372 research outputs found
Hyperfine induced spin and entanglement dynamics in Double Quantum Dots: A homogeneous coupling approach
We investigate hyperfine induced electron spin and entanglement dynamics in a
system of two quantum dot spin qubits. We focus on the situation of zero
external magnetic field and concentrate on approximation-free theoretical
methods. We give an exact solution of the model for homogeneous hyperfine
coupling constants (with all coupling coefficients being equal) and varying
exchange coupling, and we derive the dynamics therefrom. After describing and
explaining the basic dynamical properties, the decoherence time is calculated
from the results of a detailed investigation of the short time electron spin
dynamics. The result turns out to be in good agreement with experimental data.Comment: 10 pages, 8 figure
Perturbative regimes in central spin models
Central spin models describe several types of solid state nanostructures
which are presently considered as possible building blocks of future quantum
information processing hardware. From a theoretical point of view, a key issue
remains the treatment of the flip-flop terms in the Hamiltonian in the presence
of a magnetic field. We systematically study the influence of these terms, both
as a function of the field strength and the size of the spin baths. We find
crucial differences between initial states with central spin configurations of
high and such of low polarizations. This has strong implications with respect
to the influence of a magnetic field on the flip-flop terms in central spin
models of a single and more than one central spin. Furthermore, the
dependencies on bath size and field differ from those anticipated so far. Our
results might open the route for the systematic search for more efficient
perturbative treatments of central spin problems.Comment: 7 pages, 3 figure
Nuclear spin state narrowing via gate--controlled Rabi oscillations in a double quantum dot
We study spin dynamics for two electrons confined to a double quantum dot
under the influence of an oscillating exchange interaction. This leads to
driven Rabi oscillations between the --state and the
--state of the two--electron system. The width of the
Rabi resonance is proportional to the amplitude of the oscillating exchange. A
measurement of the Rabi resonance allows one to narrow the distribution of
nuclear spin states and thereby to prolong the spin decoherence time. Further,
we study decoherence of the two-electron states due to the hyperfine
interaction and give requirements on the parameters of the system in order to
initialize in the --state and to perform a
operation with unit fidelity.Comment: v1:9 pages, 1 figure; v2: 13 pages, 2 figures, added section on
measurement, to appear in Phys. Rev.
Universal phase shift and non-exponential decay of driven single-spin oscillations
We study, both theoretically and experimentally, driven Rabi oscillations of
a single electron spin coupled to a nuclear spin bath. Due to the long
correlation time of the bath, two unusual features are observed in the
oscillations. The decay follows a power law, and the oscillations are shifted
in phase by a universal value of ~pi/4. These properties are well understood
from a theoretical expression that we derive here in the static limit for the
nuclear bath. This improved understanding of the coupled electron-nuclear
system is important for future experiments using the electron spin as a qubit.Comment: Main text: 4 pages, 3 figures, Supplementary material: 2 pages, 3
figure
ESCAP CovCAP survey of heads of academic departments to assess the perceived initial (April/May 2020) impact of the COVID-19 pandemic on child and adolescent psychiatry services.
In April 2020, the European Society for Child and Adolescent Psychiatry (ESCAP) Research Academy and the ESCAP Board launched the first of three scheduled surveys to evaluate the impact of the coronavirus disease 2019 (COVID-19) pandemic on child and adolescent psychiatry (CAP) services in Europe and to assess the abilities of CAP centers to meet the new challenges brought on by the crisis. The survey was a self-report questionnaire, using a multistage process, which was sent to 168 heads of academic CAP services in 24 European countries. Eighty-two responses (56 complete) from 20 countries, representing the subjective judgement of heads of CAP centers, were received between mid-April and mid-May 2020. Most respondents judged the impact of the crisis on the mental health of their patients as medium (52%) or strong (33%). A large majority of CAP services reported no COVID-19 positive cases among their inpatients and most respondents declared no or limited sick leaves in their team due to COVID-19. Outpatient, daycare, and inpatient units experienced closures or reductions in the number of treated patients throughout Europe. In addition, a lower referral rate was observed in most countries. Respondents considered that they were well equipped to handle COVID-19 patients despite a lack of protective equipment. Telemedicine was adopted by almost every team despite its sparse use prior to the crisis. Overall, these first results were surprisingly homogeneous, showing a substantially reduced patient load and a moderate effect of the COVID-19 crisis on psychopathology. The effect on the organization of CAP services appears profound. COVID-19 crisis has accelerated the adoption of new technologies, including telepsychiatry
Nuclear spin dynamics and Zeno effect in quantum dots and defect centers
We analyze nuclear spin dynamics in quantum dots and defect centers with a
bound electron under electron-mediated coupling between nuclear spins due to
the hyperfine interaction ("J-coupling" in NMR). Our analysis shows that the
Overhauser field generated by the nuclei at the position of the electron has
short-time dynamics quadratic in time for an initial nuclear spin state without
transverse coherence. The quadratic short-time behavior allows for an extension
of the Overhauser field lifetime through a sequence of projective measurements
(quantum Zeno effect). We analyze the requirements on the repetition rate of
measurements and the measurement accuracy to achieve such an effect. Further,
we calculate the long-time behavior of the Overhauser field for effective
electron Zeeman splittings larger than the hyperfine coupling strength and
find, both in a Dyson series expansion and a generalized master equation
approach, that for a nuclear spin system with a sufficiently smooth
polarization the electron-mediated interaction alone leads only to a partial
decay of the Overhauser field by an amount on the order of the inverse number
of nuclear spins interacting with the electron.Comment: 11 pages, 3 figure
Developing a Critical Understanding of Smart Urbanism?
Smart urbanism is emerging at the intersection of visions for the future of urban places, new technologies and infrastructures. Smart urbanism discourses are deeply rooted in seductive and normative visions of the future where digital technology stands as the primary driver for change. Yet our understanding of the opportunities, challenges, and implications of smart urbanism is limited. Research in this field is in its infancy, fragmented along disciplinary lines and often based on single city case studies. As a result, we lack both the theoretical insight and empirical evidence required to assess the implications of this potentially transformative phenomenon. Given the significant implications of smart urbanism there is an urgent need to critically engage with why, how, for whom and with what consequences smart urbanism is emerging in different urban contexts. The aim of this review is to unpack the different logics and rationales behind smart urbanism discourses and proposals, and in this way understand the ways by which imaginaries of urban futures are currently being constructed, along with their socio-technical and political implications for future research priorities
Driven coherent oscillations of a single electron spin in a quantum dot
The ability to control the quantum state of a single electron spin in a
quantum dot is at the heart of recent developments towards a scalable
spin-based quantum computer. In combination with the recently demonstrated
exchange gate between two neighbouring spins, driven coherent single spin
rotations would permit universal quantum operations. Here, we report the
experimental realization of single electron spin rotations in a double quantum
dot. First, we apply a continuous-wave oscillating magnetic field, generated
on-chip, and observe electron spin resonance in spin-dependent transport
measurements through the two dots. Next, we coherently control the quantum
state of the electron spin by applying short bursts of the oscillating magnetic
field and observe about eight oscillations of the spin state (so-called Rabi
oscillations) during a microsecond burst. These results demonstrate the
feasibility of operating single-electron spins in a quantum dot as quantum
bits.Comment: Total 25 pages. 11 pages main text, 5 figures, 9 pages supplementary
materia
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