First discovery silicon is the main factor limiting the performance of superconducting quantum processors

Silicon is a material used in a wide range of computing applications: including computer chips, circuits, displays and other modern computing devices. Silicon is also used as a substrate, or the basis for quantum computing chips.

However, researchers at the U.S. Department of Energy's Fermi National Accelerator Laboratory's Center for Superconducting Quantum Materials and Systems (SQMS) have demonstrated that silicon substrates can be detrimental to the performance of quantum processors. They measured the effect of silicon on the lifetime of quantum bits with one-billionth accuracy. These findings have been published in the Review of Applied Physics [1]. It was also found that the sapphire used in the Zuchon was a better substrate than silicon.

Silicon is a material used in a wide range of computing applications: including computer chips, circuits, displays and other modern computing devices. Silicon is also used as a substrate, or the basis for quantum computing chips.

However, researchers at the U.S. Department of Energy's Fermi National Accelerator Laboratory's Center for Superconducting Quantum Materials and Systems (SQMS) have demonstrated that silicon substrates can be detrimental to the performance of quantum processors. They measured the effect of silicon on the lifetime of quantum bits with one-billionth accuracy. These findings have been published in the Review of Applied Physics [1]. It was also found that the sapphire used in the Zuchon was a better substrate than silicon.

A quantum processor based on superconductivity, consisting of several thin film materials deposited on top of a silicon substrate. Image credit: Rigetti Computing

01Factors affecting the lifetime of superconducting quantum bits

Quantum computers offer a new approach to computing based on quantum mechanics. These devices can perform calculations that would take traditional computers years to complete or are practically impossible to perform.

Using the power of quantum mechanics, a quantum bit - the fundamental unit of quantum information held in a quantum computing chip - can be both a 1 and a 0. Processing and storing information in a quantum bit is challenging and requires a well-controlled environment. Tiny environmental disturbances or defects in the quantum bit material can corrupt the information.

Quantum bits require near-perfect conditions to maintain the integrity of their quantum state, but certain material properties can shorten the lifetime of a quantum bit. This phenomenon is known as quantum decoherence and is a key hurdle to overcome to operate quantum processors.

The first step in reducing or eliminating quantum decoherence is to understand its root cause. scientists at the SQMS Center are studying a widely used type of quantum bit - transmon. it is made of several layers of different materials with unique properties. Each layer and each interface between these layers plays an important role in quantum decoherence. They create "traps" where microwave photons - the key to storing and processing quantum information - can be absorbed and disappear.

Based on measurements of quantum bits alone, researchers cannot clearly distinguish where the traps are located or which of the various materials or interfaces are driving decoherence. scientists at the SQMS Center use uniquely sensitive tools to study these effects in the materials that make up the transmon quantum bits.

Alexander Romanenko, Chief Technical Officer, Head of the Applied Physics and Superconductivity Division at Fermilab and Head of the Technical Thrust at the SQMS Center, said [2]: "We are decomposing the system to understand how a single factor affects the decoherence of quantum bits. A few years ago, we realized that our (superconducting RF) cavities could be used as a tool to evaluate microwave losses in these materials with an accuracy of more than one part per billion."

A scientist demonstrates the process of assembling the silicon samples used in the study. Image credit: SQMS Center

Researchers typically use the same technique to make silicon-based microelectronic devices, placing quantum bits on a silicon substrate. Sapphire is therefore rarely used for quantum computing.

It took years of materials science and device physics research to develop the niobium material specifications to ensure the continued high performance of the SRF cavity," Romanenko said. Similarly, similar research is needed for materials that contain superconducting quantum bits. This work includes collaboration between researchers and material industry suppliers."

Regardless of the material used for quantum bits, eliminating loss and increasing coherence time are critical to the success of quantum computing. No one material is perfect. Through rigorous testing and research, researchers are building a more comprehensive understanding of the materials and properties that are best suited for quantum computing.

This loss-angle tangent measurement is a substantial step forward in the search for the best materials for quantum computing. scientists at the SQMS Center have derived a question that can now explore whether finer silicon or sapphire can harness the computing power of quantum bits.

Reference links：

[1]https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.18.034013

[2]https://news.fnal.gov/2022/09/new-measurements-point-to-silicon-as-a-major-contributor-to-performance-limitations-in-superconducting-quantum-processors/