Tyler O'Neal, Staff Editor INDUSTRY

Osaka Metropolitan University improves quantum algorithm performance

Osaka Metropolitan University in Japan addresses issues with the quantum simulation of time evolution found in the conventional quantum phase estimation algorithm by applying its recently developed Bayesian phase difference estimation quantum algorithm to perform full-CI calculations free from controlled time evolutions in quantum simulations and chemical calculations. Overseas Press Release Figure 34359

A team of researchers at the Osaka Metropolitan University Graduate School of Science continue to improve their recently developed quantum algorithm, this time by applying it to calculate the potential energy curves of the H2 molecule without the need for controlled time evolutions. By including a controlled-state preparatory step to the Bayesian phase difference estimation (BPDE) quantum algorithm they previously developed, the team calculated the superposition of the “vacuum” wave function with zero electrons and the wave function of the desired electronic state, bypassing the need to simulate the time evolution of the wave function conditional on an ancillary qubit. This addresses an issue common to conventional quantum algorithms, namely the parallel processing of quantum gates and a high number of quantum gates between two non-neighboring qubits – demonstrating itself as a quantum algorithm executable on quantum supercomputers that can perform full configuration interaction (full-CI) calculations of atoms and molecules. Note that the full-CI calculations are capable of giving the optimized solutions of Schrödinger equations for microscopic systems, but not intractable for the sizable systems with classical computers because of the exponential explosion of required computing time.

Their research was published in The Journal of Physical Chemistry Letters as an open-access article; https://doi.org/10.1021/acs.jpclett.1c03214

In the race to accurately solve the Schrödinger equation to show the electronic state of an atom or molecule, bringing a paradigm shift in scientific research and materials development, scientists have turned to quantum supercomputers to carry out chemical calculations in polynomial time. The quantum phase estimation (QPE) algorithm has become a well-known and powerful tool in giving full-CI calculations of wave functions for small molecules, and various attempts have been made to have QPE-based methods account for the computational costs that exponentially accumulate against the system size under study.

“QPE-based methods simulate the time evolution of a wave function on an ancillary qubit, which requires many controlled quantum gates, the presence of which also hinders parallel execution of these gates and compression of quantum circuits,” states lead author, Specially-Appointed Lecturer Kenji Sugisaki. “In this research, we have applied our Bayesian phase difference estimation (BPDE) algorithm, which is a modification of QPE and capable of bypassing the need of controlled time evolution operations, to perform full-CI calculations. We emphasize that our approach invokes the energy calculation of many-electron systems as referring to the counterpart of many-electron ionized systems.”

“Evolving the wave function only when the first qubit is in the |1⟩ state makes it difficult to parallelize the quantum gates,“ says research advisor, Professor Kazunobu Sato. “Rewriting the quantum logic circuit so that the time evolution operator can be applied regardless of whether the first qubit is in the |0⟩ or |1⟩ state would allow easy parallel processing of quantum gates and increase the likelihood of implementing the algorithm in actual quantum computers.”

To do this, the team introduced a controlled-state preparation that constructs the quantum superposition of the “vacuum” wave function |vac⟩ with zero electrons and the wave function |Ψ⟩ of the target electronic state – “in other words, we computed the full-CI energy of an atom or molecule as its ionization energy,” explains Professor Emeritus Takeji Takui – with the following logic circuit (|0⟩|vac⟩+|1⟩|Ψ⟩)⁄√2.  The team exemplified the efficiency of their BPDE-based full-CI method by calculating the potential energy curves of four valence electronic states of the H2 molecule. “The numerical simulations revealed that the BPDE-based method reproduces the full-CI energy within 3 kcal mol-1 of errors for all the electronic states”, continues Prof. Takui, who also served as an advisor to the study.

Importantly, being free from controlled time evolution makes parallelization of quantum gates and implementation on real quantum devices easier, which gives the team hopes that their Bayesian phase difference estimation algorithm paves the way for more practical full-CI calculations and becomes synonymous with precise quantum chemistry.

Finnish physicists show how new artificial material mimics quantum entangled rare-earth compounds

Tyler O'Neal, Staff Editor INDUSTRY

By combining two-dimensional materials, researchers create a macroscopic quantum entangled state emulating rare-earth compounds

Physicists at Aalto University in Espoo, Finland have created a new ultra-thin two-layer material with quantum properties that normally require rare earth compounds. This material, which is relatively easy to make and does not contain rare earth metals, could provide a new platform for quantum supercomputing and advance research into unconventional superconductivity and quantum criticality.

The researchers showed that by starting from seemingly common materials, a radically new quantum state of matter can appear. The discovery emerged from their efforts to create a quantum spin liquid which they could use to investigate emergent quantum phenomena such as gauge theory. This involves fabricating a single layer of atomically thin tantalum disulfide, but the process also creates islands that consist of two layers. Physicists have created a new ultra-thin two-layer material with quantum properties that normally require rare earth compounds. CREDIT Heikka Valja

When the team examined these islands, they found that interactions between the two layers induced a phenomenon known as the Kondo effect, leading to a macroscopically entangled state of matter producing a heavy-fermion system.

The Kondo effect is an interaction between magnetic impurities and electrons that causes a material’s electrical resistance to change with temperature. This results in the electrons behaving as though they have more mass, leading these compounds to be called heavy fermion materials. This phenomenon is a hallmark of materials containing rare earth elements.

Heavy fermion materials are important in several domains of cutting-edge physics, including research into quantum materials. "Studying complex quantum materials is hindered by the properties of naturally occurring compounds. Our goal is to produce artificial designer materials that can be readily tuned and controlled externally to expand the range of exotic phenomena that can be realized in the lab," says Professor Peter Liljeroth.

For example, heavy fermion materials could act as topological superconductors, which could be useful for building qubits that are more robust to noise and perturbation from the environment, reducing error rates in quantum computers. "Creating this in real life would benefit enormously from having a heavy-fermion material system that can be readily incorporated into electrical devices and tuned externally,’ explains Viliam Vaňo, a doctoral student in Liljeroth’s group and the paper’s lead author. 

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Although both layers in the new material are tantalum sulfide, there are subtle but important differences in their properties. One layer behaves like a metal, conducting electrons, while the other layer has a structural change that causes electrons to be localized into a regular lattice. The combination of the two results in the appearance of heavy fermion physics, which neither layer exhibits alone.

This new heavy fermion material also offers a powerful tool for probing quantum criticality. "The material can reach a quantum-critical point when it begins to move from one collective quantum state to another, for example, from a regular magnet towards an entangled heavy-fermion material," explains Professor Jose Lado. "Between these states, the entire system is critical, reacting strongly to the slightest change, and providing an ideal platform to engineer even more exotic quantum matter."

"In the future, we will explore how the system reacts to the rotation of each sheet relative to the other and try to modify the coupling between the layers to tune the material towards quantum critical behavior," says Liljeroth.

A new artificial material mimics quantum entangled rare earth compounds

Tyler O'Neal, Staff Editor INDUSTRY
Viliam Vaňo and his colleagues created a new ultra-thin two-layer material with quantum properties that normally require rare earth compounds.This material could improve quantum supercomputers and advance research into superconductivity and quantum criticality.In this interview, Vaňo tells the story of how this discovery was made.
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