Magic-angle twisted bilayer graphene Josephson junctions (MATBG-JJ) with a gate-tunable valley-polarized weak link exhibit an intrinsic Josephson diode effect originating from broken symmetries associated with valley polarization and band-structure anisotropy. Exploiting this nonreciprocal superconducting platform, we construct quantum Stirling (QSC), Otto (QOC), and Carnot (QCC) thermodynamic cycles, where the valley-polarization potential $Δ_v$ acts as the principal control parameter, in contrast to conventional Josephson thermal machines driven by superconducting phase bias. We systematically compare the performance of MATBG-based Josephson diode thermal machines (MATBG-JDTM) with MATBG-based Josephson junction thermal machines (MATBG-JJTM) and AA-stacked bilayer graphene Josephson junction thermal machines (AABLG-JJTM). Owing to the flat-band-enhanced density of states and electrically tunable nonreciprocal transport in MATBG, both MATBG-JDTM and MATBG-JJTM exhibit significantly enhanced work output and efficiency over a broad operating regime compared to AABLG-JJTM. In particular, the gate-controlled MATBG-JDTM provides a flux-free alternative to conventional phase-driven architectures, mitigating limitations associated with magnetic-flux control and flux-noise effects. Our results establish MATBG Josephson diode platforms as a promising route toward electrically tunable quantum thermal machines and nonreciprocal superconducting caloritronics.
Magic-angle twisted bilayer graphene Josephson junctions (MATBG-JJ) with a gate-tunable valley-polarized weak link exhibit an intrinsic Josephson diode effect originating from broken symmetries associated with valley polarization and band-structure anisotropy. Exploiting this nonreciprocal superconducting platform, we construct quantum Stirling (QSC), Otto (QOC), and Carnot (QCC) thermodynamic cycles, where the valley-polarization potential Δv acts as the principal control parameter, in contrast to conventional Josephson thermal machines driven by superconducting phase bias. We systematically compare the performance of MATBG-based Josephson diode thermal machines (MATBG-JDTM) with MATBG-based Josephson junction thermal machines (MATBG-JJTM) and AA-stacked bilayer graphene Josephson junction thermal machines (AABLG-JJTM). Owing to the flat-band-enhanced density of states and electrically tunable nonreciprocal transport in MATBG, both MATBG-JDTM and MATBG-JJTM exhibit significantly enhanced work output and efficiency over a broad operating regime compared to AABLG-JJTM. In particular, the gate-controlled MATBG-JDTM provides a flux-free alternative to conventional phase-driven architectures, mitigating limitations associated with magnetic-flux control and flux-noise effects. Our results establish MATBG Josephson diode platforms as a promising route toward electrically tunable quantum thermal machines and nonreciprocal superconducting caloritronics.
Abstract Recently, gate-defined Josephson junctions based on magic-angle twisted bilayer graphene (MATBG) have been fabricated. In such a junction, local electrostatic gating can create two superconducting regions connected by an interaction-driven valley-polarized state as the weak link. Due to the spontaneous time-reversal and inversion symmetry breaking, novel phenomena such as the Josephson diode effect have been observed with zero external magnetic fields. Importantly, when the so-called nonreciprocity efficiency (which measures the sign and strength of the Josephson diode effect) changes sign, the energy-phase relation of the junction is approximately F(ϕ)∝cos(2ϕ) F ( ϕ ) ∝ cos ( 2 ϕ ) where F is the free energy and ϕ is the phase difference of the two superconductors. In this work, we show that such a MATBG-based Josephson junction, when shunted by a capacitor, can be used to realize the long-sought-after 0- π qubits which are protected from local perturbation-induced decoherence. Interestingly, by changing the junction parameters to the regime where a large nonreciprocity efficiency is obtainable, transmon-like qubits with large anharmonicity can also be realized. The gate-defined Josephson junctions can be employed as platforms for realizing qubits that are protected from local perturbations.
Valley-polarized Josephson Junctions as gate-tunable 0-π qubit platforms
Recently, gate-defined Josephson junctions based on magic-angle twisted bilayer graphene (MATBG) have been fabricated. In such a junction, local electrostatic gating can create two superconducting regions connected by an interaction-driven valley-polarized state as the weak link. Due to the spontaneous time-reversal and inversion symmetry breaking of the valley-polarized state, novel phenomena such as the Josephson diode effect have been observed without applying external fields. Importantly, when the so-called nonreciprocity efficiency (which measures the sign and strength of the Josephson effect) changes sign, the energy-phase relation of the junction is approximate F(ϕ)≈cos(2ϕ) where F is the free energy and ϕ is the phase difference of the two superconductors. In this work, we show that such a MATBG-based Josephson junction, when shunted by a capacitor, can be used to realize the long-sought-after 0-π qubits which are protected from local perturbation-induced decoherence. Interestingly, by changing the junction parameters, transmon-like qubits with large anharmonicity can also be realized. In short, by utilizing the novel interaction-driven valley-polarized state in MATBG, a single gate-defined Josephson junction can be used to replace complicated superconducting circuits for realizing qubits that are protected from local perturbations.
The superconducting (SC) state of magic-angle twisted bilayer graphene (MATBG) shows exceptional properties, as it consists of an unprecedentedly small electron (hole) ensemble of only ~ 10 power 11 carriers per square centimeter, which is five orders of magnitude lower than in traditional superconductors. This results in an ultra-low electronic heat capacity and kinetic inductance of this truly two-dimensional SC, and provides record-breaking key parameters for a variety of quantum sensing applications, in particular in thermal sensing and single photon detection (SPD), which traditionally exploit thermal effects in nanostructured superconducting thin films. In this work, we systematically study the interaction of the superconducting state of MATBG with individual light quanta. We discover full destruction of the SC state upon absorption of a single infrared photon even in a 16 square micrometer sized device, which showcases its exceptional bolometric sensitivity. Upon voltage biasing close to its critical current, we further show that this non-optimized device can be used as a SPD, whose click-rate is proportional to the number of absorbed photons, following Poissonian statistics. Our work offers insights into the MATBG-photon interaction and shows up pathways to use low-carrier density graphene-based superconductors as a new platform for developing revolutionary new quantum devices and sensors.
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.
We propose a novel quantum computing architecture that combines the engineered electromagnetic properties of quantum metamaterials with the strongly correlated electron physics of magic-angle systems. By integrating magic-angle twisted bilayer graphene (MATBG) into superconducting metamaterial waveguides, we demonstrate theoretically that the resulting hybrid system exhibits: (1) enhanced qubit coherence through geometric control of electromagnetic modes, (2) tunable qubit-qubit coupling via metamaterial dispersion engineering, and (3) topologically protected quantum states arising from the interplay of moiré band structure and metamaterial boundary conditions. Our framework provides design equations for a scalable quantum computing platform that leverages both the flat-band physics of magic-angle materials and the controllable photon-matter interactions of quantum metamaterials.