Ultrafast high-fidelity initialization of a quantum-dot spin qubit without magnetic fields

Abstract

Ultrafast initialization enables fault-tolerant processing of quantum information while QND read-out enables scalable quantum computation. By spatially assembling photon resonators and wave-guides around an n-doped nanodot and by temporally designing optical pump pulses, an efficient quantum pathway can be established from an electron spin to a charged exciton to a cavity photon and finally to a flying photon in the waveguide. Such control of vacuum-nanodot coupling can be exploited for ultrafast initialization and QND readout of the spin, which are particularly compatible with the optically driven spin quantum computers. Vacuum electromagnetic (EM) fluctuation plays an important role in quantum dynamics of nuclei [1], atoms [2], electrons [3], and solid-state quantum structures [4]. Recent advances in optical micro-structures, such as micro-spheres [5, 6], micro-rings [7, 8], micro-pillars [9], and engineered defects in photonic crystals [10], offer an opportunity of modifying the vacuum EM environment of various systems with a great extent of controllability. Novel ideas have been demonstrated such as engineering the Casimir force in microelectromechanical systems [11] and assembling semiconductor quantum dots inside photon resonators for efficient single photon sources [9]. At the same time, modern technology in ultrafast optics allows almost arbitrary design of laser pulses for coherent control [12]. Optical control of excitons in quantum dots has become one of the methods for building solid-state processors of quantum information [13, 14, 15]. Possessing ultra-long coherence time, electron spins in quantum dots are among the top candidates for quantum computing [16, 17]. Recent advances in quantum optics experiments have encouraged the proposals of optical manipulation of spins on the picosecond scale. In particular, Raman process [15] and optical RKKY interaction [14] mediated by charged excitons have been proposed for single-spin and two-spin operations, respectively , which constitute a set of gates for universal quantum computing. To make quantum computing complete , reading (measurement) and writing (initialization) of the qubits are two basic steps. Because the direct coupling between spins and their EM environment is extremely weak (which favors the long coherence time), spin readout has been a formidable task under investigation [18, 19, 20, 21, 22, 23, 24]. In most existing schemes [18, 19, 20, 21, 22], the spin state is first mapped into an orbit state which is then detected by electric sensors. These readout schemes, limited by the clock speed of the electric measurement and/or requiring local magnetic control, is not ideally suited for optically operated spin quantum computers. The ultrafast initialization of an individual qubit is essential in quantum error correction and fault-tolerant quantum computing [25], for the error (entropy) has to be erased as it is generated during processing [26]. Much less attention has been paid to the speed of initialization [22] than its importance would suggest...

Ultrafast Spin Initialization in a Gated InSb Nanowire Quantum Dot

“…The most difficult operation to accomplish turns out to be the spin initialization, that is, orienting spin in a chosen direction before any computations are executed. In self-assembled QDs spin of a single electron [13][14][15] or hole [16][17][18] can be set by using optical transitions to excitonic or trionic (charged excitons) states. We can proceed in a similar way in nanowire QDs based on InAsP/InP heterojunctions 19 .…”

Controlled tunneling-induced dephasing of Rabi rotations for high-fidelity hole spin initialization

“…For all associated quantum protocols the high fidelity initialization of single spin states that can be generated on-demand on ultra short timescales [13][14][15] and the reliable storage of the spin state is crucial 16,17 . The initialization of single spins can either be performed by spin-pumping of a charged QD 14 or by tunneling ionization of photo-generated excitons 13,[18][19][20][21] . While spin pumping is very convenient since it requires only a single continuous wave laser, it is rather slow with reported initialization fidelities of > 90% after ∼ 1ns.…”

“…In both cases, the high fidelity is only achieved for specific experimental conditions that limit the hole lifetime. Moreover, the experiments presented in reference 21 were performed with continuous wave excitation that is unsuitable for highly precise on-demand generation of hole spin qubits and allows the dynamics of the system only to be inferred with indirect techniques such as linewidth broadening 22 due to the incoherent nature of the continuous wave -quantum dot interaction.…”

All-electric single electron spin initialization

“…In QDs, in which confinement is created by heterojunctions of semiconductors, spin can also be controlled with photons. In self-assembled QDs, it is possible to initialize spin of an electron [15][16][17] or a hole [18][19][20][21][22] along the direction of magnetic field using optical transitions through trionic states. Similarly spin can be initialized in QDs formed by heterojunctions in catalytically grown nanowires [23].…”