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RF-QPC Charge Detector and S-T₊ Qubit in a Lateral Double Quantum Dot Device

J.D. Mason

Published Jan 18, 2017• 1 citation
OpenAlex

Abstract

A measurement system is developed for studying a lateral double quantum dot (DQD) device formed in the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. Copper powder and RC filters are constructed to isolate the device from sources of instrumentation and thermal noise, allowing the temperature of the 2DEG to reach 70 mK. Coaxial cables and bias tees allow Gaussian shaped pulses to be delivered to several depletion gates of the device for the purpose of quantum dot qubit manipulation. A conventional charge detector based on a quantum point contact (QPC) and a room temperature current preamplifier is implemented. This readout has a bandwidth of 25 kHz. To improve readout bandwidth, a radio-frequency QPC (RF-QPC) circuit is also developed. A superconducting niobium inductor, manufactured using photolithography techniques, is the basis of a 520 MHz matching network. The RF-QPC system noise temperature is 5.2 K, limited by the cryogenic semiconductor amplifier. The RF-QPC has a bandwidth of 15 MHz. A summary of the first quantum dot physics measurements is provided. Data related to fundamental phenomena such as conductance quantization of a QPC, Coulomb blockade for a single dot, and spin blockade for a DQD are presented. A qubit based on two-electron spin states in a DQD (S-T₊ qubit) is created and coherent oscillations of the qubit known as Landau-Zener-Stückelberg oscillations are observed. Analysis of the spin-to-charge conversion mechanism associated with the qubit readout reveals the role played by metastable charge states, a result published in Physical Review B 92, 125434 (2015).

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 18, 2005•0 citations
A measurement system is developed for studying a lateral double quantum dot (DQD) device formed in the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. Copper powder and RC filters are constructed to isolate the device from sources of instrumentation and thermal noise, allowing the temperature of the 2DEG to reach 70 mK. Coaxial cables and bias tees allow Gaussian shaped pulses to be delivered to several depletion gates of the device for the purpose of quantum dot qubit manipulation. A conventional charge detector based on a quantum point contact (QPC) and a room temperature current preamplifier is implemented. This readout has a bandwidth of 25 kHz. To improve readout bandwidth, a radio-frequency QPC (RF-QPC) circuit is also developed. A superconducting niobium inductor, manufactured using photolithography techniques, is the basis of a 520 MHz matching network. The RF-QPC system noise temperature is 5.2 K, limited by the cryogenic semiconductor amplifier. The RF-QPC has a bandwidth of 15 MHz. A summary of the first quantum dot physics measurements is provided. Data related to fundamental phenomena such as conductance quantization of a QPC, Coulomb blockade for a single dot, and spin blockade for a DQD are presented. A qubit based on two-electron spin states in a DQD (S-T₊ qubit) is created and coherent oscillations of the qubit known as Landau-Zener-Stückelberg oscillations are observed. Analysis of the spin-to-charge conversion mechanism associated with the qubit readout reveals the role played by metastable charge states, a result published in Physical Review B 92, 125434 (2015).
Charge detection at microsecond time-scales has far reaching consequences in both technology and in our understanding of electron dynamics in nanoscale devices such as quantum dots. Radio-frequency superconducting single electron transistors (RF-SET) and quantum point contacts (QPC) are ultra sensitive charge sensors operating near the quantum limit. The operation of RF-SETs outside the superconducting gap has been a topic of study; the sub-gap operation, especially in the presence of large quantum fluctuations of quasiparticles remains largely unexplored, both theoretically and experimentally. We have investigated the effects of quantum fluctuations of quasiparticles on the operation of 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, sub-gap RF-SET operation is still feasible despite quantum fluctuations that wash out quasiparticle tunneling thresholds. 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.\nWe have operated a QPC charge detector in a radio frequency mode that allows fast charge detection in a bandwidth of several megahertz. The noise limiting the sensitivity of the charge detector is not the noise of a secondary amplifier, but the non-equilibrium device noise of the QPC itself. The noise power averaged over a measurement bandwidth of about 10MHz around the carrier frequency is in agreement with the theory of photon-assisted shot noise. Frequency-resolved measurements, however show several significant discrepancies with the theoretical predictions. The measurement techniques developed can also be used to investigate the noise of other semiconductor nanostructures such as quantum dots in the Kondo regime.\nA study of the noise characteristics alone can not determine whether the device is operating at the quantum limit; a characterization of back action is also necessary. The inelastic current through a double quantum dot system (DQD) is sensitive to the spectral density of voltage fluctuations in its electromagnetic environment. Electrical transport studies on a DQD system electrostatically coupled to an SET shows qualitative evidence of back-action of SET. The design and fabrication of a few electron DQD device with integrated RF-SET and QPC charge sensors for the study of back action of the sensors and real-time electron dynamics in the DQD are also discussed.
We study the dynamics of two electron spins in coupled quantum dots (CQDs) monitored by a quantum point contact (QPC) detector. Their quantum state can be measured by embedding the QPC in an LC circuit. We derive the Bloch-type rate equations of the reduced density matrix for CQDs. Special attention is paid to the numerical results for the weak measurement condintion under a strong Coulomb interaction. It is shown that the evolution of QPC current always follows that of electron occupation in the right dot. In addition, we find that the output voltage of the circuit can reflect the evolution of QPC current when the circuit and QPC are approximately equal in frequency. In particular, the wave shape of the output voltage can be improved by adjusting the circuit resonance frequency and bandwidth.
We report on charge sensing measurements of a GaAs semiconductor quantum dot device using a radio frequency quantum point contact (rf-QPC). The rf-QPC is fully characterized at 4K and millikelvin temperatures and found to have a bandwidth exceeding 20MHz. For single shot charge sensing, we achieve a charge sensitivity of ∼2×10−4e∕Hz referred to the neighboring dot’s charge. The rf-QPC compares favorably with rf single electron transistor electrometers and promises to be an extremely useful tool for characterizing and measuring semiconductor quantum systems on fast time scales.
There is currently a worldwide effort towards the realisation of large-scale quantum computers that exploit quantum phenomena for information processing. While these computing systems could potentially redefine the technological landscape, harnessing quantum effects is challenging due to their inherently fragile nature and the experimentally demanding environments in which they arise. In order for quantum computation to be viable it is first necessary to demonstrate the operation of two-level quantum systems (qubits) which have long coherence times, can be quickly read out, and can be controlled with high fidelity. Focusing on these key requirements, this thesis presents four experiments towards scalable solid state quantum computing using gate-defined quantum dot devices based on gallium arsenide (GaAs) heterostructures. The first experiment investigates a phonon emission process that limits the charge coherence in GaAs and potentially complicates the microwave control of multi-qubit devices. We show that this microwave analogy to Raman spectroscopy can provide a means of detecting the unique phonon spectral density created by a nanoscale device. Experimental results are compared to a theoretical model based on a non-Markovian master equation and approaches to suppressing electron-phonon coupling are discussed. The second experiment demonstrates a technique involving in-situ gate electrodes coupled to lumped-element resonators to provide high-bandwidth dispersive read-out of the state of a double quantum dot. We characterise the charge sensitivity of this method in the few-electron regime and benchmark its performance against quantum point contact charge sensors. The third experiment implements a low-loss, chip-level frequency multiplexing scheme for the readout of scaled-up spin qubit arrays. Dispersive gate-sensing is realised in combination with charge detection based on two radio frequency quantum point contacts to perform multiplexed readout of a double quantum dot in the few-electron regime. Demonstration of a 10-channel multiplexing device is achieved and limitations in scaling spin qubit readout to large numbers using multiplexed channels discussed. The final experiment ties previously presented results together by realising a micro-architecture for controlling and reading out qubits during the execution of a quantum algorithm. The basic principles of this architecture are demonstrated via the manipulation of a semiconductor qubit using control pulses that are cryogenically routed using a high-electron mobility transistor switching matrix controlled by a field programmable gate array. Finally, several technical results are also presented including the development of printed circuit board solutions to allow the high-frequency measurement of nanoscale devices at cryogenic temperatures and the design of on-chip interconnects used to suppress electromagnetic crosstalk in high-density spin qubit device architectures.
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.