Josephson junctions (JJ) and their tunable properties, including their nonlinearities, form the core of superconducting circuit quantum electrodynamics (cQED). In quantum circuits, low-noise amplification of feeble microwave signals is essential and the Josephson parametric amplifiers (JPA) are the widely used devices. The existing JPAs are based on Al-AlOx-Al tunnel junctions realized in a superconducting quantum interference device geometry, where magnetic flux is the knob for tuning the frequency. Recent experimental realizations of 2D van der Waals JJs provide an opportunity to implement various cQED devices with the added advantage of tuning the junction properties and the operating point using a gate potential. While other components of a possible 2D van der Waals cQED architecture have been demonstrated -- quantum noise limited amplifier, an essential component, has not been realized. Here we implement a quantum noise limited JPA, using a graphene JJ, that has linear resonance gate tunability of 3.5 GHz. We report 24 dB amplification with 10 MHz bandwidth and -130 dBm saturation power; performance on par with the best single-junction JPAs. Importantly, our gate tunable JPA works in the quantum-limited noise regime which makes it an attractive option for highly sensitive signal processing. Our work has implications for novel bolometers -- the low heat capacity of graphene together with JJ nonlinearity can result in an extremely sensitive microwave bolometer embedded inside a quantum noise-limited amplifier. In general, our work will open up exploration of scalable device architecture of 2D van der Waals materials by integrating a sensor with the quantum amplifier.
Josephson junctions (JJ) and their tunable properties, including their nonlinearities, form the core of superconducting circuit quantum electrodynamics (cQED). In quantum circuits, low-noise amplification of feeble microwave signals is essential and the Josephson parametric amplifiers (JPA) are the widely used devices. The existing JPAs are based on Al-AlOx-Al tunnel junctions realized in a superconducting quantum interference device geometry, where magnetic flux is the knob for tuning the frequency. Recent experimental realizations of 2D van der Waals JJs provide an opportunity to implement various cQED devices with the added advantage of tuning the junction properties and the operating point using a gate potential. While other components of a possible 2D van der Waals cQED architecture have been demonstrated -- quantum noise limited amplifier, an essential component, has not been realized. Here we implement a quantum noise limited JPA, using a graphene JJ, that has linear resonance gate tunability of 3.5 GHz. We report 24 dB amplification with 10 MHz bandwidth and -130 dBm saturation power; performance on par with the best single-junction JPAs. Importantly, our gate tunable JPA works in the quantum-limited noise regime which makes it an attractive option for highly sensitive signal processing. Our work has implications for novel bolometers -- the low heat capacity of graphene together with JJ nonlinearity can result in an extremely sensitive microwave bolometer embedded inside a quantum noise-limited amplifier. In general, our work will open up exploration of scalable device architecture of 2D van der Waals materials by integrating a sensor with the quantum amplifier.
With a large portfolio of elemental quantum components, superconducting quantum circuits have contributed to dramatic advances in microwave quantum optics. Of these elements, quantum-limited parametric amplifiers have proven to be essential for low noise readout of quantum systems whose energy range is intrinsically low (tens of µeV). They are also used to generate non classical states of light that can be a resource for quantum enhanced detection. Superconducting parametric amplifiers, like quantum bits, typically utilize a Josephson junction as a source of magnetically tunable and dissipation-free nonlinearity. The magnetic control is not an industry standard for devices and starts already to be an issue in large scale circuits. In recent years, efforts have been made to introduce semiconductor weak links as electrically tunable nonlinear elements, with demonstrations of microwave resonators and quantum bits using semiconductor nanowires, a two dimensional electron gas, carbon nanotubes and graphene. However, given the challenge of balancing nonlinearity, dissipation, participation, and energy scale, parametric amplifiers have not yet been implemented with a semiconductor weak link. The work presented in this PhD thesis demonstrates the design, fabrication and performances of a parametric amplifier leveraging a graphene Josephson junction.The graphene is encapsulated in between hBN flakes in order to improve its quality. Using such high quality junctions, we demonstrate amplification with 20 dB gain which is the milestone for overcoming noise of the classical amplifiers in the rest of the amplification chain. The Josephson parametric amplifier being based on a resonant structure (around 6 GHz) suffers from the gain bandwidth product limiting the amplification to a few MegaHertz frequency range. The use of a side gate to electrically tune the graphene gives control of the Josephson junction inductance and thus the resonant frequency of the amplifier. We demonstrate a near 1 GHz tunability of the amplification frequency with the use of a side gate which is as good as the tunability offered by using a magnetic flux on a SQUID for similar amplifier designs. Moreover, graphene has shown to exhibit nonlinear loss under a strong microwave irradiation arising because of dynamics in the Andreev states. Dissipation is known to add intrinsic noise on quantum amplifiers and could be a problem for quantum limited parametric amplification. We demonstrate that despite the presence of nonlinear loss, graphene based Josephson parametric amplifiers can reach near quantum limited amplification. A two photon loss model was used to describe the behaviour of the device but we show that a more complex model is required in order to take into account the nonlinear dissipation and the non-sinusoidal current phase relation. We also studied the dynamic range and showed that the graphene based Josephson parametric amplifier can reach a 1 dB compression point of -123 dBm which is as good as single tunnel junctions based parametric amplifiers. Our results expand the toolset for electrically tunable superconducting quantum circuits and offer new opportunities for the development of quantum technologies such as quantum computing, quantum sensing and fundamental science.
Josephson junction (JJ) is a key nonlinear element in superconducting devices such as qubits, amplifiers, and bolometers. Recently, gate-tunable JJs based on graphene and semiconductors have gained interest due to their rich Andreev physics and wide applications in circuit quantum electrodynamics devices. In addition to gate tunability, it offers many advantages over conventional JJs, such as exceptional thermal properties for bolometric sensors, magnetic-field compatibility, and operability at elevated temperatures above 1 K. Like conventional Al–AlOx–Al JJs, graphene JJs also act as nonlinear inductors, and at their heart lies the Kerr nonlinearity. Additionally, in graphene JJs, the nonlinearity is tunable via external knobs in a single device. However, a detailed experimental exploration of the tunable Kerr nonlinearity in graphene JJs has never been performed. In this work, we study the dependence of the Kerr nonlinearity on gate voltage, temperature, and DC bias—an interesting knob that has been less explored. Using these parameters, we show that the magnitude of the Kerr coefficient can be tuned over a wide range, from ∼200 kHz to 1.2 MHz. Our work will be a valuable resource for further understanding of the nonlinearity in graphene JJs and for the design of next-generation amplifiers and sensors, with enhanced performance.
Bolometers are radiation sensors that are central to wide areas such as dark matter search, radio astronomy, material science, and qubit readouts, among others. There have been different kinds of bolometer realizations in the recent past. The challenge is to have a single device that combines high sensitivity, broad bandwidth, and a fast readout scheme. Here we demonstrate the usage of Josephson parametric amplifiers (JPA) as highly sensitive bolometers. Our key finding is that the Kerr non-linearity of the JPA boosts the device’s sensitivity. When the bolometer is biased in the non-linear regime, it enhances the up-converted signals (~100 times), resulting in an order of magnitude improvement in sensitivity compared to the linear regime. In the non-linear biasing, we achieve a NEP ~ 500 aW/ Hz . Our device offers a fast detection scheme with a thermal time constant of 4.26 μs and an intrinsic JPA time constant of 70 ns. Our work integrates a JPA into a bolometer, enabling a fast and sensitive operation compared to previously studied graphene-based bolometers. Our study demonstrates a way forward to improve the quantum sensors based on 2D materials by leveraging the inherent non-linear response. Graphene-based Josephson junction bolometers hold promise as sensitive single-photon detectors, but they are normally limited by slow readout schemes and narrow operational bandwidths. Here, the authors report the application of Josephson parametric amplifiers as fast, tunable bolometers with sensitivity enhanced by the intrinsic Kerr non-linearity.
Cryogenic, ultralow-noise microwave amplification is critical in superconducting qubit readout, microwave detection, and space technology. Josephson parametric amplifiers (JPAs) are widely used in microwave amplification. Parametric amplification occurs because a nonlinear element and a strong pump enable frequency mixing. The core component of a JPA is a Josephson junction, which acts as a nonlinear inductor and is formed by a thin layer of insulator or normal metal sandwiched between two superconductors. However, gate-controlled Josephson field-effect transistors (JoFETs) have been less studied in this context. In particular, graphene-based JoFETs have shown promising results in terms of scalability and electrostatic control, which could provide an alternative to magnetic field control. The aim of this thesis is to study graphene-based JoFETs as a nonlinear element in superconducting parametric amplifiers. The study was carried out as a literature review and circuit-level simulations using Keysight Advanced Design System (ADS) software. At first, an LC resonator-based JPA was implemented, in which the nonlinear element was a Verilog-A model of a nonlinear Josephson inductance. Harmonic balance (HB) simulation was performed to study the bandwidth, and gain as a function of signal frequency and the dependence on pump power. It showed that parametric gain occurs only when the pump is on. 30 dB gain and 290 MHz bandwidth were achieved. As the gain increased, the bandwidth became narrower. Finally, a gate-pumped JPA using a JoFET was simulated. The JoFET was implemented using a Verilog-A model, which was based on measurements by VTT of their graphene JoFETs. HB simulations showed the electrostatic control of the gate-pumped JPA and gain-bandwidth trade-off typical of JPAs. 15–35 dB gain and 150 MHz bandwidth were obtained, and the resonance frequency could be adjusted with the DC gate voltage. Overall, the observed behavior corresponds well with the reported results for JPAs and fabricated JoFET JPAs. These findings suggest that graphene JoFETs enable parametric gain that can be controlled electrostatically. Although their noise levels may not meet the requirements of quantum computers, they could be useful in other demanding applications, such as space technology.
Abstract Over the last two decades, the superconducting qubits have become a promising and rapidly growing area of research. However, there are specific challenges when working with such a delicate system. One of them is the qubit state readout. Since a very small signal (of the order of 1 photon) is used for the readout, its detection is challenging and it requires low-noise cryogenic amplifiers working in the 3–12 GHz frequency range. Josephson parametric amplifiers (JPAs) are one of the most studied and developed candidates. Currently, several JPA designs offer low noise performance and instantaneous bandwidth from tens of MHz to several GHz; they are also becoming an integral part of circuit quantum electrodynamics (cQED) setups. In the current work, results are presented of the measurements of a JPA made of a coplanar waveguide with an array of three SQUIDs. Nondegenerate amplification and an amplifier added noise being close to the quantum limit are demonstrated.
Josephson junctions (JJ) are a fundamental component of microwave quantum circuits, such as tunable cavities, qubits, and parametric amplifiers. Recently developed encapsulated graphene JJs, with supercurrents extending over micron distance scales, have exciting potential applications as a new building block for quantum circuits. Despite this, the microwave performance of this technology has not been explored. Here, we demonstrate a microwave circuit based on a ballistic graphene JJ embedded in a superconducting cavity. We directly observe a gate-tunable Josephson inductance through the resonance frequency of the device and, using a detailed RF model, we extract this inductance quantitatively. We also observe the microwave losses of the device, and translate this into sub-gap resistances of the junction at μeV energy scales, not accessible in DC measurements. The microwave performance we observe here suggests that graphene Josephson junctions are a feasible platform for implementing coherent quantum circuits.