arXiv subjects

I. Schwartz

Publications and source records attributed to I. Schwartz.

10 recordsLinked to original sources

arXiv2018

Shaped Pulses for Energy Efficient High-Field NMR at the Nanoscale

The realisation of optically detected magnetic resonance via nitrogen vacancy centers in diamond faces challenges at high magnetic fields which include growing energy consumption of control pulses as well as decreasing sensitivities. Here we address these challenges with the design of shaped pulses in microwave control sequences that achieve orders magnitude reductions in energy consumption and concomitant increases in sensitivity when compared to standard top-hat microwave pulses. The method proposed here is general and can be applied to any quantum sensor subjected to pulsed control sequences.

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arXiv2018

Sharp reversals of steady-state nuclear polarisation as a tool for quantum sensing

The polarisation dynamics of nuclear spins weakly coupled to an NV center is highly sensitive to the parameters of the microwave control and the nuclear Larmor frequency. What is commonly regarded as a challenge, we propose here as a resource for quantum sensing. By varying a single experimental parameter in a suitable set-up, i.e., the Rabi frequency of a continual microwave driving or the nuclear Larmor frequency, we predict periodic reversals of the steady-state polarisation of the nuclear spin. Crucially, interference between the transverse and longitudinal dipolar interaction of electron and nuclear spins results in remarkably sharp steady-state polarisation reversals of nuclear spins within only a few tens of Hz change in the nuclear Larmor frequency. Our method is particularly robust against imperfections such as decoherence of the electron spin and the frequency resolution of the sensor is not limited by the coherence time $T_2$ of the electron sensor.

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arXiv2017

On-demand source of maximally entangled photon-pairs using the biexciton-exciton radiative cascade

We perform full time resolved tomographic measurements of the polarization state of pairs of photons emitted during the radiative cascade of the confined biexciton in a semiconductor quantum dot. The biexciton was deterministically initiated using a $π$-area pulse into the biexciton two-photon absorption resonance. Our measurements demonstrate that the polarization states of the emitted photon pair are maximally entangled. We show that the measured degree of entanglement depends solely on the temporal resolution by which the time difference between the emissions of the photon pair is determined. A route for fabricating an on demand source of maximally polarization entangled photon pairs is thereby provided.

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arXiv2016

Selection Rules for Non-Radiative Carrier Relaxation Processes in Semiconductor Quantum Dots

Time resolved intensity cross-correlation measurements of radiative cascades are used for studying non-radiative relaxation processes of excited carriers confined in semiconductor quantum dots. We spectrally identify indirect radiative cascades which include intermediate phonon assisted relaxations. The energy of the first photon reveals the multicarrier configuration prior to the non-radiative relaxation, while the energy of the second photon reveals the configuration after the relaxation. The intensity cross correlation measurements thus provide quantitative measures of the non-radiative processes and their selection rules. We construct a model which accurately describes the experimental observations in terms of the electron-phonon and electron-hole exchange interactions. Our measurements and model provide a new tool for engineering relaxation processes in semiconductor nanostructures.

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arXiv2016

Controlling the Dark Exciton Spin Eigenstates by External Magnetic Field

We study the dark exciton's behavior as a coherent physical two-level spin system (qubit) using an external magnetic field in the Faraday configuration. Our studies are based on polarization-sensitive intensity autocorrelation measurements of the optical transition resulting from the recombination of a spin-blockaded biexciton state, which heralds the dark exciton and its spin state. We demonstrate control over the dark exciton eigenstates without degrading its decoherence time. Our observations agree well with computational predictions based on a master equation model.

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arXiv2015

Deterministic Coherent Writing of a Long-Lived Semiconductor Spin Qubit Using One Ultrafast Optical Pulse

We use one single, few-picosecond-long, variably polarized laser pulse to deterministically write any selected spin state of a quantum dot confined dark exciton whose life and coherence time are six and five orders of magnitude longer than the laser pulse duration, respectively. The pulse is tuned to an absorption resonance of an excited dark exciton state, which acquires non-negligible oscillator strength due to residual mixing with bright exciton states. We obtain a high fidelity one-to-one mapping from any point on the Poincaré sphere of the pulse polarization to a corresponding point on the Bloch sphere of the spin of the deterministically photogenerated dark exciton.

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arXiv2015

All-Optical Depletion of Dark Excitons from a Semiconductor Quantum Dot

Semiconductor quantum dots are considered to be the leading venue for fabricating on-demand sources of single photons. However, the generation of long-lived dark excitons imposes significant limits on the efficiency of these sources. We demonstrate a technique that optically pumps the dark exciton population and converts it to a bright exciton population, using intermediate excited biexciton states. We show experimentally that our method considerably reduces the DE population while doubling the triggered bright exciton emission, approaching thereby near-unit fidelity of quantum dot depletion.

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arXiv2014

Determinstic Generation of a Quantum Dot-Confined Triexciton and its Radiative Decay via Three-Photon Cascade

Semiconductor quantum dots (QDs) have potential applications in quantum information processing due to the fact that they are potential on-demand sources of single and entangled photons. Generation of polarization-entangled photon pairs was demonstrated using the biexciton-exciton radiative cascade. One obvious way to increase the number of quantum correlated photons that the QDs emit is to use higher-order multiexcitons, in particular the triexciton. Towards achieving this goal, we first demonstrate deterministic generation of the QD-confined triexciton in a well-definedcoherent state and then spectrally identify and directly measure a three-photon radiative cascade resulting from the sequential triexciton-biexciton-exciton radiative recombination.

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arXiv2014

Determinisitic Writing and Control of the Dark Exciton Spin using Short Single Optical Pulses

We demonstrate that the quantum dot-confined dark exciton forms a long-lived integer spin solid state qubit which can be deterministically on-demand initiated in a pure state by one optical pulse. Moreover, we show that this qubit can be fully controlled using short optical pulses, which are several orders of magnitude shorter than the life and coherence times of the qubit. Our demonstrations do not require an externally applied magnetic field and they establish that the quantum dot-confined dark exciton forms an excellent solid state matter qubit with some advantages over the half-integer spin qubits such as the confined electron and hole, separately. Since quantum dots are semiconductor nanostructures that allow integration of electronic and photonic components, the dark exciton may have important implications on implementations of quantum technologies consisting of semiconductor qubits.

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arXiv2012

Complete control of a matter qubit using a single picosecond laser pulse

We demonstrate for the first time that a matter physical two level system, a qubit, can be fully controlled using one ultrafast step. We show that the spin state of an optically excited electron, an exciton, confined in a quantum dot, can be rotated by any desired angle, about any desired axis, during such a step. For this we use a single, resonantly tuned, picosecond long, polarized optical pulse. The polarization of the pulse defines the rotation axis, while the pulse detuning from a non-degenerate absorption resonance, defines the magnitude of the rotation angle. We thereby achieve a high fidelity, universal gate operation, applicable to other spin systems, using only one short optical pulse. The operation duration equals the pulse temporal width, orders of magnitude shorter than the qubit evolution life and coherence times.

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