P. Zawadzki

Bipolar spin blockade and coherent state superpositions in a triple quantum dot

Spin qubits based on interacting spins in double quantum dots have been demonstrated successfully. 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 Paulis exclusion principle. The archetypal spin blockade transport signature in double quantum dots takes the form of a rectified current. At present, more complex spin qubit circuits including triple quantum dots are being developed. Here we show, both experimentally and theoretically, that in a linear triple quantum dot circuit the spin blockade becomes bipolar 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.

Long-Range Spin Transfer in Triple Quantum Dots

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.

Classical and quantum transmission effects in submicron-size dots

We investigate both low and high field magnetotransport through submicron quantum dots using a six-gate coupled dot structure. At low fields we have studied classical magnetic focusing and coherent effects in this confined geometry. At higher fields we have observed a new phenomenon, the resonant backscattering of an edge state modulated by zero-dimensional states. At the highest fields we do not observe oscillations associated with the zero-dimensional states.

Enhanced charge detection of spin qubit readout via an intermediate state

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

3He Film flow: 4He substrate coating effect

Abstract Superfluid 3He film flow out of a copper beaker was measured without and then with a 4He coating on the copper surface. The effect of the 4 He was to convert the substrate from a purely diffuse to a purely specular surface for the 3He quasiparticles.

3He Film Flow

Following our earlier observation of 3He film flow, a new experiment has been designed to avoid possible temperature gradients along the 3He film. The film substrate, a 4 mm stainless steel tube, is physically separate from the level detectors, and the 3He both inside and outside the beaker is cooled with sinter. The onset of film flow is now below 1 mK, as distinct from ~3 mK in our previous work. At our lowest temperature, 0.55 mK, the flow rate decreases almost exponentially with the height of the film, from ~5 mm3/hour at the rim to ~0.1 mm3/hour with the 3He 1.5 mm below the rim. As T approaches Tc the flow rate decreases. The present results suggest flow of the bulk or 3-D superfluid 3He, with the decrease in flow with height resulting from the suppression of the order parameter as the film thickness approaches the coherence length.

Consequences of spin-orbit coupling at the single hole level: Spin-flip tunneling and the anisotropic g factor

Hole transport experiments were performed on a gated double quantum dot device defined in a p-GaAs/AlGaAs heterostructure with a single hole occupancy in each dot. The charging diagram of the device was mapped out using charge detection confirming that the single hole limit is reached. In that limit, a detailed study of the two-hole spin system was performed using high bias magnetotransport spectroscopy. In contrast to electron systems, the hole spin was found not to be conserved during interdot resonant tunneling. This allows one to fully map out the two-hole energy spectrum as a function of the magnitude and the direction of the external magnetic field. The heavy-hole g factor was extracted and shown to be strongly anisotropic, with a value of 1.45 for a perpendicular field and close to zero for an in-plane field as required for hybridizing schemes between spin and photonic quantum platforms.

Coulomb and spin blockade of two few-electron quantum dots in series in the cotunneling regime

We present Coulomb Blockade measurements of two few-electron quantum dots in series which are configured such that the electrochemical potential of one of the two dots is aligned with spin-selective leads. The charge transfer through the system requires co-tunneling through the second dot which is

Fabrication of tunable antidot structures with submicron airbridges

not

Quantum Hall effect and inter-edge-state tunneling within a barrier.

in resonance with the leads. The observed amplitude modulation of the resulting current is found to reflect spin blockade events occurring through either of the two dots. We also confirm that charge redistribution events occurring in the off-resonance dot are detected indirectly via changes in the electrochemical potential of the aligned dot.