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.
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 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.
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.
Superconductor-semiconductor hybrid materials have been extensively used for experiments on electrically tunable quantum devices. Notably, Josephson junctions utilizing nanowire weak links have enabled a number of new gate-tunable qubits, including gatemons, Andreev level qubits and spin qubits. Conversely, superconducting parametric amplifiers based on Josephson junctions have not yet been implemented using nanowires, even though such nearly quantum limited amplifiers are key elements in experiments on quantum circuits. Here we present Josephson parametric amplifiers based on arrays of parallel InAs nanowires that feature a large critical current as required for linear amplification. The resonance frequency of the devices is gate-tunable by almost 1 GHz, with a gain exceeding 20 dB in multiple frequencies and noise approaching the quantum-limit. This new platform enables on-chip integration of gate-tunable qubits with quantum limited amplifiers using the same hybrid materials and on any substrate.
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.
A requirement for quantum information experiments using superconducting quantum circuits is the readout of extremely weak microwave signals. Quantum limited amplifiers are essential for such task and this is precisely why they occupy an always-growing importance in such experiments, particularly for superconducting quantum bits readout.In this thesis, we present two quantum limited amplifiers architectures: the first one is based on a resonator while the second one leverages a traveling-wave construction. These amplifiers both take advantage of tunable Josephson junctions (squid). It is the ideal component for quantum limited parametric amplification: it is both nonlinear and dissipationless (given its superconducting nature). This thesis demonstrates the operation of these Josephson quantum limited amplifiers and how they go beyond the state-of-the-art thanks to original design choices.An important limitation of resonant Josephson parametric amplifiers is their poor dynamic range. We identified the Kerr nonlinearity as the main cause, and we successfully managed to tailor it using Josephson junction arrays. We also developed a model in good agreement with the experimental data, therefore validating our initial assumption that the Kerr nonlinearity and saturation are linked. However, most of the resonant Josephson parametric amplifiers suffer from a fundamental issue, arising from their architecture. It is the conservation of the gain bandwidth product, therefore limiting their bandwidth. Traveling-wave architectures are well suited to overcome this limitation. However, two new issues must be dealt with: impedance matching and phase matching. To tackle the former, we developed an original and simple fabrication process in order to obtain a 50ohm matched 2000 squid array. As for the phase, we matched it by periodically modulating the impedance of the squid transmission line in order to open a photonic band gap. Therefore, we locally distort the dispersion relation. This step does not add any complexity to the initial fabrication process. Moreover, we developed a model in good agreement with experimental data while considering the nonlinear behavior of the photonic gap in a Josephson metamaterial. In addition to demonstrating high performance amplifiers poised for numerous quantum technological applications, this thesis opens the door to more fundamental quantum optics experiments taking advantage of these highly nonlinear transmission lines.With the resonant amplifiers, we measured a 1dB compression point around -117dBm and 45MHz bandwidth (at 20 db gain). With the traveling-wave amplifiers, we measured 18dB gain on a 2.25GHz bandwidth, a 1dB compression point reaching -103dBm and near quantum-limited noise performances.
A quantum-limited amplifier enables the amplification of weak signals while introducing minimal noise dictated by the principles of quantum mechanics. Such amplifiers serve a broad spectrum of applications in quantum computing, including fast and accurate readout of superconducting qubits and spins, as well as various uses in quantum sensing and metrology. Parametric amplification, primarily developed with use of Josephson junctions, has evolved into the leading technology for highly effective microwave measurements within quantum circuits. Despite their significant contributions, these amplifiers face fundamental limitations, such as their inability to handle high powers, their sensitivity to parasitic magnetic fields, and particularly their limitation to operate only at millikelvin temperatures. To tackle these challenges, here we experimentally develop a novel quantum-limited amplifier based on superconducting kinetic inductance and present an extensive theoretical model to describe this nonlinear coupled-mode system. Our device surpasses the conventional constraints associated with Josephson-junction amplifiers by operating at much higher temperatures up to 4.5 K. With two distinct spectral modes and tunability through bias current, this amplifier can operate selectively in both the single-mode-amplification regime and the double-mode-amplification regime near the quantum noise limit. Using a nonlinear thin film exhibiting kinetic inductance, our device attains gain exceeding 50 dB in a single-mode configuration and 32 dB in a double-mode configuration, while adding 0.82 input-referred noise quanta. Importantly, this amplifier eliminates the need for Josephson junctions, resulting in the capability to handle significantly higher powers than Josephson junction--based amplifiers. It also demonstrates resilience in the presence of magnetic fields, offers a straightforward design, and increases reliability. This positions the amplifier as a versatile solution for quantum applications and facilitates its integration into future superconducting quantum computers.