2,525 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
Almost Euclidean sections of the N-dimensional cross-polytope using O(N) random bits
It is well known that R^N has subspaces of dimension proportional to N on
which the \ell_1 norm is equivalent to the \ell_2 norm; however, no explicit
constructions are known. Extending earlier work by Artstein--Avidan and Milman,
we prove that such a subspace can be generated using O(N) random bits.Comment: 16 pages; minor changes in the introduction to make it more
accessible to both Math and CS reader
Quantum thermometry using the ac Stark shift within the Rabi model
This work was supported by the EPSRC, the National Research Foundation and Ministry of Education, Singapore, and the Royal Society.A quantum two-level system coupled to a harmonic oscillator represents a ubiquitous physical system. New experiments in circuit QED and nanoelectromechanical systems (NEMS) achieve unprecedented coupling strength at large detuning between qubit and oscillator, thus requiring a theoretical treatment beyond the Jaynes-Cummings model. Here we present a new method for describing the qubit dynamics in this regime, based on an oscillator correlation function expansion of a non-Markovian master equation in the polaron frame. Our technique yields a new numerical method as well as a succinct approximate expression for the qubit dynamics. These expressions are valid in the experimentally interesting regime of strong coupling at low temperature. We obtain a new expression for the ac Stark shift and show that this enables practical and precise qubit thermometry of an oscillator.Peer reviewe
Differential Evolution for Many-Particle Adaptive Quantum Metrology
We devise powerful algorithms based on differential evolution for adaptive
many-particle quantum metrology. Our new approach delivers adaptive quantum
metrology policies for feedback control that are orders-of-magnitude more
efficient and surpass the few-dozen-particle limitation arising in methods
based on particle-swarm optimization. We apply our method to the
binary-decision-tree model for quantum-enhanced phase estimation as well as to
a new problem: a decision tree for adaptive estimation of the unknown bias of a
quantum coin in a quantum walk and show how this latter case can be realized
experimentally.Comment: Fig. 2(a) is the cover of Physical Review Letters Vol. 110 Issue 2
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
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