With a large portfolio of elemental quantum components, superconducting quantum circuits have contributed to dramatic advances in microwave quantum optics. Of these elements, quantum-limitedparametric 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. 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. Here we demonstrate a parametric amplifier leveraging a graphene Josephson junction and show that its working frequency is widely tunable with a gate voltage. We report gain exceeding 20 dB and noise performance close to the standard quantum limit. Our results complete 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.
Quantum computers can potentially achieve an exponential speedup versus classical computers on certain computational tasks, as was recently demonstrated in systems of superconductingqubits. However, these qubits have large footprints due to the need of ultra low-loss capacitors. The large electric field volume of \textit{quantum compatible} capacitors stems from their distributed nature. This hinders scaling by increasing parasitic coupling in circuit designs, degrading individual qubit addressability, and limiting the minimum achievable circuit area. Here, we report the use of van der Waals (vdW) materials to reduce the qubit area by a factor of >1000. These qubit structures combine parallel-plate capacitors comprising crystalline layers of superconducting niobium diselenide (NbSe2) and insulating hexagonal-boron nitride (hBN) with conventional aluminum-based Josephson junctions. We measure a vdW transmon T1 relaxation time of 1.06 μs, demonstrating that a highly-compact capacitor can reach a loss-tangent of <2.83×10−5. Our results demonstrate a promising path towards breaking the paradigm of requiring large geometric capacitors for long quantum coherence in superconducting qubits, and illustrate the broad utility of layered heterostructures in low-loss, high-coherence quantum devices.[/expand]
Dielectrics with low loss at microwave frequencies are imperative for high-coherence solid-state quantum computing platforms. We study the dielectric loss of hexagonal boron nitride(hBN) thin films in the microwave regime by measuring the quality factor of parallel-plate capacitors (PPCs) made of NbSe2-hBN-NbSe2 heterostructures integrated into superconducting circuits. The extracted microwave loss tangent of hBN is bounded to be at most in the mid-10-6 range in the low temperature, single-photon regime. We integrate hBN PPCs with aluminum Josephson junctions to realize transmon qubits with coherence times reaching 25 μs, consistent with the hBN loss tangent inferred from resonator measurements. The hBN PPC reduces the qubit feature size by approximately two-orders of magnitude compared to conventional all-aluminum coplanar transmons. Our results establish hBN as a promising dielectric for building high-coherence quantum circuits with substantially reduced footprint and, with a high energy participation that helps to reduce unwanted qubit cross-talk.
We perform extensive analysis of graphene Josephson junctions embedded in microwave circuits. By comparing a diffusive junction at 15 mK with a ballistic one at 15 mK and 1 K, we areable to reconstruct the current-phase relation.
Two-dimensional (2D) transition-metal dichalcogenide (TMD) superconductors have unique and desirable properties for integration with conventional superconducting circuits. These includethe ability to form atomically-flat and clean interfaces with stable tunnel barriers, increased kinetic inductance due to the atomically-thin geometry, and resilience to very high in-plane magnetic fields. However, integration of 2D TMD superconductors in conventional superconducting circuits, particularly those employing microwave drive and readout of qubits, requires that a fully superconducting contact be made between the 2D material and a three-dimensional (3D) superconductor. Here, we present an edge contact method for creating zero-resistance contacts between 2D \nbse and 3D aluminum. These hybrid Al-NbSe_2 Josephson junctions (JJs) display a Fraunhofer response to magnetic field with micron2-scale effective areas as the thin NbSe_2 allows field to uniformly penetrate the flake. We present a model for the supercurrent flow in a 2D-3D superconducting structure by numerical solution of the Ginzburg-Landau equations and find good agreement with experiment. The devices formed from 2D TMD superconductors are strongly influenced by the geometry of the flakes themselves as well as the placement of the contacts to bulk 3D superconducting leads. These results demonstrate our ability to graft 2D TMD superconductors and nano-devices onto conventional 3D superconducting materials, opening the way to a new generation of hybrid superconducting quantum circuits.