Quantum point contacts (QPC) are the building blocks of quantum dot qubits and semiconducting quantum electrical metrology circuits. QPCs also make highly sensitive electrical amplifiers with the potential to operate in the quantum-limited regime. Though the inherent operational bandwidth of QPCs can eclipse the THz regime, the impedance mismatch with the external circuitry limits the operation to a few kHz regimes. Lumped-element impedance-matching circuits are successful only up to a few hundreds of MHz. QPCs are characterised by a complex impedance consisting of quantized resistance, capacitance, and inductance elements. Characterising the complex admittance at higher frequencies and understanding the coupling of QPC to other circuit elements and electromagnetic environments will provide valuable insight into its sensing and backaction properties. In this work, we couple a QPC galvanically to a superconducting stub tuner impedance matching circuit realised in a coplanar waveguide architecture to enhance the operation frequency into the GHz regime and investigate the electrical amplification and complex admittance characteristics. The device, operating at ∼ 1.96GHz exhibits a conductance sensitivity of 2.92×10−5(e2/h)/Hz and a bandwidth of 13MHz. Besides, the RF reflected power unambiguously reveals the complex admittance characteristics of the QPC, shining more light on the behaviour of quantum tunnel junctions at higher operational frequencies.
Quantum point contacts (QPC) are the building blocks of quantum dot qubits and semiconducting quantum electrical metrology circuits. QPCs also make highly sensitive electrical amplifiers with the potential to operate in the quantum-limited regime. Though the inherent operational bandwidth of QPCs can eclipse the THz regime, the impedance mismatch with the external circuitry limits the operation to a few kHz regimes. Lumped-element impedance-matching circuits are successful only up to a few hundreds of MHz. QPCs are characterised by a complex impedance consisting of quantized resistance, capacitance, and inductance elements. Characterising the complex admittance at higher frequencies and understanding the coupling of QPC to other circuit elements and electromagnetic environments will provide valuable insight into its sensing and backaction properties. In this work, we couple a QPC galvanically to a superconducting stub tuner impedance matching circuit realised in a coplanar waveguide architecture to enhance the operation frequency into the GHz regime and investigate the electrical amplification and complex admittance characteristics. The device, operating at ∼ 1.96GHz exhibits a conductance sensitivity of 2.92×10−5(e2/h)/Hz and a bandwidth of 13MHz. Besides, the RF reflected power unambiguously reveals the complex admittance characteristics of the QPC, shining more light on the behaviour of quantum tunnel junctions at higher operational frequencies.
Abstract Probing single charge dynamics in solids can give insights into various quantum transport phenomena, most of which are fragile and short-time-scaled. Detection of these events in real-time requires a mesoscopic electrical amplifier with unprecedented sensitivity and operational bandwidth. In this work, we explore a hybrid electrical amplifier consisting of a semiconducting quantum point contact galvanically coupled to a superconducting λ /2 transmission-line resonator for ultra-fast and ultra-sensitive charge amplification. The resonator, made of aluminium with a coplanar waveguide geometry, is designed to operate at its first harmonic resonant mode ∼3.4 GHz, where the reflected power from the resonator is amplitude-modulated by the conductance changes in the quantum point contact channel. From the sidebands of the amplitude modulated reflected signal we extract a conductance sensitivity of 2.85 × 1 0 − 7 e 2 / h / H z 11.05 p S H z − 1 / 2 ). This sensitivity translates to a unit signal-to-noise measurement time ∼1.62 ns for a variation of 0.01 ( e 2 / h in the conductance. From the analysis of the noise characteristics of the device, we find that up to a few MHz of signal frequency the sensitivity is limited by the photon assisted shot-noise associated with the electron tunnelling in the QPC channel. The optimization of various operational parameters of the device reveals a bandwidth of ∼155 MHz which corresponds to a rise-time ∼2.2 ns. Both the sensitivity and bandwidth that we obtain are greater by an order compared to the existing reports. In addition, the device also exhibits very good sensitivities ∼ 3.58 × 1 0 − 4 e 2 / h / H z up to a measurable frequency of 240 MHz. The extremely high sensitivity, shot-noise limited detection, ultra-fast operation reaching the nanosecond time-scales, and the circuit QED architecture makes this scheme an attractive choice for single charge detection and counting experiments for spin-qubit readout and quantum electrical metrology.
Abstract Probing single charge dynamics in solids can give insights into various quantum transport phenomena, most of which are fragile and short-time-scaled. Detection of these events in real-time requires a mesoscopic electrical amplifier with unprecedented sensitivity and operational bandwidth. In this work, we explore a hybrid electrical amplifier consisting of a semiconducting quantum point contact galvanically coupled to a superconducting λ/2 transmission-line resonator for ultra-fast and ultra-sensitive charge amplification. The resonator, made of Aluminium with a coplanar waveguide geometry, is designed to operate at its first harmonic resonant mode ~ 3.4 GHz, where the reflected power from the resonator is amplitude-modulated by the conductance changes in the quantum point contact channel. From the sidebands of the amplitude modulated reflected signal we extract a conductance sensitivity of 2.85×10-7 (e2/h)/√Hz (11.05 pS∕√Hz). This sensitivity translates to a unit signal-to-noise measurement time ~ 1.62 ns for a variation of 0.01 (e2/h) in the conductance. The optimization of various operational parameters of the device reveals a bandwidth of ~ 155 MHz which corresponds to a rise-time ~ 2.2 ns. Both the sensitivity and bandwidth that we obtain are greater by an order compared to the existing reports. In addition, the device also exhibits very good sensitivities ~ 3.58×10-4 (e2/h)/√Hz up to a measurable frequency of 240 MHz. The extremely high sensitivity, ultra-fast operation reaching the nanosecond time-scales, and the circuit QED architecture makes this scheme an attractive choice for single charge detection and counting experiments for spin-qubit readout and quantum electrical metrology.
Fast measurement of small currents and even smaller fluctuations in devices with large impedance is not trivial. Standard low frequency lock-in measurements suffer from stray capacitance of the order of nF reducing the bandwidth to merely kHz and a time resolution of ms. One approach of increasing the bandwidth is to connect the high-impedance device next to a low input impedance amplifier, though, the signal is decreased by the ratio of the impedances of the device and the amplifier, easily a factor of a thousand. Therefore, long averaging times are required to detect the small signal. Moreover, 1/f noise still affects the measurements. The issues of small bandwidth, impedance mismatch and 1/f noise can be simultaneously addressed using resonant LC circuits. The success of this impedance transformer lies in replacing the slow operation of applying voltages and measuring currents by applying fast radio frequency signals and measuring the reflectance. LC circuits remain appealing because of a rather simple assembly. Nonetheless, the challenge to reproducibly achieve the resonance frequencies in gigahertz (GHz) range while matching to high impedance still remains due to the large parallel stray capacitance of the measurement setup. Resonance frequency in GHz range is desirable for two reasons. Firstly, measurements are faster and 1/f noise smaller. Yet more importantly, one can profit from the wide range of measurement techniques developed in the context of circuit quantum electrodynamics, such as FPGA based hardware and quantum limited Josephson parametric amplifiers. \n \nIn this thesis, a matching circuit namely stub tuner based on coplanar transmission lines is designed, fabricated and integrated with mesoscopic devices. Owing to the simple design, we could predict the frequency response of a stub tuner in terms of the standard circuit model (Chapter 3). We initiated certain necessary fabrication adaptations to achieve integration of niobium circuits with devices. In particular, a reliable stamping of carbon nanotubes from the growth substrate to the target substrate was accomplished, improving both the device quality and yield (Chapter 4). RF reflectometry is employed to perform admittance measurements on CNT quantum dots. We could reliably deduce all circuit and device properties using the resonance response. The measurement bandwidth is shown to be in MHz range which is three orders of magnitude larger than that for DC measurements. In particular, using the phase response of the circuit, double-dot qubit energy and dephasing rate are studied (Chapter 5). We could even get rid of conventional contacts that could degrade the device properties, and still extract quantum capacitance and dissipation in graphene p-n junctions. The residual doping is shown to affect not only the mean free path but also the Fermi velocity. We further see that at small doping, electron-electron interactions could modify the Fermi velocity and that the dispersion relation could deviate from linearity (Chapter 6). \n \nIn the second half of the scientific results, real strength of stub tuner is tested in transmission measurements. The large bandwidth and signal to noise ratio is utilized to measure extremely clean noise power densities at high speed and draw comparisons with the corresponding average current. Clear changes in the shot noise spectrum are seen when a transport channel is added or removed from the bias window of the quantum dot. We especially observe noise enhancement outside the Coulomb diamonds due to blocking states (Chapter 7). Finally, we investigate noise properties of quantum dots when leads are superconducting. Distinct charge transfers due to multiple Andreev reflections are visible in the shot noise. The main finding, however, was the thermal noise at zero bias. We observed its periodic suppression and emergence across the quantum dot resonance. In another device, shot noise due to the Kondo effect is found to be enhanced in the superconducting state when the Kondo temperature exceeds the value of the superconducting gap (Chapter 8). Simple design and reliable calibration of the GHz matching circuit in both reflectance and transmission pave a way for fast and non-invasive measurements of mesoscopic conductors. \n \n \n \n \n
Quantum-limited Josephson parametric amplifiers play a pivotal role in advancing the field of circuit quantum electrodynamics by enabling the fast and high-fidelity measurement of weak microwave signals. Therefore, it is necessary to develop robust parametric amplifiers with low noise, broad bandwidth, and reduced design complexity for microwave detection. However, current broadband parametric amplifiers either have degraded noise performance or rely on complex designs. Here, we present a device based on the broadband impedance-transformed Josephson parametric amplifier that integrates a hornlike coplanar waveguide transmission line, which significantly decreases the design and fabrication complexity while keeping comparable performance. The device shows an instantaneous bandwidth of 700 (200) MHz for 15 (20) dB gain with an average input saturation power of −110 dBm and near quantum-limited added noise. The operating frequency can be tuned over 1.4 GHz using an external flux bias. We further demonstrate the negligible backaction from our device on a transmon qubit. The amplification performance and simplicity of our device promise its wide adaptation in quantum metrology, quantum communication, and quantum information processing. Published by the American Physical Society 2024
Quantum point contact or QPC -- a constriction in a semiconducting two-dimensional (2D) electron system with a quantized conductance -- has been found as the building block of novel spintronic, and topological electronic circuits. They can also be used as readout electronic, charge sensor or switch in quantum nanocircuits. A short and impurity-free constriction with superconducting contacts is a Cooper pairs QPC analogue known as superconducting quantum point contact (SQPC). The technological development of such quantum devices has been prolonged due to the challenges of maintaining their geometrical requirement and near-unity superconductor-semiconductor interface transparency. Here, we develop advanced nanofabrication, material and device engineering techniques and report on an innovative realisation of nanoscale SQPC arrays with split gate technology in semiconducting 2D electron systems, exploiting the special gate tunability of the quantum wells, and report the first experimental observation of conductance quantization in hybrid InGaAs-Nb SQPCs. We observe reproducible quantized conductance at zero magnetic fields in multiple quantum nanodevices fabricated in a single chip and systematically investigate the quantum transport of SQPCs at low and high magnetic fields for their potential applications in quantum metrology, for extremely accurate voltage standards, and fault-tolerant quantum technologies.