Session B33: Nonreciprocal Superconducting Devices

Nonreciprocal and reciprocal information processing at the quantum level

Preserving the quantum coherence of signals is of paramount importance for components utilized in quantum information processing, quantum computation and quantum measurement setups. In recent years a tremendous progress has been made in the development of quantum-limited components, such as reciprocal and nonreciprocal amplifiers, circulators and isolators. A promising way to design these devices is based on parametric modulation of coupled modes, where the required mode-mixing processes are realized by utilizing Josphson junction-based tunable couplers or via coupling to mechanical degrees of freedom. All designs come with their unique challenges, such as higher-order nonlinearities -- limiting the dynamical range, or high thermal occupations -- leading to noise contamination of the signal. In this talk we present possible ways to face these challenges, complemented with an introduction of the basic concept on how to engineer nonreciprocal interactions and devices based on balancing a coherent interaction with the corresponding dissipative interaction. Furthermore, we present possible implementations in superconducting circuit and optomechanical architectures and discuss routes for optimizing the design of microwave circuits enabling nonreciprocal and reciprocal quantum information processing.   

Scalable broadband circulators

Lorentz reciprocity is the symmetry in an electrical network under exchange of source and detector. Devices that break reciprocity, such as circulators or isolators, are general measurement tools, especially in the field of superconducting circuits where they are ubiquitous. Ferrite-based circulators have broad (octave) bandwidths, but their scalability is hindered by large permanent magnets. Chip-based replacement technologies have recently been developed, which achieve scalability at the cost of reduced bandwidth. In this talk, we’ll describe a general-purpose prescription for breaking Lorentz reciprocity with a combination of frequency conversion and delay. This approach allows for on-chip devices with octave bandwidths and low insertion loss. Measurements of room-temperature prototypes and design considerations for a superconducting implementation will also be discussed.   

Multi-Path Interferometric Josephson Directional Amplifier for Qubit Readout

We realize and characterize a quantum-limited, directional Josephson amplifier suitable for qubit readout. The device consists of two nondegenerate, three-wave-mixing amplifiers that are coupled together in an interferometric scheme, embedded in a printed circuit board. Nonreciprocity is generated by applying a phase gradient between the same-frequency pumps feeding the device, which plays the role of the magnetic field in a Faraday medium. Directional amplification and reflection-gain elimination are induced via wave interference between multiple paths in the system. We measure and discuss the main figures of merit of the device and show that the experimental results are in good agreement with theory. An improved version of this directional amplifier is expected to eliminate the need for bulky, off-chip isolation stages that generally separate quantum systems and preamplifiers in high-fidelity, quantum-nondemolition measurement setups.   

A superconducting, on-chip, microwave directional amplifier

Amplification based on parametric processes in superconducting circuits has revolutionized the measurement of feeble microwave signals. These devices, which operate near the quantum limit, are routinely used to readout superconducting qubits. However, they often require ferrite circulators to separate incoming and outgoing traveling waves, limiting measurement efficiency and scalability. To facilitate the routing of quantum signals we have recently created a superconducting, on-chip circulator without permanent magnets [https://arxiv.org/abs/1707.04565]. In this talk, we describe the integration of the circulator with a Josephson parametric amplifier on a single chip. We also present preliminary measurements of its performance as a directional amplifier.   

Broadband bi-directional amplification via paired simultaneous parametric drives

Quantum-limited parametric amplifiers have become a crucial tool for quantum information processing. However, such amplifiers generally suffer from absence of directionality, a fixed gain-bandwidth product and limited dynamic range. One proposed solution is to use paired parametric drives for gain and photon conversion in order to create an amplifier free from such limitations. This mode of operation will have bi-directional transmission amplification, unity reflection gain, a large, gain-independent bandwidth, and better dynamic range due to the linear dependence of gain on pump power. Previous attempts to realize this method via three-wave mixing in the Josephson Ring Modulator (JRM) met with limited success due to fourth order terms in the JRM which shift mode frequencies with pump strength, preventing accurate balancing of the simultaneous parametric processes. We avoid this complication by cancelling all fourth order terms at special flux biases in a JRM shunted with linear inductances. We will present data and discuss performance optimization of our JRM hardware for this multi-parametrically driven mode of operation.   

Nonreciprocal microwave amplifier for efficient qubit measurements

In a typical circuit QED architecture, the dispersive coupling between a superconducting qubit and microwave resonator leads to state-dependent frequency shifts. By probing the cavity with a weak coherent state one can achieve a QND qubit readout, whose fidelity is directly tied to the measurement efficiency of the photons leaving the readout cavity. While developments in quantum-limited parametric amplifiers have enabled rapid single-shot qubit readout, these measurements require circulators to protect the device under test from amplifier backaction and to control signal flow. The losses inherent to the use of these nonreciprocal commercial components are now the primary bottleneck in the overall measurement efficiency.
In this talk I will describe a compact and lossless superconducting circuit that can be programmed in situ by a set of microwave drives to perform nonreciprocal frequency conversion and both phase-insensitive and phase-sensitive amplification [1]. We will discuss demonstrated device performance as well as our progress towards direct integration with superconducting qubits.
[1] F. Lecocq. et al, ‘Nonreciprocal Microwave Signal Processing with a Field-Programmable Josephson Amplifier’, Phys. Rev. Applied 7, 024028 (2017).   

High Kinetic Inductance Based Fluxonium Circuit

Inductively shunted Fluxonium circuits have shown promise as long lived qubits. The large inductance needed in the Fluxonium circuit has been achieved using Josephson junction arrays which have several desirable features such as ease of fabrication and small physical footprints. However, the self resonant modes of large Josephson junction arrays set a practical limit for how large of an inductance can be achieved. Using high kinetic inductance materials to fabricate superinductors is one possible solution. Here, we fabricate and measure a fluxonium circuit containing a NbTiN superinductor to demonstrate that these materials can be integrated into standard superconducting circuit fabrication processes.   

High Kinetic Inductance NbN Nanowire Superinductance

Decoherence due to charge fluctuations is a still outstanding problem in many device architectures for quantum information processing and quantum metrology. Charge fluctuations can be suppressed by embedding the circuit in a high-impedance microwave environment - a superinductance - i.e. a non-dissipative, inductive circuit element with impedance greater than the quantum resistance (6.5 kΩ) and low losses at the frequency of the circuit (several GHz). This requirement cannot be met using the ordinary, geometric inductance of a wire due to its unavoidable shunt capacitance.

Superinductance was realized using a Josephson-junction array. An attractive alternative to arrays is offered by the high kinetic inductance of strongly disordered superconducting thin films. We have fabricated NbN nanowires with widths down to 40 nm and thickness typically 20 nm, implementing a superinductance with impedance Z = 6.8 kΩ. We demonstrate internal quality factors Q_i = 25,000 at single-photon excitation, significantly higher than values reported in devices with similar materials and geometries. Moreover, we show that the dominant dissipation in our nanowires is not an intrinsic property of the disordered films, but can instead be understood within the well-studied framework of two-level systems.   

Electrically driven, singly degenerate three-wave mixing in the Josephson Ring Modulator

Parametric amplifiers are now a required component of superconducting quantum information processors. One of the most conceptually attractive ways to produce parametric amplification is the modulation of a single mode’s resonant frequency with a drive at twice that frequency. This is regularly achieved in superconducting circuits via flux pumping a SQUID-based resonator. However, this typically requires multi-layer fabrication and crossovers to tightly couple a microwave flux into the amplifying mode. As an alternative, we exploit broken symmetries in a flux-biased, inductively-shunted Josephson Ring Modulator (JRM). In this circuit, the multiple spatial modes of the JRM play the roles of the drive line and amplifying mode, providing the equivalent of tight flux coupling with good spectral and spatial pump isolation in a simple, single-layer device. Moreover, this adds yet another primitive to the library of parametric couplings available to power next-generation, multiply-driven JRM-based circuits. We will present data on this mode of operation and discuss its performance as compared against conventional phase-preserving amplification in the same device.   

Squeezed Light Generation Using a Josephson Traveling Wave Parametric Amplifier in Non-Degenerate Four Wave Mixing

The generation of highly-squeezed states using superconducting amplifiers is a valuable tool for quantum optics and quantum metrology in the microwave domain. However, conventional Josephson parametric amplifiers can be limited with respect to their dynamic range, often due to the relatively strong nonlinearity of cavity-based devices. Recently, Josephson traveling-wave parametric amplifiers (JTWPAs) have been developed exhibiting large dynamic range with a 1dB compression point around -100dBm at the input of the JTWPA and over 3 GHz bandwidth. The high saturation power of these devices makes them promising candidates for generating highly squeezed states. In this talk, we present a novel scheme for operating a JTWPA in a non-degenerate four-wave mixing mode, and discuss our investigations of the JTWPA squeezing performance.