OSAKA, Japan. Quantum computers have received a lot of attention recently as they are expected to solve specific problems that are beyond the capabilities of conventional computers. The basis of these problems is to determine the electronic states of atoms and molecules so that they can be used more efficiently in a number of industries – from lithium-ion battery designs to *in silico* technologies in drug development. A common way scientists have tackled this problem is by measuring the total energy of the individual states of a molecule or atom and then determining the energy difference between these states. In nature, many molecules grow in size and complexity, and the cost of measuring this stable motion is beyond the capacity of any traditional computer or the establishment of quantum algorithms right now. Thus, it has not been possible to make a theoretical prediction of the total energy if molecules have not been moved and separated from the natural environment.

“To produce quantum computers, its algorithms need to be robust enough to accurately predict the electronic states of atoms and molecules, as they are in nature,” said Kenji Sugisaki and Takeji Takui from Graduate School of Science, Osaka City University.

In December 2020, Sugisaki and Takui, together with their colleagues, led a team of researchers to develop a quantum algorithm called the Bayesian eXchange pair parameter calculator with Broken-symmetry (BxB) wave functions, which tell the electronic states of atoms and molecules by directly calculating the energy differences. They noted that energy differences in atoms and molecules remain stable, no matter how complex and large they are despite the total energy growing as the size of the system. “With BxB, we avoided the common practice of measuring total energy and focused on the energy differences directly, keeping computing costs within polynomial time,” they say. . “Since then, our goal has been to improve the efficiency of our BxB software so that we can predict the electronic devices of atoms and molecules with chemical precision.”

Using the computational costs of a well-known algorithm called Quantum Phase Estimation (QPE) as a benchmark, “we calculated the direct ionization energy of small molecules such as CO, O_{2}, CN, F._{2}, H._{2}O, NH_{3} within 0.1 electron volts (eV) of accuracy, “the team says, using half the number of squares, giving the computational cost equal to QPE.

Their findings will be published online in the March edition of *Letters Journal of Corporate Chemistry*.

Ionization energy is one of the most fundamental physical properties of atoms and molecules and an important signal for understanding the strength and properties of chemical bonds and reactions. In short, correctly predicting the ionization energy will allow us to use chemicals beyond the conventional range. In the past, it was necessary to work out the energy of the neutral and ionized states, but with the quantum BxB algorithm, the ionization energy can be obtained in a single calculation without examining the individual total energy of the neutral states. and ionized. “From numerical simulations of the quantum logic circuit in BxB, we found that the computational cost for reading out the ionization energy is constant regardless of the atomic number or size of the molecule,” the team states, “and that the ionization energy can be obtained with an actual accuracy of 0.1 eV after changing the entire quantum logic cycle to be less than one-tenth of QPE.” (See image for change details)

With the development of quantum computer hardware, Sugisaki and Takui, together with their team, expect the BxB quantum algorithm to perform high-precision energy calculations for large molecules that cannot be handled in real time by conventional computers.

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