Madhu Thalakulam

Charge Sensing and Controllable Tunnel Coupling in a Si/SiGe Double Quantum Dot

We report integrated charge sensing measurements on a Si/SiGe double quantum dot. The quantum dot is shown to be tunable from a single, large dot to a well-isolated double dot. Charge sensing measurements enable the extraction of the tunnel coupling t between the quantum dots as a function of the voltage on the top gates defining the device. Control of the voltage on a single such gate tunes the barrier separating the two dots. The measured tunnel coupling is an exponential function of the gate voltage. The ability to control t is an important step toward controlling spin qubits in silicon quantum dots.

Single-electron quantum dot in Si∕SiGe with integrated charge sensing

Single-electron occupation is an essential component to the measurement and manipulation of spin in quantum dots, capabilities that are important for quantum information processing. Si∕SiGe is of interest for semiconductor spin qubits, but single-electron quantum dots have not yet been achieved in this system. We report the fabrication and measurement of a top-gated quantum dot occupied by a single electron in a Si∕SiGe heterostructure. Transport through the quantum dot is directly correlated with charge sensing from an integrated quantum point contact, and this charge sensing is used to confirm single-electron occupancy in the quantum dot.

Fast tunnel rates in Si/SiGe one-electron single and double quantum dots

We report the fabrication and measurement of one-electron single and double quantum dots with fast tunnel rates in a Si/SiGe heterostructure. Achieving fast tunnel rates in few-electron dots can be challenging, in part due to the large electron effective mass in Si. Using charge sensing, we identify signatures of tunnel rates in and out of the dot that are fast or slow compared to the measurement rate. Such signatures provide a means to calibrate the absolute electron number and verify single electron occupation. Pulsed gate voltage measurements are used to validate the approach.

A macroscopic mechanical resonator driven by mesoscopic electrical back-action

Systems with coupled mechanical and optical or electrical degrees of freedom have fascinating dynamics that, through macroscopic manifestations of quantum behaviour, provide new insights into the transition between the classical and quantum worlds. Of particular interest is the back-action of electrons and photons on mechanical oscillators, which can lead to cooling and amplification of mechanical motion. Furthermore, feedback, which is naturally associated with back-action, has been predicted to have significant consequences for the noise of a detector coupled to a mechanical oscillator. Recently it has also been demonstrated that such feedback effects lead to strong coupling between single-electron transport and mechanical motion in carbon nanotube nanomechanical resonators. Here we present noise measurements which show that the mesoscopic back-action of electrons tunnelling through a radio-frequency quantum point contact causes driven vibrations of the host crystal. This effect is a remarkable macroscopic manifestation of microscopic quantum behaviour, where the motion of a mechanical oscillator—the host crystal, which consists of on the order of 1020 atoms—is determined by statistical fluctuations of tunnelling electrons.

Sensitivity and Linearity of Superconducting Radio-Frequency Single-Electron Transistors: Effects of Quantum Charge Fluctuations

We have investigated the effects of quantum fluctuations of quasiparticles on the operation of superconducting radio-frequency single-electron transistors (rf-SETs) for large values of the quasiparticle cotunneling parameter alpha = 8EJ/Ec, where EJ and Ec are the Josephson and charging energies. We find that, for alpha > 1, subgap rf-SET operation is still feasible despite quantum fluctuations that wash out quasiparticle tunneling thresholds. Surprisingly, such rf-SETs show linearity and signal-to-noise ratio superior to those obtained when quantum fluctuations are weak, while still demonstrating excellent charge sensitivity.

Pauli spin blockade and lifetime-enhanced transport in a Si/SiGe double quantum dot

We analyze electron-transport data through a Si/SiGe double quantum dot in terms of spin blockade and lifetime-enhanced transport LET, which is transport through excited states that is enabled by long spinrelaxation times. We present a series of low-bias voltage measurements showing the sudden appearance of a strong tail of current that we argue is an unambiguous signature of LET appearing when the bias voltage becomes greater than the singlet-triplet splitting for the 2,0 electron state. We present eight independent data sets, four in the forward-bias spin-blockade regime and four in the reverse-bias lifetime-enhanced transport regime and show that all eight data sets can be fit to one consistent set of parameters. We also perform a detailed analysis of the reverse-bias LET regime, using transport rate equations that include both singlet and triplet transport channels. The model also includes the energy-dependent tunneling of electrons across the quantum barriers and resonant and inelastic tunneling effects. In this way, we obtain excellent fits to the experimental data, and we obtain quantitative estimates for the tunneling rates and transport currents throughout the reverse-bias regime. We provide a physical understanding of the different blockade regimes and present detailed predictions for the conditions under which LET may be observed.

Real-time electron counting in semiconductor nanostructures

By coupling a radio-frequency single-electron transistor (RF-SET) to a quantum dot (QD) in a GaAs/AlGaAs heterostructure, we have succeeded in detecting the tunneling of individual electrons on and off the QD on time scales as short as one microsecond. Using charge detection to probe the state of the QD allows us to nearly isolate the dot from its leads, thereby minimizing decoherence-inducing effects of the environment. We have extended these charge detection techniques to double quantum dots (DQDs) that can simultaneously be used to characterize the backaction of the RF-SET. The combined RF-SET/DQD system is well-suited to the development of charge- or spin-based quantum bits, and to investigation of the quantum measurement problem.