Y. Don

Deterministic generation of a cluster state of entangled photons

Weaving an entangled cluster Entanglement is a powerful resource for quantum computation and information processing. One requirement is the ability to entangle multiple particles reliably. Schwartz et al. created an on-demand entangled cluster state of several photons by addressing a quantum dot with a sequence of laser pulses (see the Perspective by Briegel). They used an internal state of the quantum dot, a dark exciton, and its association with another internal state, a biexciton, to weave successive photons into an entangled cluster, generating entanglement between up to five photons. Science, this issue p. 434; see also p. 416 A quantum dot is used to realize entangled cluster states of up to five photons. Photonic cluster states are a resource for quantum computation based solely on single-photon measurements. We use semiconductor quantum dots to deterministically generate long strings of polarization-entangled photons in a cluster state by periodic timed excitation of a precessing matter qubit. In each period, an entangled photon is added to the cluster state formed by the matter qubit and the previously emitted photons. In our prototype device, the qubit is the confined dark exciton, and it produces strings of hundreds of photons in which the entanglement persists over five sequential photons. The measured process map characterizing the device has a fidelity of 0.81 with that of an ideal device. Further feasible improvements of this device may reduce the resources needed for optical quantum information processing.

Deterministic Writing and Control of the Dark Exciton Spin Using Single Short Optical Pulses

We experimentally demonstrate deterministic optical writing of a quantum dot-confined dark exciton, in a pure quantum state using one optical pulse. We then control the spin state of this long-lived exciton using picosecond optical pulses.

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

Controlling the dark exciton spin eigenstates by external magnetic field

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Coherent Control of Dark Excitons in Semiconductor Quantum Dots

-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.

Selection rules for nonradiative carrier relaxation processes in semiconductor quantum dots

We study the dark excitons 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.

Coherent writing of the dark exciton state using one picosecond long optical pulse

We review studies of the quantum dot confined dark exciton and demonstrate its use as a matter qubit. The dark exciton is an optically forbidden semiconductor electronic excitation, in which an electron-hole pair is generated with parallel spin projections. This optcally inactive excitation lives orders of magnitude longer than the corresponding optically active excitation, the bright exciton, in which the pair has anti-parallel spins. We show that despite its optical inactivity, the dark exciton can be deterministically generated in any desired coherent superposition of its two eigenstates using a single picosecond optical pulse. We provide lower bounds for the dark exciton life and coherence times and show that its coherent state can be fully controlled using short optical pulses. We also study its behavior in an externally applied magnetic field and present a method for its optical depletion from the quantum dot. Our results demonstrate that the dark exciton is an excellent matter spin qubit.

Atomistic theory of dark excitons in self-assembled quantum dots of reduced symmetry

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.

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

We demonstrate a one to one correspondence between the polarization of a picosecond optical pulse and the coherent spin state of the long lived dark exciton that it deterministically photogenerates in a single quantum dot.