivity and biological function
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