Manipulating qubits via electrical pulses in a piezoelectric material such as GaAs can be expected to generate incidental acoustic phonons. In this Letter we determine theoretically and experimentally the consequences of these phonons for semiconductor spin qubits using Landau-Zener-Stückelberg interferometry. Theoretical calculations predict that phonons in the presence of the spin-orbit interaction produce both phonon-Rabi fringes and accelerated evolution at the singlet-triplet anticrossing. Observed features confirm the influence of these mechanisms. Additionally, evidence is found that the pulsed gates themselves act as phonon cavities increasing the influence of phonons under specific resonant conditions.

The authors present an experimental mechanism for converting the state of a spin qubit in a quantum dot device into different charge states. This mechanism depends on metastable charge states present when the device has very asymmetric couplings to the external leads. This approach is used to observe Landau-Zener-Stuckelberg oscillations of the (S-${T}_{+}$) qubit.

Probabilities deduced from quantum information studies are usually based on averaging many identical experiments separated by an initialization step. Such initialization steps become experimentally more challenging to implement as the complexity of quantum circuits increases. To better understand the consequences of imperfect initialization on the deduced probabilities, we study the effect of not initializing the system between measurements. For this we utilize Landau-Zener-St\"uckelberg oscillations in a double quantum dot circuit. Experimental results are successfully compared to theoretical simulations.

We utilize magnetic field dependencies to identify two hitherto unobserved quantum interference processes in a triplequantumdotcircuit.ThefirstobservationinvolvestheinterplayofLandau-Zener-St¨uckelbergbehaviorfrom two separate anticrossings between two energy levels that anticross twice as a function of a detuning parameter. The second process involves quantum interference between all-exchange and hyperfine qubits activated in a three-spin system.

Tunneling in a quantum coherent structure is not restricted to only nearest neighbors. Hopping between distant sites is possible via the virtual occupation of otherwise avoided intermediate states. Here we report the observation of long-range transitions in the transport through three quantum dots coupled in series. A single electron is delocalized between the left and right quantum dots, while the center one remains always empty. Superpositions are formed, and both charge and spin are exchanged between the outermost dots. The delocalized electron acts as a quantum bus transferring the spin state from one end to the other. Spin selection is enabled by spin correlations. The process is detected via the observation of narrow resonances which are insensitive to Pauli spin blockade.

Spin qubits based on interacting spins in double quantum dots have been demonstrated successfully 1 , 2 . Readout of the qubit state involves a conversion of spin to charge information, which is universally achieved by taking advantage of a spin blockade phenomenon resulting from Pauli's exclusion principle. The archetypal spin blockade transport signature in double quantum dots takes the form of a rectified current 3 . At present, more complex spin qubit circuits including triple quantum dots are being developed 4 . Here we show, both experimentally and theoretically, that in a linear triple quantum dot circuit the spin blockade becomes bipolar 5 with current strongly suppressed in both bias directions and also that a new quantum coherent mechanism becomes relevant. In this mechanism, charge is transferred non-intuitively via coherent states from one end of the linear triple dot circuit to the other, without involving the centre site. Our results have implications for future complex nanospintronic circuits.

In this paper we describe our recent experiments on coherent manipulation of electron spin states formed in a highly tunable GaAs/AlGaAs triple quantum dot device. The coherent evolution of spin states is achieved by using fast pulses from an initialization point in the (201) charge configuration region of the stability diagram. We demonstrate the versatility of the triple dot system capable of tuning to different regimes controlled by the width of the (111) region and pulse parameters. In particular we observe Delta(1/2)'-Q(3/2) (analogue of S-T+ in a double dot) and Delta(1/2)'-Delta(1/2) exchange driven oscillations from both sides of the stability diagram involving all three spins. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Studenikin et al.[1] recently demonstrated a significant enhancement of the fringe contrast of coherent Landau-Zener-Stuckelberg (LZS) oscillations between singlet S and triplet T+ two-spin states using a modified charge detection technique called enhanced charge detection (ECD). In this paper we explain the amplitude phase reversal and confirm the magnitude of the effect is consistent with our calibrations. We also show that the enhancement cannot be explained by a T-1 effect.

To quantify the effects of local magnetic fields on triple quantum dots, the Heisenberg Hamiltonian has been diagonalized for three electrons coupled via the exchange interaction. In particular, we have investigated different geometries of micro-magnets located on top of the triple dot in order to optimize the field gradient characteristics. In this paper, we focus on two geometries which are candidates for an addressable EDSR triple quantum dot device.

Qubits based on the singlet (S) and the triplet (T(0), T(+)) states in double quantum dots have been demonstrated in separate experiments. It has been recently proposed theoretically that under certain conditions a quantum interference could occur from the interplay between these two qubit species. Here we report experiments and modeling that confirm these theoretical predictions and identify the conditions under which this interference occurs. Density matrix calculations show that the interference pattern manifests primarily via the occupation of the common singlet state. The S/T(0) qubit is found to have a much longer coherence time as compared to the S/T(+) qubit.

Induced currents associated with the quantum Hall effect are studied in the temperature range 39 mK to 1.6 K, and at Landau-level filling factors nu = 1,2,3,4, and 6, using torsion-balance magnetometry. A quantitative link is demonstrated between (nonlinear induced current) vs (inducing electromotive force) curves, and the subexponential decay of the induced current in a static magnetic field. The energy storage in the induced currents is reexamined with the conclusion that the predominant mechanism for storage is inductive, through the mutual inductance between the sample and the magnet, not capacitive as previous reports have assumed. The temperature dependencies of the currents are consistent with previous models, except for a low-temperature saturation at filling factors nu = 1 and nu = 2, which we attribute to electron heating.

Recent experiments with an electrostatically gated structure demonstrated coherent oscillations, involving all three spins of a linear triple dot system. In a combined experimental and theoretical study, we demonstrate coherent doublet/doublet exchange oscillations, leading to arbitrary rotation on the relevant Bloch sphere, and we reveal an interplay between exchange and hyperfine-driven "double coherent beam splitter" oscillations.

This paper reports on the observation and analysis of magnetotransport phenomena in the nonlinear differential resistance $r_{xx}=dV_{xx}/dI$ of high-mobility InGaAs/InP and GaAs/AlGaAs Hall bar samples driven by direct current, $\Idc$. Specifically, it is observed that Shubnikov -de Haas (SdH) oscillations at large filling factors invert their phase at sufficiently large values of $\Idc$. This phase inversion is explained as being due to an electron heating effect. In the quantum Hall effect regime the $r_{xx}$ oscillations transform into diamond-shaped patterns with different slopes corresponding to odd and even filling factors. The diamond-shaped features at odd filling factors can be used as a probe to determine spin energy gaps. A Zero Current Anomaly (ZCA) which manifests itself as a narrow dip in the $r_{xx}(\Idc)$ characteristics at zero current, is also observed. The ZCA effect strongly depends upon temperature, vanishing above 1 K while the transport diamonds persist to higher temperatures. The transport diamonds and ZCA are fully reproduced in a higher mobility GaAs/AlGaAs Hall bar structure confirming that these phenomena reflect intrinsic properties of two-dimensional systems.

Spin qubits have been successfully realized in electrostatically defined, lateral few-electron quantum dot circuits. Qubit readout typically involves spin to charge information conversion, followed by a charge measurement made using a nearby biased quantum point contact. It is critical to understand the back-action disturbances resulting from such a measurement approach. Previous studies have indicated that quantum point contact detectors emit phonons which are then absorbed by nearby qubits. We report here the observation of a pronounced back-action effect in multiple dot circuits where the absorption of detector-generated phonons is strongly modified by a quantum interference effect, and show that the phenomenon is well described by a theory incorporating both the quantum point contact and coherent phonon absorption. Our combined experimental and theoretical results suggest strategies to suppress back-action during the qubit readout procedure.

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-Stückelberg oscillations between singlet S and triplet T+ two-spin states.

We study the electron transport properties of gated quantum dots formed in InGaAs/InP quantum well structures grown by chemical-beam epitaxy on prepatterned substrates. Quantum dots form directly underneath narrow gates due to potential fluctuations. We measure the Coulomb-blockade diamonds in the few-electron regime of a single dot and observe photon-assisted tunneling peaks under microwave irradiation. A singlet-triplet transition at high magnetic field and Coulomb-blockade in the quantum Hall regime are also observed.

We report on the observation and study of quantum transport diamonds and Zero Current Anomaly (ZCA) in the non-linear differential resistance r(xx)=dV(xx)/dI of high-mobility InxGa1-xAs/InP structures in quantizing magnetic fields. The diamond-shaped features are observed in the grey-scale plots of r(xx) as a function of magnetic field and dc current. Spin diamonds are revealed at higher magnetic fields when spin levels at odd filling factors are well resolved. Unexpectedly, a narrow dip is observed in differential resistance vs. current at I-dc=0 in quantizing magnetic fields, which we refer to as the ZCA effect.