1,402 research outputs found
A Non-Demolition Single Spin Meter
We present the theory of a single spin meter consisting of a quantum dot in a
magnetic field under microwave irradiation combined with a charge counter. We
show that when a current is passed through the dot, a change in the average
occupation number occurs if the microwaves are resonant with the on-dot Zeeman
splitting. The width of the resonant change is given by the microwave induced
Rabi frequency, making the quantum dot a sensitive probe of the local magnetic
field and enabling the detection of the state of a nearby spin. If the dot-spin
and the nearby spin have different g-factors a non-demolition readout of the
spin state can be achieved. The conditions for a reliable spin readout are
found.Comment: 4 pages, 5 figure
Probing charge fluctuator correlations using quantum dot pairs
We study a pair of quantum dot exciton qubits interacting with a number of
fluctuating charges that can induce a Stark shift of both exciton transition
energies. We do this by solving the optical master equation using a numerical
transfer matrix method. We find that the collective influence of the charge
environment on the dots can be detected by measuring the correlation between
the photons emitted when each dot is driven independently. Qubits in a common
charge environment display photon bunching, if both dots are driven on
resonance or if the driving laser detunings have the same sense for both
qubits, and antibunching if the laser detunings have in opposite signs. We also
show that it is possible to detect several charges fluctuating at different
rates using this technique. Our findings expand the possibility of measuring
qubit dynamics in order to investigate the fundamental physics of the
environmental noise that causes decoherence.Comment: 9 pages, 13 figure
Exchange coupling between silicon donors: the crucial role of the central cell and mass anisotropy
Donors in silicon are now demonstrated as one of the leading candidates for
implementing qubits and quantum information processing. Single qubit
operations, measurements and long coherence times are firmly established, but
progress on controlling two qubit interactions has been slower. One reason for
this is that the inter donor exchange coupling has been predicted to oscillate
with separation, making it hard to estimate in device designs. We present a
multivalley effective mass theory of a donor pair in silicon, including both a
central cell potential and the effective mass anisotropy intrinsic in the Si
conduction band. We are able to accurately describe the single donor properties
of valley-orbit coupling and the spatial extent of donor wave functions,
highlighting the importance of fitting measured values of hyperfine coupling
and the orbital energy of the levels. Ours is a simple framework that can
be applied flexibly to a range of experimental scenarios, but it is nonetheless
able to provide fast and reliable predictions. We use it to estimate the
exchange coupling between two donor electrons and we find a smoothing of its
expected oscillations, and predict a monotonic dependence on separation if two
donors are spaced precisely along the [100] direction.Comment: Published version. Corrected b and B values from previous versio
Quantum-enhanced capture of photons using optical ratchet states
Natural and artificial light harvesting systems often operate in a regime
where the flux of photons is relatively low. Besides absorbing as many photons
as possible it is therefore paramount to prevent excitons from annihilation via
photon re-emission until they have undergone an irreversible energy conversion
process. Taking inspiration from photosynthetic antenna structures, we here
consider ring-like systems and introduce a class of states we call ratchets:
excited states capable of absorbing but not emitting light. This allows our
antennae to absorb further photons whilst retaining the excitations from those
that have already been captured. Simulations for a ring of four sites reveal a
peak power enhancement by up to a factor of 35 under ambient conditions owing
to a combination of ratcheting and the prevention of emission through
dark-state population. In the slow extraction limit the achievable power
enhancement due to ratcheting alone exceeds 20%.Comment: major revision with improved model (all data and figures updated
Surface code architecture for donors and dots in silicon with imprecise and nonuniform qubit couplings
A scaled quantum computer with donor spins in silicon would benefit from a
viable semiconductor framework and a strong inherent decoupling of the qubits
from the noisy environment. Coupling neighbouring spins via the natural
exchange interaction according to current designs requires gate control
structures with extremely small length scales. We present a silicon
architecture where bismuth donors with long coherence times are coupled to
electrons that can shuttle between adjacent quantum dots, thus relaxing the
pitch requirements and allowing space between donors for classical control
devices. An adiabatic SWAP operation within each donor/dot pair solves the
scalability issues intrinsic to exchange-based two-qubit gates, as it does not
rely on sub-nanometer precision in donor placement and is robust against noise
in the control fields. We use this SWAP together with well established global
microwave Rabi pulses and parallel electron shuttling to construct a surface
code that needs minimal, feasible local control.Comment: Published version - more detailed discussions, robustness to
dephasing pointed out additionall
Spin detection at elevated temperatures using a driven double quantum dot
We consider a double quantum dot in the Pauli blockade regime interacting
with a nearby single spin. We show that under microwave irradiation the average
electron occupations of the dots exhibit resonances that are sensitive to the
state of the nearby spin. The system thus acts as a spin meter for the nearby
spin. We investigate the conditions for a non-demolition read-out of the spin
and find that the meter works at temperatures comparable to the dot charging
energy and sensitivity is mainly limited by the intradot spin relaxation.Comment: 8 pages, 6 figure
Entanglement distribution for a practical quantum-dot-based quantum processor architecture
We propose a quantum dot (QD) architecture for enabling universal quantum information processing. Quantum registers, consisting of arrays of vertically stacked self-assembled semiconductor QDs, are connected by chains of in-plane self-assembled dots. We propose an entanglement distributor, a device for producing and distributing maximally entangled qubits on demand, communicated through in-plane dot chains. This enables the transmission of entanglement to spatially separated register stacks, providing a resource for the realization of a sizeable quantum processor built from coupled register stacks of practical size. Our entanglement distributor could be integrated into many of the present proposals for self-assembled QD-based quantum computation (QC). Our device exploits the properties of simple, relatively short, spin-chains and does not require microcavities. Utilizing the properties of self-assembled QDs, after distribution the entanglement can be mapped into relatively long-lived spin qubits and purified, providing a flexible, distributed, off-line resource. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
Superabsorption of light via quantum engineering
Almost 60 years ago Dicke introduced the term superradiance to describe a
signature quantum effect: N atoms can collectively emit light at a rate
proportional to N^2. Even for moderate N this represents a significant increase
over the prediction of classical physics, and the effect has found applications
ranging from probing exciton delocalisation in biological systems, to
developing a new class of laser, and even in astrophysics. Structures that
super-radiate must also have enhanced absorption, but the former always
dominates in natural systems. Here we show that modern quantum control
techniques can overcome this restriction. Our theory establishes that
superabsorption can be achieved and sustained in certain simple nanostructures,
by trapping the system in a highly excited state while extracting energy into a
non-radiative channel. The effect offers the prospect of a new class of quantum
nanotechnology, capable of absorbing light many times faster than is currently
possible; potential applications of this effect include light harvesting and
photon detection. An array of quantum dots or a porphyrin ring could provide an
implementation to demonstrate this effect
Optimal power generation using dark states in dimers strongly coupled to their environment
Dark state protection has been proposed as a mechanism to increase the power
output of light harvesting devices by reducing the rate of radiative
recombination. Indeed many theoretical studies have reported increased power
outputs in dimer systems which use quantum interference to generate dark
states. These models have typically been restricted to particular geometries
and to weakly coupled vibrational baths. Here we consider the
experimentally-relevant strong vibrational coupling regime with no geometric
restrictions on the dimer. We analyze how dark states can be formed in the
dimer by numerically minimizing the emission rate of the lowest energy excited
eigenstate, and then calculate the power output of the molecules with these
dark states. We find that there are two distinct types of dark states depending
on whether the monomers form homodimers, where energy splittings and dipole
strengths are identical, or heterodimers, where there is some difference.
Homodimers, which exploit destructive quantum interference, produce high power
outputs but strong phonon couplings and perturbations from ideal geometries are
extremely detrimental. Heterodimers, which are closer to the classical picture
of a distinct donor and acceptor molecule, produce an intermediate power output
that is relatively stable to these changes. The strong vibrational couplings
typically found in organic molecules will suppress destructive interference and
thus favour the dark-state enhancement offered by heterodimers.Comment: 20+18 pages, 5+5 figures. We have updated Figures 4, 5, F1 and G1 to
correct for a minor error, however the correction is small and does not
change the message of the paper. We have also added a paragraph to the
appendix to detail how the rotating wave approximation and double excited
state affect the master equatio
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