New Algorithm Removes Spin Contaminants from Quantum Computers

A group of scientists from Osaka City University have developed a new quantum algorithm that eliminates pesky spin contaminants from chemical calculations on quantum computers.

A quantum circuit on quantum computers for a probabilistic spin clean-up/annihilation method. If the measurement outcome in the quantum circuit is the |0⟩ state, the spin contaminated wave function |ΨCont⟩ is projected out onto the spin annihilated one |ΨAnni⟩. The rightmost part of the top line denotes the measurement. Image Credit: © Kenji Sugisaki, Kazunobu Sato, and Takeji Takui, Osaka City University.

This algorithm can help predict molecular and electronic behavior with levels of precision that otherwise cannot be achieved with traditional computers and brings practical quantum computers closer to reality.

Overview

Quantum computers have become the new frontier in next-generation research technology, with a number of promising applications, like protecting financial assets, performing crucial calculations, or estimating molecular behavior in pharmaceuticals.

At Osaka City University, scientists have now solved a crucial problem that prevents the practical application of large-scale quantum computers—that is, accurate and precise predictions of molecular and atomic behavior. The new method eliminates superfluous data from quantum chemical calculations.

The researchers have recently published their new technique as an advanced online article in Physical Chemistry Chemical Physics—a journal of the Royal Society of Chemistryon September 17th, 2020.

One of the most anticipated applications of quantum computers is electronic structure simulations of atoms and molecules.

Kenji Sugisaki, Paper Author and Lecturer; and Takeji Takui, Paper Author and Professor Emeritus, Department of Chemistry and Molecular Materials Science, Graduate School of Science, Osaka City University

Quantum chemical calculations are pervasive across scientific disciplines, such as materials research and pharmaceutical therapy development. The entire calculations are based on solving the equation of physicist Erwin Schrödinger. This equation utilizes molecular and electronic interactions that lead to a specific property to explain the state of a quantum-mechanical system.

Schrödinger equations govern any behavior of electrons in molecules, including all chemical properties of molecules and materials, including chemical reactions.

Kenji Sugisaki, Paper Author and Lecturer; and Takeji Takui, Paper Author and Professor Emeritus, Department of Chemistry and Molecular Materials Science, Graduate School of Science, Osaka City University

Such accurate equations would take a considerable amount of time on traditional computers. While this accuracy on quantum computers can be achieved in realistic time, it needs “cleaning” at the time of calculations to achieve the actual nature of the system, believe the researchers.

A quantum system at a particular moment in time, called a wave function, exhibits a property explained as spin, which represents the total of the spin of every electron in the system.

Owing to mathematical errors or hardware faults, incorrect spins might be informing the spin calculation of the system. Hence, to eliminate these “spin contaminants,” the team implemented a new quantum algorithm that enables them to choose the required spin quantum number.

This cleanses the spin, eliminating contaminants at the time of each calculation, which according to the researchers, is a first on quantum computers.

Quantum chemical calculations based on exactly solving Schrödinger equations for any behavior of atoms and molecules can afford predictions of their physical-chemical properties and complete interpretations on chemical reactions and processes,” the researchers added, noting that this cannot be achieved with presently available traditional algorithms and computers.

The present paper has given a solution by implementing a quantum algorithm on quantum computers.

Kenji Sugisaki, Paper Author and Lecturer; and Takeji Takui, Paper Author and Professor Emeritus, Department of Chemistry and Molecular Materials Science, Graduate School of Science, Osaka City University

The next plan of the researchers is to create and apply algorithms that are specifically developed to establish the state of electrons in molecules with the same level of precision for both ground and excited-state electrons.

The study was funded by the AOARD Scientific Project on “Molecular Spins for Quantum Technologies” (Grant No. FA2386-17-1-4040, 4041), USA, and by KAKENHI Scientific Research B (Grant No. 17H03012) and Scientific Research C (Grant No. 18K03465) from JSPS, Japan, and PRESTO project “Quantum Software" (Grant No. JPMJPR1914) from JST, Japan.

Journal Reference:

Sugisaki, K., et al. (2020) A probabilistic spin annihilation method for quantum chemical calculations on quantum computers. Physical Chemistry Chemical Physics. doi.org/10.1039/d0cp03745a.

Source: https://www.osaka-cu.ac.jp/en/

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