Design and Testing of Superconducting Microwave Passive Components for Quantum Information Processing
Abstract
We report on the design, fabrication and testing of
two superconducting passive microwave components, a quadrature
hybrid and a 20 dB directional coupler. These components
are designed to be integrated with superconducting qubits or
Josephson parametric amplifiers and used in quantum information
processing applications. For the coupler, we measure return
loss and isolation 20 dB, and insertion loss 0.3 dB in a 2 GHz
band around 6 GHz. For the hybrid performance, we measure
isolation 20 dB and insertion loss 0.3 dB in a 10% band
around 6.5 GHz. These values are within the design specifications
of our application; however, we find a 7% difference between the
designed and measured center frequency for the hybrid.
Index Terms—Directional coupler, microwave, passive microwave
components, quadrature hybrid, quantum information,
TRL calibration.
I. INTRODUCTION
MICROWAVE signals and superconducting quantum bits
(qubits) provide a promising architecture for realizing
quantum computing and related quantum information processing
tasks. To fully exploit the power of microwave circuits,
it is important to be able to combine and split propagating
modes of microwave fields. These tasks are accomplished
using passive microwave components, particularly directional
couplers and hybrids, which act as beam splitters for microwave
photons. These linear optics elements provide the
ability to manipulate quantum states of light. For example, a
directional coupler can be used as a tool to shift a quantum
state of light in phase space and a two-mode squeezed vacuum
state can be generated at the outputs of a 50:50 beam splitter
with two single-mode squeezed states as inputs [1]. Although
passive microwave components are routinely combined into
monolithic microwave integrated circuits [2], [3], they are only
now beginning to appear in integrated quantum information
processing circuits. The requirements of a quantum processor
yield different optimization of these circuits [4].
In this paper, we describe the design and testing of two
microwave components, a 20 dB directional coupler and a
quadrature hybrid, which are suitable for integration with
superconducting qubits and related devices. These components
are built to meet the requirements of a particular continuous
variables quantum information processing scheme, which
exploits the squeezed states prepared by Josephson Parametric
Amplifiers (JPAs) [5]. As such, they must have low loss and
high isolation, but only in a 10% band around a specific
frequency of 7 GHz. While these design constraints are specific
to a particular scheme, in general quantum circuits are
relatively narrowband, but exceptionally intolerant to loss.
Superconducting qubit circuits and JPAs are necessarily built
from superconducting metals, operated at low temperatures,
and often use a CPW architecture.


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