The reconfigurable Josephson circulator/directional amplifier (1503.00209v4)

Abstract: Circulators and directional amplifiers are crucial non-reciprocal signal routing and processing components involved in microwave readout chains for a variety of applications. They are particularly important in the field of superconducting quantum information, where the devices also need to have minimal photon losses to preserve the quantum coherence of signals. Conventional commercial implementations of each device suffer from losses and are built from very different physical principles, which has led to separate strategies for the construction of their quantum-limited versions. However, as recently proposed theoretically, by establishing simultaneous pairwise conversion and/or gain processes between three modes of a Josephson-junction based superconducting microwave circuit, it is possible to endow the circuit with the functions of either a phase-preserving directional amplifier or a circulator. Here, we experimentally demonstrate these two modes of operation of the same circuit. Furthermore, in the directional amplifier mode, we show that the noise performance is comparable to standard non-directional superconducting amplifiers, while in the circulator mode, we show that the sense of circulation is fully reversible. Our device is far simpler in both modes of operation than previous proposals and implementations, requiring only three microwave pumps. It offers the advantage of flexibility, as it can dynamically switch between modes of operation as its pump conditions are changed. Moreover, by demonstrating that a single three-wave process yields non-reciprocal devices with reconfigurable functions, our work breaks the ground for the development of future, more-complex directional circuits, and has excellent prospects for on-chip integration.

• The paper introduces a dual-mode non-reciprocal microwave circuit that can be reconfigured to serve as either a circulator or directional amplifier.
• Experimental results show an insertion loss below 0.5 dB, an 11 MHz bandwidth for circulation, and a directional amplifier gain around 13 dB with near-quantum limited noise.
• This innovation simplifies superconducting circuit design by using only three microwave pumps, enabling flexible on-chip integration for quantum information systems.

Overview of "The Reconfigurable Josephson Circulator/Directional Amplifier"

The paper "The Reconfigurable Josephson Circulator/Directional Amplifier" presents a novel approach to creating non-reciprocal devices, specifically focusing on a Josephson-junction based microwave circuit capable of functioning either as a phase-preserving directional amplifier or a circulator. This work is significant in the field of superconducting quantum information, where minimizing photon losses is crucial to maintaining quantum coherence.

Key Contributions

The authors experimentally demonstrate two primary operational modes of the circuit: as a circulator and as a directional amplifier. Both are realized within the same device architecture by adjusting the pump conditions, which introduces flexibility and efficiency over separate devices for each function. A standout feature is the device’s simplicity and reduced complexity in comparison with previous implementations, requiring only three microwave pumps.

Circulator and Directional Amplifier Implementation

The circulator function is achieved through pairwise photon conversion processes between three circuit modes using Josephson junctions. By simultaneously driving these modes at specific frequencies, the device achieves full conversion with minimal insertion loss and high isolation, manipulating the total pump phase to switch the sense of circulation. The device demonstrates a low insertion loss of less than 0.5 dB and a bandwidth of 11 MHz for effective non-reciprocal circulation.

For the directional amplifier, the configuration involves two gain processes and one conversion process among the modes. The authors achieve gains comparable to traditional non-directional superconducting amplifiers, around 13 dB, while retaining a noise performance close to quantum limits. The paper emphasizes the precise control required for effective directional amplification, where the conversion coefficient must be near unity to maintain directionality.

Implications and Future Perspectives

This development presents substantial implications for on-chip integration with superconducting qubits, offering a viable pathway towards compact and efficient superconducting circuits without the need for bulky magnetic components. Additionally, the ability to dynamically switch operational modes could facilitate more complex signal routing strategies in quantum information systems.

Future research could focus on extending this work to explore other non-reciprocal devices, potentially including directional phase-sensitive amplifiers. Exploration into how higher-order nonlinearities affect device performance, particularly concerning dynamic range and off-resonant behaviors, would be beneficial. Further theoretical and experimental efforts are needed to refine the system’s robustness against phase drift and to enhance device fidelity.

In conclusion, the paper outlines a significant advancement in the design of microwave quantum devices, indicating promising directions for their integration into broader quantum technologies. This work not only demonstrates the potential for reconfigurable microwave circuits but also sets a foundation for the development of more sophisticated non-reciprocal circuit architectures.