75 research outputs found
Full coherent control of nuclear spins in an optically pumped single quantum dot
Highly polarized nuclear spins within a semiconductor quantum dot (QD) induce
effective magnetic (Overhauser) fields of up to several Tesla acting on the
electron spin or up to a few hundred mT for the hole spin. Recently this has
been recognized as a resource for intrinsic control of QD-based spin quantum
bits. However, only static long-lived Overhauser fields could be used. Here we
demonstrate fast redirection on the microsecond time-scale of Overhauser fields
of the order of 0.5 T experienced by a single electron spin in an optically
pumped GaAs quantum dot. This has been achieved using full coherent control of
an ensemble of 10^3-10^4 optically polarized nuclear spins by sequences of
short radio-frequency (rf) pulses. These results open the way to a new class of
experiments using rf techniques to achieve highly-correlated nuclear spins in
quantum dots, such as adiabatic demagnetization in the rotating frame leading
to sub-micro K nuclear spin temperatures, rapid adiabatic passage, and spin
squeezing
Cerenkov Radiation Energy Transfer (CRET) Imaging: A Novel Method for Optical Imaging of PET Isotopes in Biological Systems
Positron emission tomography (PET) allows sensitive, non-invasive analysis of the distribution of radiopharmaceutical tracers labeled with positron (β(+))-emitting radionuclides in small animals and humans. Upon β(+) decay, the initial velocity of high-energy β(+) particles can momentarily exceed the speed of light in tissue, producing Cerenkov radiation that is detectable by optical imaging, but is highly absorbed in living organisms.To improve optical imaging of Cerenkov radiation in biological systems, we demonstrate that Cerenkov radiation from decay of the PET isotopes (64)Cu and (18)F can be spectrally coupled by energy transfer to high Stokes-shift quantum nanoparticles (Qtracker705) to produce highly red-shifted photonic emissions. Efficient energy transfer was not detected with (99m)Tc, a predominantly γ-emitting isotope. Similar to bioluminescence resonance energy transfer (BRET) and fluorescence resonance energy transfer (FRET), herein we define the Cerenkov radiation energy transfer (CRET) ratio as the normalized quotient of light detected within a spectral window centered on the fluorophore emission divided by light detected within a spectral window of the Cerenkov radiation emission to quantify imaging signals. Optical images of solutions containing Qtracker705 nanoparticles and [(18)F]FDG showed CRET ratios in vitro as high as 8.8±1.1, while images of mice with subcutaneous pseudotumors impregnated with Qtracker705 following intravenous injection of [(18)F]FDG showed CRET ratios in vivo as high as 3.5±0.3.Quantitative CRET imaging may afford a variety of novel optical imaging applications and activation strategies for PET radiopharmaceuticals and other isotopes in biomaterials, tissues and live animals
Electric-field-induced coherent coupling of the exciton states in a single quantum dot
The signature of coherent coupling between two quantum states is an
anticrossing in their energies as one is swept through the other. In single
semiconductor quantum dots containing an electron-hole pair the eigenstates
form a two-level system that can be used to demonstrate quantum effects in the
solid state, but in all previous work these states were independent. Here we
describe a technique to control the energetic splitting of these states using a
vertical electric field, facilitating the observation of coherent coupling
between them. Near the minimum splitting the eigenstates rotate in the plane of
the sample, being orientated at 45{\deg} when the splitting is smallest. Using
this system we show direct control over the exciton states in one quantum dot,
leading to the generation of entangled photon pairs
Measurement of the spin temperature of optically cooled nuclei and GaAs hyperfine constants in GaAs/AlGaAs quantum dots
Deep cooling of electron and nuclear spins is equivalent to achieving polarization degrees close to 100% and is a key requirement in solid state quantum information technologies. While polarization of individual nuclear spins in diamond and SiC reaches 99% and beyond, it has been limited to 60-65% for the nuclei in quantum dots. Theoretical models have attributed this limit to formation of coherent "dark" nuclear spin states but experimental verification is lacking, especially due to the poor accuracy of polarization degree measurements. Here we measure the nuclear polarization in GaAs/AlGaAs quantum dots with high accuracy using a new approach enabled by manipulation of the nuclear spin states with radiofrequency pulses. Polarizations up to 80% are observed - the highest reported so far for optical cooling in quantum dots. This value is still not limited by nuclear coherence effects. Instead we find that optically cooled nuclei are well described within a classical spin temperature framework. Our findings unlock a route for further progress towards quantum dot electron spin qubits where deep cooling of the mesoscopic nuclear spin ensemble is used to achieve long qubit coherence. Moreover, GaAs hyperfine material constants are measured here experimentally for the first time
Ultrafast optical control of entanglement between two quantum dot spins
The interaction between two quantum bits enables entanglement, the
two-particle correlations that are at the heart of quantum information science.
In semiconductor quantum dots much work has focused on demonstrating single
spin qubit control using optical techniques. However, optical control of
entanglement of two spin qubits remains a major challenge for scaling from a
single qubit to a full-fledged quantum information platform. Here, we combine
advances in vertically-stacked quantum dots with ultrafast laser techniques to
achieve optical control of the entangled state of two electron spins. Each
electron is in a separate InAs quantum dot, and the spins interact through
tunneling, where the tunneling rate determines how rapidly entangling
operations can be performed. The two-qubit gate speeds achieved here are over
an order of magnitude faster than in other systems. These results demonstrate
the viability and advantages of optically controlled quantum dot spins for
multi-qubit systems.Comment: 24 pages, 5 figure
Isotope sensitive measurement of the hole-nuclear spin interaction in quantum dots
Decoherence caused by nuclear field fluctuations is a fundamental obstacle to
the realization of quantum information processing using single electron spins.
Alternative proposals have been made to use spin qubits based on valence band
holes having weaker hyperfine coupling. However, it was demonstrated recently
both theoretically and experimentally that the hole hyperfine interaction is
not negligible, although a consistent picture of the mechanism controlling the
magnitude of the hole-nuclear coupling is still lacking. Here we address this
problem by performing isotope selective measurement of the valence band
hyperfine coupling in InGaAs/GaAs, InP/GaInP and GaAs/AlGaAs quantum dots.
Contrary to existing models we find that the hole hyperfine constant along the
growth direction of the structure (normalized by the electron hyperfine
constant) has opposite signs for different isotopes and ranges from -15% to
+15%. We attribute such changes in hole hyperfine constants to the competing
positive contributions of p-symmetry atomic orbitals and the negative
contributions of d-orbitals. Furthermore, we find that the d-symmetry
contribution leads to a new mechanism for hole-nuclear spin flips which may
play an important role in hole spin decoherence. In addition the measured
hyperfine constants enable a fundamentally new approach for verification of the
computed Bloch wavefunctions in the vicinity of nuclei in semiconductor
nanostructures
High resolution nuclear magnetic resonance spectroscopy of highly-strained quantum dot nanostructures
Much new solid state technology for single-photon sources, detectors,
photovoltaics and quantum computation relies on the fabrication of strained
semiconductor nanostructures. Successful development of these devices depends
strongly on techniques allowing structural analysis on the nanometer scale.
However, commonly used microscopy methods are destructive, leading to the loss
of the important link between the obtained structural information and the
electronic and optical properties of the device. Alternative non-invasive
techniques such as optically detected nuclear magnetic resonance (ODNMR) so far
proved difficult in semiconductor nano-structures due to significant
strain-induced quadrupole broadening of the NMR spectra. Here, we develop new
high sensitivity techniques that move ODNMR to a new regime, allowing high
resolution spectroscopy of as few as 100000 quadrupole nuclear spins. By
applying these techniques to individual strained self-assembled quantum dots,
we measure strain distribution and chemical composition in the volume occupied
by the confined electron. Furthermore, strain-induced spectral broadening is
found to lead to suppression of nuclear spin magnetization fluctuations thus
extending spin coherence times. The new ODNMR methods have potential to be
applied for non-invasive investigations of a wide range of materials beyond
single nano-structures, as well as address the task of understanding and
control of nuclear spins on the nanoscale, one of the central problems in
quantum information processing
Study protocol to investigate the effect of a lifestyle intervention on body weight, psychological health status and risk factors associated with disease recurrence in women recovering from breast cancer treatment
Background
Breast cancer survivors often encounter physiological and psychological problems related to their diagnosis and treatment that can influence long-term prognosis. The aim of this research is to investigate the effects of a lifestyle intervention on body weight and psychological well-being in women recovering from breast cancer treatment, and to determine the relationship between changes in these variables and biomarkers associated with disease recurrence and survival.
Methods/design
Following ethical approval, a total of 100 patients will be randomly assigned to a lifestyle intervention (incorporating dietary energy restriction in conjunction with aerobic exercise training) or normal care control group. Patients randomised to the dietary and exercise intervention will be given individualised healthy eating dietary advice and written information and attend moderate intensity aerobic exercise sessions on three to five days per week for a period of 24 weeks. The aim of this strategy is to induce a steady weight loss of up to 0.5 Kg each week. In addition, the overall quality of the diet will be examined with a view to (i) reducing the dietary intake of fat to ~25% of the total calories, (ii) eating at least 5 portions of fruit and vegetables a day, (iii) increasing the intake of fibre and reducing refined carbohydrates, and (iv) taking moderate amounts of alcohol. Outcome measures will include body weight and body composition, psychological health status (stress and depression), cardiorespiratory fitness and quality of life. In addition, biomarkers associated with disease recurrence, including stress hormones, estrogen status, inflammatory markers and indices of innate and adaptive immune function will be monitored.
Discussion
This research will provide valuable information on the effectiveness of a practical, easily implemented lifestyle intervention for evoking positive effects on body weight and psychological well-being, two important factors that can influence long-term prognosis in breast cancer survivors. However, the added value of the study is that it will also evaluate the effects of the lifestyle intervention on a range of biomarkers associated with disease recurrence and survival. Considered together, the results should improve our understanding of the potential role that lifestyle-modifiable factors could play in saving or prolonging lives
Optical control of one and two hole spins in interacting quantum dots
A single hole spin in a semiconductor quantum dot has emerged as a quantum
bit that is potentially superior to an electron spin. A key feature of holes is
that they have a greatly reduced hyperfine interaction with nuclear spins,
which is one of the biggest difficulties in working with an electron spin. It
is now essential to show that holes are viable for quantum information
processing by demonstrating fast quantum gates and scalability. To this end we
have developed InAs/GaAs quantum dots coupled through coherent tunneling and
charged with controlled numbers of holes. We report fast, single qubit gates
using a sequence of short laser pulses. We then take the important next step
toward scalability of quantum information by optically controlling two
interacting hole spins in separate dots.Comment: 5 figure
Empowerment and Parent Gain as Mediators and Moderators of Distress in Mothers of Children with Autism Spectrum Disorders
Mothers of children with Autism Spectrum Disorders (ASD) experience considerable amounts of distress and experiences of crisis. The Family Adjustment and Adaptation Response model provides a theory for understanding the experience of distress and family crisis in families, and the purpose of the present study was to examine experiences of distress in mothers of individuals with ASD using this framework. We specifically investigated how parent empowerment and positive gain are related to their experiences of distress, whether as mediators or as moderators of child aggression. Participants included 156 mothers of children with ASD ranging in age from 4 – 21 years. Mothers completed an online survey of demographics, problem behaviors, family empowerment, positive gain, and distress. We conducted path analyses of multiple mediation and moderation. Results indicated that greater child problem behavior was related to less parent empowerment, which was related to greater maternal distress, supporting empowerment as a partial mediator. At the same time, greater child aggression was not related to maternal distress in mothers who report high rates of positive gain, suggesting that parent gain functions as a moderator. The implications for how and when clinicians intervene with families of children with ASD are discussed
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