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Physical review. B./Physical review. B

High-fidelity optical quantum gates based on type-II double quantum dots in a nanowire

Masoomeh Taherkhani, Morten Willatzen, Emil V. Denning, Игорь Евгеньевич Проценко, Niels Gregersen

Published Apr 10, 2019• 11 citations
Publisher

Abstract

We propose an optical gating scheme for quantum computing based on crystal-phase type II double quantum dots in an InP nanowire. The qubit is encoded on the electron spin and the gate operations are performed using stimulated Raman adiabatic passage (STIRAP), using the orbital degree of freedom in double quantum dots to form an auxiliary ground state. Successful STIRAP gating processes require an efficient coupling of both qubit ground states of the double quantum dot to the gating auxiliary ground state, and we demonstrate that this can be achieved using a charged exciton state. Crucially, by using type II dots, the hole is localized between the two spatially separated electrons in the charged exciton complex, thereby efficiently coupling the electron ground-state orbitals. By taking advantage of the high-fidelity state transfer by means of STIRAP in type II double quantum dots, we propose a protocol for coherently manipulating the spin-orbital quantum state of confined electrons in a quantum dot chain of an InP nanowire. We subsequently exploit the protocol to realize single- and two-qubit gates with fidelity above 0.99.

DOI
10.1103/physrevb.99.165305

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A numerical tool is developed to calculate the exciton energy and oscillator strength in newly emerged type-II nanowire quantum-dots. For a singlequantum-dot, the poor overlap of the electron part and the weakly confined hole part of the exciton wavefunction leads to a small oscillator strength compared to type-I systems. To increase the oscillator strength, we propose a double-quantum-dot structure featuring a strongly localized exciton wavefunction and a corresponding four-fold relative enhancement of the oscillator strength, paving the way towards efficient optically controlled quantum gate applications in the type-II nanowire system. Next, an optical gating scheme for quantum computing based on type-II double-quantum-dots is proposed. The qubit is encoded on the electron spin and the gate operations are performed by stimulated Raman adiabatic passage (STIRAP) using the position degree of freedom in double-quantum-dots to form an auxiliary ground-state. Successful STIRAP gating processes require an efficient coupling of both qubit ground-states of the double-quantum-dot to the gating auxiliary state and we demonstrate that this can be achieved using a charged exciton state. Crucially, by using type-II quantum-dots, the hole is localized between the two spatially separated electrons in the chargedexciton complex, thereby efficiently coupling the electron states orbitals. We subsequently exploit the scheme to realize single- and two-qubit gates for quantum computation. The conditional operation is performed by using Coulomb coupling to induce a shift of the STIRAP transition frequencies leading to a conditional violation of the STIRAP two-photon resonance. We calculate the fidelity of gates and show their performance is robust against the spin and charge noises.
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The spin states of electrons and holes confined in InAs quantum dot molecules have recently come to fore as a promising system for the storage or manipulation of quantum information. We describe here a feasible scheme for complete quantum optical control of two electron spin qubits in two vertically-stacked singly-charged InAs quantum dots coupled by coherent electron tunneling. With an applied magnetic field transverse to the growth direction, we construct a universal set of gates that corresponds to the possible Raman transitions between the spin states. We detail the procedure to decompose a given two-qubit unitary operation, so as to realize it with a successive application of up to 8 of these gates. We give the pulse shapes for the laser pulses used to implement this universal set of gates and demonstrate the realization of the two-qubit quantum Fourier transform with fidelity of 0.881 and duration of 414 ps. Our proposal therefore offers an accessible path to universal computation in quantum dot molecules and points to the advantages of using pulse shaping incoherent manipulation of optically active quantum dots to mitigate the negative effects of unintended dynamics and spontaneous emission.