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 q
uantum 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-Stuckelberg oscillations in a double quantum dot circuit. Experimental results are successfully compared to theoretical simulations.
Spin qubits based on interacting spins in double quantum dots have been successfully demonstrated. Readout of the qubit state involves a conversion of spin to charge information, universally achieved by taking advantage of a spin blockade phenomenon
resulting from Paulis exclusion principle. The archetypal spin blockade transport signature in double quantum dots takes the form of a rectified current. Currently more complex spin qubit circuits including triple quantum dots are being developed. Here we show both experimentally and theoretically (a) that in a linear triple quantum dot circuit, the spin blockade becomes bipolar with current strongly suppressed in both bias directions and (b) that a new quantum coherent mechanism becomes relevant. Within 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 in future complex nano-spintronic circuits.
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 q
uantum 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.
Spin qubits involving individual spins in single quantum dots or coupled spins in double quantum dots have emerged as potential building blocks for quantum information processing applications. It has been suggested that triple quantum dots may provid
e additional tools and functionalities. These include the encoding of information to either obtain protection from decoherence or to permit all-electrical operation, efficient spin busing across a quantum circuit, and to enable quantum error correction utilizing the three-spin Greenberger-Horn-Zeilinger quantum state. Towards these goals we demonstrate for the first time coherent manipulation between two interacting three-spin states. We employ the Landau-Zener-Stuckelberg approach for creating and manipulating coherent superpositions of quantum states. We confirm that we are able to maintain coherence when decreasing the exchange coupling of one spin with another while simultaneously increasing its coupling with the third. Such control of pairwise exchange is a requirement of most spin qubit architectures but has not been previously demonstrated.
We study experimentally the electron transport properties of gated quantum dots formed in InGaAs/InP and InAsP/InP quantum well structures grown by chemical-beam epitaxy. For the case of the InGaAs quantum well, quantum dots form directly underneath
narrow gate electrodes due to potential fluctuations. We measure the Coulomb-blockade diamonds in the few-electron regime of a single quantum dot and observe photon-assisted tunneling peaks under microwave irradiation. A singlet-triplet transition at high magnetic field and Coulomb-blockade effects in the quantum Hall regime are also observed. For the InAsP quantum well, an incidental triple quantum dot forms also due to potential fluctuations within a single dot layout. Tunable quadruple points are observed via transport measurements.
We measure a triple quantum dot in the regime where three addition lines, corresponding to the addition of an electron to each of three dots, pass through each other. In particular, we probe the interplay between transport and the tridimensional natu
re of the stability diagram. We choose the regime most pertinent for spin qubit applications. We find that at low bias transport through the triple quantum dot circuit is only possible at six quadruple point locations. The results are consistent with an equivalent circuit model.
