30 research outputs found
Heat pumping with optically driven excitons
We present a theoretical study showing that an optically driven excitonic
two-level system in a solid state environment acts as a heat pump by means of
repeated phonon emission or absorption events. We derive a master equation for
the combined phonon bath and two-level system dynamics and analyze the
direction and rate of energy transfer as a function of the externally
accessible driving parameters. We discover that if the driving laser is detuned
from the exciton transition, cooling the phonon environment becomes possible
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
Calibration and High Fidelity Measurement of a Quantum Photonic Chip
Integrated quantum photonic circuits are becoming increasingly complex.
Accurate calibration of device parameters and detailed characterization of the
prepared quantum states are critically important for future progress. Here we
report on an effective experimental calibration method based on Bayesian
updating and Markov chain Monte Carlo integration. We use this calibration
technique to characterize a two qubit chip and extract the reflectivities of
its directional couplers. An average quantum state tomography fidelity of
93.79+/-1.05% against the four Bell states is achieved. Furthermore, comparing
the measured density matrices against a model using the non-ideal device
parameters derived from the calibration we achieve an average fidelity of
97.57+/-0.96%. This pinpoints non-ideality of chip parameters as a major factor
in the decrease of Bell state fidelity. We also perform quantum state
tomography for Bell states while continuously varying photon distinguishability
and find excellent agreement with theory
Demonstration of Free-space Reference Frame Independent Quantum Key Distribution
Quantum key distribution (QKD) is moving from research laboratories towards
applications. As computing becomes more mobile, cashless as well as cardless
payment solutions are introduced, and a need arises for incorporating QKD in a
mobile device. Handheld devices present a particular challenge as the
orientation and the phase of a qubit will depend on device motion. This problem
is addressed by the reference frame independent (RFI) QKD scheme. The scheme
tolerates an unknown phase between logical states that varies slowly compared
to the rate of particle repetition. Here we experimentally demonstrate the
feasibility of RFI QKD over a free-space link in a prepare and measure scheme
using polarisation encoding. We extend the security analysis of the RFI QKD
scheme to be able to deal with uncalibrated devices and a finite number of
measurements. Together these advances are an important step towards mass
production of handheld QKD devices
Dynamical instabilities of a resonator driven by a superconducting single-electron transistor
We investigate the dynamical instabilities of a resonator coupled to a
superconducting single-electron transistor (SSET) tuned to the Josephson
quasiparticle (JQP) resonance. Starting from the quantum master equation of the
system, we use a standard semiclassical approximation to derive a closed set of
mean field equations which describe the average dynamics of the resonator and
SSET charge. Using amplitude and phase coordinates for the resonator and
assuming that the amplitude changes much more slowly than the phase, we explore
the instabilities which arise in the resonator dynamics as a function of
coupling to the SSET, detuning from the JQP resonance and the resonator
frequency. We find that the locations (in parameter space) and sizes of the
limit cycle states predicted by the mean field equations agree well with
numerical solutions of the full master equation for sufficiently weak
SSET-resonator coupling. The mean field equations also give a good qualitative
description of the set of dynamical transitions in the resonator state that
occur as the coupling is progressively increased.Comment: 23 pages, 6 Figures, Accepted for NJ
Statistics of charge transfer in a tunnel junction coupled to an oscillator
The charge transfer statistics of a tunnel junction coupled to a quantum
object is studied using the charge projection technique. The joint dynamics of
the quantum object and the number of charges transferred through the junction
is described by the charge specific density matrix. The method allows
evaluating the joint probability distribution of the state of the quantum
object and the charge state of the junction.The statistical properties of the
junction current are derived from the charge transfer statistics using the
master equation for the charge specific density matrix. The theory is applied
to a nanoelectromechanical system, and the influence on the average current and
the current noise of the junction is obtained for coupling to a harmonic
oscillator.Comment: 18 pages, 3 figure
Vibrational spectra of C60C8H8 and C70C8H8 in the rotor-stator and polymer phases
C60-C8H8 and C70-C8H8 are prototypes of rotor-stator cocrystals. We present
infrared and Raman spectra of these materials and show how the rotor-stator
nature is reflected in their vibrational properties. We measured the
vibrational spectra of the polymer phases poly(C60C8H8) and poly(C70C8H8)
resulting from a solid state reaction occurring on heating. Based on the
spectra we propose a connection pattern for the fullerene in poly(C60C8H8),
where the symmetry of the C60 is D2h. On illuminating the C60-C8H8 cocrystal
with green or blue light a photochemical reaction was observed leading to a
similar product to that of the thermal polymerization.Comment: 26 pages, 8 figures, to appear in Journal of Physical Chemistry B 2nd
version: minor changes in wording, accepted version by journa
Reference-Frame-Independent Quantum-Key-Distribution Server with a Telecom Tether for an On-Chip Client
We demonstrate a client-server quantum key distribution (QKD) scheme. Large resources such as laser and detectors are situated at the server side, which is accessible via telecom fiber to a client requiring only an on-chip polarization rotator, which may be integrated into a handheld device. The detrimental effects of unstable fiber birefringence are overcome by employing the reference-frame-independent QKD protocol for polarization qubits in polarization maintaining fiber, where standard QKD protocols fail, as we show for comparison. This opens the way for quantum enhanced secure communications between companies and members of the general public equipped with handheld mobile devices, via telecom-fiber tethering
A GABAergic and peptidergic sleep neuron as a locomotion stop neuron with compartmentalized Ca2+dynamics.
Animals must slow or halt locomotion to integrate sensory inputs or to change direction. In Caenorhabditis elegans, the GABAergic and peptidergic neuron RIS mediates developmentally timed quiescence. Here, we show RIS functions additionally as a locomotion stop neuron. RIS optogenetic stimulation caused acute and persistent inhibition of locomotion and pharyngeal pumping, phenotypes requiring FLP-11 neuropeptides and GABA. RIS photoactivation allows the animal to maintain its body posture by sustaining muscle tone, yet inactivating motor neuron oscillatory activity. During locomotion, RIS axonal Ca2+ signals revealed functional compartmentalization: Activity in the nerve ring process correlated with locomotion stop, while activity in a branch correlated with induced reversals. GABA was required to induce, and FLP-11 neuropeptides were required to sustain locomotion stop. RIS attenuates neuronal activity and inhibits movement, possibly enabling sensory integration and decision making, and exemplifies dual use of one cell across development in a compact nervous system