No abstract

Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit. Highly controlled tunnelling between neighbouring dots has been demonstrated, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times-a cumulative distance of 0.25 mm-without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices.

In a one-dimensional (1D) system of interacting electrons, excitations of spin and charge travel at different speeds, according to the theory of a Tomonaga-Luttinger Liquid (TLL) at low energies. However, the clear observation of this spin-charge separation is an ongoing challenge experimentally. We have fabricated an electrostatically-gated 1D system in which we observe spin-charge separation and also the predicted power-law suppression of tunnelling into the 1D system. The spin-charge separation persists even beyond the low-energy regime where the TLL approximation should hold. TLL effects should therefore also be important in similar, but shorter, electrostatically gated wires, where interaction effects are being studied extensively worldwide.The effects of interactions are almost impossible to calculate in a general many-particle system, though they cannot be ignored. However, for a one-dimensional (1D) system, Luttinger, building on an approximation scheme of Tomonaga, constructed a soluble 1D model with infinite linear dispersion and a restricted set of interactions. The solution has 1 arXiv:1002.2782v1 [cond-mat.str-el]

State readout is a key requirement for a quantum computer. For semiconductor-based qubit devices it is usually accomplished using a separate mesoscopic electrometer. Here we demonstrate a simple detection scheme in which a radio-frequency resonant circuit coupled to a semiconductor double quantum dot is used to probe its charge and spin states. These results demonstrate a new non-invasive technique for measuring charge and spin states in quantum dot systems without requiring a separate mesoscopic detector. PACS numbers:State readout is a key requirement for a quantum information processor. For semiconductor-based qubit devices this is usually accomplished using a separate mesoscopic electrometer such as a quantum point contact (QPC) [1]. Non-invasive detection using a QPC has been critical to recent advances in coherent control of few-electron double quantum dots [2], allowing their charge configurations to be mapped in regimes where transport through the double dot itself is not measurable [3]. Furthermore, through spin-to-charge conversion techniques, spin state measurements of single and double dots have been demonstrated using QPCs [4,5].Resonant microwave circuits have played an important role in performing sensitive measurements of mesoscopic devices. In the case of superconducting charge qubits, earlier schemes used a proximal single electron transistor (SET) charge detector to perform state readout [6,7]. This SET was coupled to an impedance matching rf resonant circuit to form an rf-SET which had dramatically improved sensitivity and bandwidth [8]. Similarly, an rf-QPC has also been realised for broadband charge detection of semiconductor quantum dot devices [9,10].More recently, state readout of a superconducting charge qubit has been accomplished by directly coupling it to a microwave resonant circuit [11][12][13]. Working in the dispersive regime where the resonator and qubit bandgap energies are detuned, the qubit has a state-dependent 'quantum capacitance' which causes a shift in the resonator frequency [14]. This frequency shift can then be detected using standard homodyne detection techniques.In this Letter, we demonstrate dispersive detection using a resonant circuit coupled directly to an AlGaAs/GaAs few-electron double quantum dot device. This allows us to probe the charge state of a single electron confined to a double quantum dot. We also perform a spin-state measurement for a pair of electrons using the resonant circuit. These results demonstrate a new and non-invasive technique for measuring charge and spin states in semiconductor quantum dot systems without the need for a separate mesoscopic detector.In using the resonator to probe the state of a double quantum dot we consider the double dot as a charge qubit where a single electron occupies either the ground state of one dot or the other [16]. The Hamiltonian for this two level system is given by H = 1 2 σ z + tσ x , where is the detuning between the two dot chemical potential energies and t is the interdot tunnel coupling energy which m...

Measurements are presented of a device designed to cool a 6 microm;{2} region of 2D electron gas using quantum dots. Electrostatic effects are found to be significant in the device, and a model that accounts for them is developed. At ambient electron temperatures above 120 mK the results are consistent with the model and the base temperature of the cooled region is estimated. At an ambient electron temperature of 280 mK, the 6 microm;{2} region is found to be cooled below 190 mK. Below 120 mK the results deviate from predictions, which is attributed to reduced electron-electron scattering rates.