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arXiv:1301.3153 · Review of Scientific Instruments

Microstrip filters for measurement and control of superconducting qubits

Luigi Longobardi, D. A. Bennett, Vijay Patel, Wei Chen, James E. Lukens

Published Jan 1, 2013• 13 citations
arXivHTMLPDF

Abstract

Careful filtering is necessary for observations of quantum phenomena in superconducting circuits at low temperatures. Measurements of coherence between quantum states require extensive filtering to protect against noise coupled from room temperature electronics. We demonstrate distributed transmission line filters which cut off exponentially at GHz frequencies and can be anchored at the base temperature of a dilution refrigerator. The compact design makes them suitable to filter many different bias lines in the same setup, necessary for the control and measurement of superconducting qubits.

DOI
10.1063/1.4789310

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Careful filtering is necessary for observations of quantum phenomena in superconducting circuits at low temperatures. Measurements of coherence between quantum states require extensive filtering to protect against noise coupled from room temperature electronics. We demonstrate distributed transmission line filters which cut off exponentially at GHz frequencies and can be anchored at the base temperature of a dilution refrigerator. The compact design makes them suitable to filter many different bias lines in the same setup, necessary for the control and measurement of superconducting qubits.
We consider a typical circuit QED setup where an artificial atom encodes a qubit and is dispersively coupled to a measurement resonator that in turn is coupled to a transmission line. We show theoretically that by placing another artificial atom in this transmission line to act as a filter, the Purcell decay of the qubit into the transmission line is suppressed. When strong control fields are applied in the transmission line, the filter is saturated and effectively switched off. Such a Purcell filtering capability permits both the control and measurement of the qubit using the single transmission line, while maintaining the long coherence time of the qubit in the absence of the control pulses. We show that high fidelity Pauli ${\ensuremath{\sigma}}_{x}$ gates on the qubit can be realized using simple pulse shapes. For devices that already use one transmission line both for control and measurement of the qubit, our paper provides a way to completely filter out the qubit frequency without removing the possibility of controlling the system. Further, combining the proposed filter with frequency multiplexing potentially enables both control and measurement of several qubits using a single Purcell-filtered transmission line. This will enhance the scalability of superconducting quantum processors by decreasing the number of the required transmission lines.
We develop a small-signal measurement system on cryogen-free dilution refrigerator which is suitable for superconducting qubit studies. Cryogen-free refrigerators have several advantages such as less manpower for system operation and large sample space for experiment, but concern remains about whether the noise introduced by the coldhead can be made sufficiently low. In this work, we demonstrate some effective approaches of acoustic isolation to reduce the noise impact. The electronic circuit that includes the current, voltage, and microwave lines for qubit coherent state measurement is described. For the current and voltage lines designed to have a low pass of dc-100 kHz, we show that the measurements of Josephson junction's switching current distribution with a width down to 1 nA, and quantum coherent Rabi oscillation and Ramsey interference of the superconducting qubit can be successfully performed.
Superconducting qubits may suffer, despite extensive shielding, from interactions with residual noise photons arriving through signal lines to the device, which leads to qubit decoherence. To mitigate the photon population at the qubit frequency due to thermal radiation, attenuators can thermalize the electromagnetic environment at the sample stage. However, to prevent quasiparticle poisoning caused by Cooper pair breaking due to IR radiation, additional filtering is required. Such IR filters are routinely achieved using Eccosorb, a commercially available castable dielectric. We manufacture Eccosorb filters of various length and grade and characterize them at room and liquid nitrogen temperatures up to 67 GHz. Based on the obtained frequency dependent attenuation, we compute the residual noise photon population at base temperature and conclude on the preferred filter and attenuator configuration for superconducting qubit experiments. We validate the suggested microwave filtering for drive and flux bias lines by comparing coherent properties of transmon qubits for different filter configurations.
In superconducting qubit measurements, stray infrared photons lead to damping processes that degrade quantum coherence. In this Letter, we show that a thermal blocking filter made of multiwalled carbon nanotubes diluted in stainless steel powder can significantly improve the energy relaxation time, T1, and the pure dephasing time, Tφ, of a qubit. By using two independent measurement lines, with and without the filter, and switching between them in situ, we observe that with the filter there is an increase of more than 61% in T1 and 291% in Tφ. We characterize the filter, demonstrating that the scattering parameters remain stable down to 8 mK over a wide range of frequencies, spanning from 10 MHz to 50 GHz. We also show that the cutoff frequency of the filter is easily controlled by selecting the concentration of nanotubes.
Electromagnetic filtering is essential for the coherent control, operation and readout of superconducting quantum circuits at milliKelvin temperatures. The suppression of spurious modes around transition frequencies of a few GHz is well understood and mainly achieved by on-chip and package considerations. Noise photons of higher frequencies - beyond the pair-breaking energies - cause decoherence and require spectral engineering before reaching the packaged quantum chip. The external wires that pass into the refrigerator and go down to the quantum circuit provide a direct path for these photons. This article contains quantitative analysis and experimental data for the noise photon flux through coaxial, filtered wiring. The attenuation of the coaxial cable at room temperature and the noise photon flux estimates for typical wiring configurations are provided. Compact cryogenic microwave low-pass filters with CR-110 and Esorb-230 absorptive dielectric fillings are presented along with experimental data at room and cryogenic temperatures up to 70 GHz. Filter cut-off frequencies between 1 to 10 GHz are set by the filter length, and the roll-off is material dependent. The relative dielectric permittivity and magnetic permeability for the Esorb-230 material in the pair-breaking frequency range of 75 to 110 GHz are measured, and the filter properties in this frequency range are calculated. The estimated dramatic suppression of the noise photon flux due to the filter proves its usefulness for experiments with superconducting quantum systems.
Quantum computers are expected to surpass current modern computers in certain types of computations by utilizing quantum mechanical properties of atoms, molecules, and photons. Among the various systems being explored for realizing such computers, superconducting qubits have enabled many pioneering experimental demonstrations due to their ability to perform nonlinear operations and be integrated using semiconductor fabrication technologies. These quantum computers require operation at extremely low temperatures, around 10 mK, using dilution refrigerators. A major challenge at these temperatures is the heat inflow caused by wiring used to control and read out the qubits. This study reports on the development of a transmission line that can operate at low temperatures and achieves both low thermal inflow and low transmission loss in the 5 GHz band, which is used for qubit readout. Here we focus on the scalability of the transmission line and design a parallel set of transmission lines with low crosstalk.