Deterministic coherent writing of a long-lived semiconductor spin qubit using one ultrafast optical pulse

Abstract

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.Future technologies based on quantum information processing (QIP) [1][2][3][4][5][6] require the ability to coherently control matter two-level systems, or qubits. Since they are ultrafast and require no contacts, optical means of control are preferred. Spins of charge carriers in semiconductors are promising matter qubits since they complement contemporary leading technologies, specifically those of light sources and detectors. Semiconductor QDs isolate single carriers and can be easily incorporated into nanophotonic devices, thereby providing an excellent interface between single spins and single photons. For these reasons, semiconductor quantum dots (QDs) have been the subject of many recent works, which demonstrated significant progress in optical writing, readout, and control of confined spins [7][8][9][10][11][12][13][14][15][16].In semiconductors, the absorption of a photon results in the promotion of an electron from the full valence band, across the forbidden band-gap, to the empty conduction band, leaving its spin unaltered. The missing valence band electron (or "hole") and the conduction band electron form an electron-hole-pair with opposite spin directions, or a bright exciton (BE). The BE forms an integer spin (total spin 1) qubit in the matter.In a previous work, we demonstrated that, in straininduced self-assembled quantum dots, the polarization of a resonantly tuned, single picosecond optical pulse can be used to deterministically write the bright exciton spin qubit in any desired coherent state [14]. Such a process is not possible for single spins, where a few pulses are required to prepare the spin in an eigenstate and then a three step (Ramsey) rotation is need ed to write the spin state at will [7,8,10], a process that can take a few nanoseconds. Moreover, while full coherent control ("rotation") of the bright exciton can be achieved by one single optical pulse [15,17], single spins require two pulses and free precession in between [7][8][9]12]. However, these advantages of the bright exciton are not very useful, since its lifetime is rather short (sub ns), limited by radiative recombination of the electron-hole pair.Since light barely interacts with the electronic spin, an electron-hole pair with parallel spin directions is almost optically inactive. Such a pair is called a dark exciton (DE). The DE is also a spin integer (t...

Vanishing fine structure splitting in highly asymmetric InAs/InP quantum dots without wetting layer

“…Dark exciton. Dark excitons are nowadays considered as important platform for various quantum-dot applications [70][71][72][83][84][85][86][87][88] . In Fig.…”

“…Finally, we study dark-exciton spectra 15 , 70 – 72 , properties of which in quantum dashes, such as the non-zero optical activity 63 , 73 and splitting, appear to be dominated by shape deformation and rather immune to alloying effects.…”

Complete state tomography of a quantum dot spin qubit

“…1c. The conduction-band electron [1,2,27], the valenceband hole [28,29], and the dark exciton (DE-optically inactive electron-hole pair) [30][31][32] are long-lived ground electronic spin qubits in a QD system. The first two are Kramers' degenerate, but the third one is not.…”

Controlling the dark exciton spin eigenstates by external magnetic field

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