We perform Landau-Zener-Stückelberg-Majorana (LZSM) spectroscopy on a system with strong spin-orbit interaction (SOI), realized as a single hole confined in a gated double quantum dot. Analogous to electron systems, at a magnetic field B=0 and high modulation frequencies, we observe photon-assisted tunneling between dots, which smoothly evolves into the typical LZSM funnel-shaped interference pattern as the frequency is decreased. In contrast to electrons, the SOI enables an additional, efficient spin-flip interdot tunneling channel, introducing a distinct interference pattern at finite B. Magnetotransport spectra at low-frequency LZSM driving show the two channels to be equally coherent. High-frequency LZSM driving reveals complex photon-assisted tunneling pathways, both spin conserving and spin flip, which form closed loops at critical magnetic fields. In one such loop, an arbitrary hole spin state is inverted, opening the way toward its all-electrical manipulation.
We employ an intermediate excited charge state of a lateral quantum dot device to increase the charge detection contrast during the qubit state readout procedure, allowing us to increase the visibility of coherent qubit oscillations. This approach amplifies the coherent oscillation magnitude but has no effect on the detector noise resulting in an increase in the signal to noise ratio. In this letter we apply this scheme to demonstrate a significant enhancement of the fringe contrast of coherent Landau-Zener-Stückleberg oscillations between singlet S and triplet T+ two-spin states.PACS numbers: 73.63. Kv, 73.21.La, The crucial step of spin qubit readout via spin to charge conversion is usually achieved through charge detection technology.[1] The maximum sensitivity is related to the difference (contrast) in the charge detection signal of the two charge state configurations. In this letter we demonstrate how this contrast can be amplified by a significant factor if a "state discriminating" relaxation process is introduced during the readout procedure. Since this modification only changes the charge detector contrast between the relevant qubit states, the signal to noise ratio is likewise enhanced.Spin qubits based on single, double or triple quantum dot circuits have been successfully demonstrated.[2-4] Following a coherent manipulation experiment the measurement is usually completed by reading out the final quantum state using a charge detector which is able to distinguish between the quantum spin states.We will concentrate on the qubit based on the singlet (S) and triplet (T + ) two-spin states recently demonstrated by Petta et al. [3] The corresponding oscillations are called Landau-Zener-Stückleberg (LZS) oscillations and are observed as fringes in the stability diagram or when plotted as a function of pulse duration, magnetic field or initial gate detuning. We stress that a similar readout advantage can be implemented for other spin qubit species. The experiments were made using a triple quantum dot device (see Fig. 1(a)).[4-6] During our measurements one quantum dot (R) was not coupled via exchange or tunnelling to the other two quantum dots making this effectively a two quantum dot experiment. [7] As a result we will use double quantum dot notation throughout. A schematic energy level diagram vs detuning is shown for a double dot in a magnetic field in Fig. 1(c). The two qubit states S and T + which differ in spin are coupled via the hyperfine interaction which creates an anticrossing in the energy level diagram.A generic S/T + spin qubit operation for this system * Electronic address: Andrew.Sachrajda@nrc.ca would proceed as follows. The quantum state preparation is first achieved by applying a pulse (in reality a combined pulse on gates 1 and 2) from the S(2,0) regime through the anticrossing to the (1,1) charge regime (where (n L , n C ) indicate the number of electrons, n L (n C ) in quantum dots L (C)). A suitable pulse rise time permits a superposition of the S and T + states to be generated via Landa...
Hole spins have recently emerged as attractive candidates for solid-state qubits for quantum computing. Their state can be manipulated electrically by taking advantage of the strong spinorbit interaction (SOI). Crucially, these systems promise longer spin coherence lifetimes owing to their weak interactions with nuclear spins as compared to electron spin qubits. Here we measure the spin relaxation time T 1 of a single hole in a GaAs gated lateral double quantum dot device. We propose a protocol converting the spin state into long-lived charge configurations by the SOI-assisted spin-flip tunneling between dots. By interrogating the system with a charge detector we extract the magnetic-field dependence of T 1 ∝ B −5 for fields larger than B = 0.5 T, suggesting the phonon-assisted Dresselhaus SOI as the relaxation channel. This coupling limits the measured values of T 1 from~400 ns at B = 1.5 T up tõ 60 μs at B = 0.5 T.
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