Study explores physical origin of errors in spin qubit processor

To achieve remarkable performance, quantum computing systems based on multiple qubits must achieve high-fidelity entanglement between their underlying qubits. Previous studies have shown that solid-state quantum platforms (quantum computing systems based on solid materials) are highly error-prone, which can harm the coherence between qubits and their overall performance.

Researchers from Diraq, a Sydney-based quantum hardware technology company, and UNSW recently set out to study errors appearing in a spin qubit processor composed of high-fidelity two-qubit gates in quantum dots of silicon.

Their findings, published in Natural physicsprovide new insight into the possibilities and challenges associated with scaling silicon spin-based qubits.

“The 21st century revolved around computerization through the semiconductor silicon chip,” Andrew Dzurak and Tuomo Tanttu, co-authors of the paper, told Phys.org.

“The humble silicon chip is essential to the digitalization and interconnectivity of our daily lives: from the phones we use, the computers we rely on, and the systems we operate to access information, share data and effectively manage infrastructure and decision-making tools. necessary for modern, thriving, interconnected economies.

“This demand has supported the growth of the complementary metal oxide semiconductor (CMOS) manufacturing industry.”

In recent years, some quantum engineers and physicists have explored the possibility of developing quantum technologies using well-established silicon-based transistors. Indeed, the manufacturing processes for these transistors are well established and highly controlled.

“Advanced manufacturing processes are at the heart of Diraq’s vision to build the world’s first fault-tolerant quantum computer based on silicon quantum dots,” said Dzurak, CEO and founder of Diraq. “Our core intellectual property lies in the design and operation of silicon spin qubits compatible with CMOS foundry manufacturing.”

Dzurak, Tanttu and their colleagues at Diraq explored ways to exploit lithographic techniques used to make silicon transistors to make quantum technologies. Researchers have recently specifically explored the potential for advanced techniques for encoding qubits in semiconductor spin carriers that could be fabricated and integrated at scale.

“Quantum computing requires many qubits that can be coherently controlled and coupled to each other,” Dzurak said. “We set ourselves the challenge of understanding whether it is possible to run high-fidelity (greater than 99%) entangled gates between qubits in the same silicon platform used for silicon transistors. The research s ‘is expanded to also understand which noise sources degrade the fidelity of the tangled gate.

By exploring the physical origin of errors in silicon-based quantum processors, Dzurak, Tanttu and their colleagues plan to contribute to the development of quantum computers built on widely available materials. In their study, they used three key diagnostic techniques to evaluate and characterize quantum states and logic gates in a silicon-based spin qubit device.

“We wanted a comprehensive methodology, so we chose three different robust techniques to support our investigation into monitoring and measuring sustainable performance,” explained Tanttu. “The three measurement techniques we used are interleaved randomized comparative analysis (IRB), gridded tomography (GST), and fast Bayesian tomography (FBT).”

All of the techniques employed by the researchers involve running a set of specific logic circuits in a quantum processor. By applying them to their processor, they collected information on the physics underlying its operation and therefore any errors produced.

“These diagnostic tools are critical to the development of quantum devices because they are key to improving system reliability and quantum logic operations, also known as logic gates,” Tanttu said.

“For example, with GST, the results indicate how to prevent errors instead of correcting them. Both tomography methods gave a detailed picture of the noise channels. In contrast, the more traditional IRB method gave a single fidelity number without additional physical information.

Dzurak, Tanttu and their colleagues also identified a promising method for extracting rich information from data connected via IRB. Specifically, they reanalyzed the IRB data by running an FBT analysis on the exact same data set.

“This provided information about the noise channels and how they changed over time,” Tanttu explained. “In summary, the combination of these different techniques allowed us to build a more complete physical image of our system and to better compare the performance of our qubit.”

The tests carried out by this research team led to two important results. First, they demonstrated the ability to achieve high operational fidelities, above 99%, for the entangled gates of their platform.

Second, they were able to identify the sources of noise in their tangled doors. This allowed them to design strategies to improve the fidelity of their gates.

“Our results imply the viability of using silicon transistor technology as a building block for manufacturing future large-scale quantum computers,” Dzurak said. “In practice, this also means that we can focus more on developing the platform.”

The Diraq team is currently working on improving and testing its spin qubit processor. For example, they plan to repeat their experiment beyond university laboratory devices, using spin processors manufactured in a semiconductor foundry.

“In addition, we think it is very interesting to explore whether understanding the underlying errors can be used to further improve the fidelity of the operation with improved materials or different control methods,” Dzurak added.

“With our operational fidelities closely aligned with those of other semiconductor platforms, we look forward to further advancements, including increasing the number of qubits in a single chip.”

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