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arXiv:1502.06041 · Physical Review Applied

On-Chip Superconducting Microwave Circulator from Synthetic Rotation

Joseph Kerckhoff, Kevin Lalumière, Benjamin J. Chapman, Alexandre Blais, Konrad W. Lehnert

Published Sep 10, 2015• 143 citations
arXivHTMLPDF

Abstract

Electrical circulators enforce the unidirectional flow of microwave signals through junctions in transmission lines, but off-the-shelf components create large magnetic fields that would interfere with nearby superconducting circuits in quantum-computing hardware. The authors design a circulator based on active modulation of its components, a design that uses no large magnets and may be integrated on the same chip as superconducting circuitry. An analogous problem exists in integrated optical networks, for both quantum and classical information applications, and this approach could be adapted to that context as well.

DOI
10.1103/physrevapplied.4.034002

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Electrical circulators enforce the unidirectional flow of microwave signals through junctions in transmission lines, but off-the-shelf components create large magnetic fields that would interfere with nearby superconducting circuits in quantum-computing hardware. The authors design a circulator based on active modulation of its components, a design that uses no large magnets and may be integrated on the same chip as superconducting circuitry. An analogous problem exists in integrated optical networks, for both quantum and classical information applications, and this approach could be adapted to that context as well.
This thesis develops theory for and experimentally demonstrates a new way to break Lorentz reciprocity—the symmetry, in an electrical network, under exchange of source and detector. The approach is based on the sequential application of frequency conversion and delay; as frequency and time are Fourier duals, these operations do not generally commute. We apply this method in the construction of an on-chip superconducting microwave circulator, a critical component for the unidirectional routing of quantum information in superconducting networks. The device requires neither permanent magnets nor microwave control tones, allowing on-chip integration with other superconducting circuits without expensive control hardware. Isolation in the device exceeds 20 dB over a bandwidth of tens of MHz, and its insertion loss is small, reaching as low as 0.9 dB at select operation frequencies. Furthermore, the device is linear with respect to input power for signal powers up to many hundreds of fW (~103 circulating photons), and the direction of circulation can be dynamically reconfigured. We demonstrate its tunability with operation at a selection of frequencies between 4 and 6 GHz. Given the current status of quantum error-correction and architectures for quantum information processing with superconducting circuits, such scalable non-reciprocal devices will almost certainly be necessary for construction of a superconducting quantum computer intended to be more than a proof-of-principle.
An on-chip microwave circulator that is compatible with superconducting devices is a key element for scale up of superconducting circuits. Previous approaches to integrating circulators on chip involve either external driving that requires extra microwave lines or a strong magnetic field that would compromise superconductivity. Here we report the first proof-of-principle realization of a passive on-chip circulator that is made from a superconducting loop interrupted by three notionally identical Josephson junctions and is tuned with only dc control fields. Our experimental results show evidence for nonreciprocal scattering, and excellent agreement with theoretical simulations. We also present a detailed analysis of quasiparticle tunneling in our device using a hidden Markov model. By reducing the junction asymmetry and utilizing the known methods of protection from quasiparticles, we anticipate that Josephson-loop circulator will become ubiquitous in superconducting circuits.
Microwave circulators play an important role in quantum technology based on superconducting circuits. The conventional circulator design, which employs ferrite materials, is bulky and involves strong magnetic fields, rendering it unsuitable for integration on superconducting chips. One promising design for an on-chip superconducting circulator is based on a passive Josephson-junction ring. In this paper, we consider two operational issues for such a device: circuit tuning and the effects of quasiparticle tunneling. We compute the scattering matrix using adiabatic elimination and derive the parameter constraints to achieve optimal circulation. We then numerically optimize the circulator performance over the full set of external control parameters, including gate voltages and flux bias, to demonstrate that this multidimensional optimization converges quickly to find optimal working points. We also consider the possibility of quasiparticle tunneling in the circulator ring and how it affects signal circulation. Our results form the basis for practical operation of a passive on-chip superconducting circulator made from a ring of Josephson junctions.
Nonreciprocal circuit elements form an integral part of modern measurement and communication systems. Mathematically they require breaking of time-reversal symmetry, typically achieved using magnetic materials and more recently using the quantum Hall effect, parametric permittivity modulation or Josephson nonlinearities. Here we demonstrate an on-chip magnetic-free circulator based on reservoir-engineered electromechanic interactions. Directional circulation is achieved with controlled phase-sensitive interference of six distinct electro-mechanical signal conversion paths. The presented circulator is compact, its silicon-on-insulator platform is compatible with both superconducting qubits and silicon photonics, and its noise performance is close to the quantum limit. With a high dynamic range, a tunable bandwidth of up to 30 MHz and an in situ reconfigurability as beam splitter or wavelength converter, it could pave the way for superconducting qubit processors with multiplexed on-chip signal processing and readout.Nonreciprocal optical elements often require magnetic materials in order to break time-reversal symmetry. Here, Barzanjeh et al. demonstrate a magnetic-free on-chip microwave circulator that utilizes the interference from six electro-mechanical signal paths.
We report on the design and performance of an on-chip microwave circulator with a widely (GHz) tunable operation frequency.Nonreciprocity is created with a combination of frequency conversion and delay, and requires neither permanent magnets nor microwave bias tones, allowing on-chip integration with other superconducting circuits without the need for high-bandwidth control lines.Isolation in the device exceeds 20 dB over a bandwidth of tens of MHz, and its insertion loss is small, reaching as low as 0.9 dB at select operation frequencies.Furthermore, the device is linear with respect to input power for signal powers up to hundreds of fW (≈10 3 circulating photons), and the direction of circulation can be dynamically reconfigured.We demonstrate its operation at a selection of frequencies between 4 and 6 GHz.
Superconducting qubits are a promising platform for quantum computing, but measurements of such circuits rely on the use of ferrite circulators, which are difficult to miniaturize or make lossless. Replacing those circulators with on-chip superconducting ones has become a major research thrust, but so far most of these replacements have been narrow-band. Thus the authors design an on-chip circulator that combines low-loss circulation with instantaneous bandwidth an octave wide. Such a device could enable the multiplexed readout of hundreds of qubits, facilitating the scale-up that is currently a major hurdle in quantum information processing with superconducting circuits.