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Tiny Quantum Computer Simulates Complex Molecules
Someday , engineers will build large quantum computers that can solve currently impossible science problems , crack unbreakable encryption , and make artificial intelligence smarter . In the meantime , companies building quantum computers are trying to figure out how to use the small ones they expect to build in the coming years . Decades of theoretical work suggest that quantum computers — perhaps even relatively small ones — will someday be able to solve important problems in chemistry that are intractable on existing computers . But before they can take on big challenges like understanding photosynthesis and improving catalysts for making renewable fuels , researchers have begun simulating small molecules and atoms . And so far , they haven ’ t gone far beyond what a math-savvy chemist can do with a pen and paper .
This week in the journal Nature , researchers at IBM describe using a small quantum computer to simulate more complex molecules . The IBM team used six of the quantum bits ( qubits ) on a seven-qubit system to push into the second row of the periodic table , simulating molecules as large as beryllium hydride ( BeH2 ). What ’ s significant , says Jerry Chow , manager of experimental computing at IBM research , is how they did it : by developing more sophisticated algorithms that could carry out the simulations on a small , noisy quantum computer . Beryllium hydride is easy to simulate on a classical computer . It ’ s the kind of thing theoretical chemists like Markus Reiher call a “ toy problem .” But this kind of work must be done in order to make useful quantum computers “ that can solve chemical problems where classical computation reaches its limits ,” says Reiher , who ’ s based at ETH Zurich . On a classical computer , the difficulty of chemical simulations scales exponentially with the size of the problem . To do useful work , researchers make approximations , which works much of the time . But sometimes , quantum effects that are impossible to simulate are key to understanding the chemistry . Quantum computers can represent quantum states such as the energy levels of electrons more naturally . So if companies can make a large one , they should be able to solve chemistry problems more exactly — and tackle problems that are currently impossible . When will quantum computers be able to solve a useful problem that classical computers can ’ t ? We don ’ t know the answer yet . Today ’ s quantum computers have fewer than 20 qubits . The more qubits engineers add , the more noise there is in the system , and the harder it becomes to do anything useful . Google has announced that its team is working on a 49-qubit system that should outperform a classical computer on some ( probably not practically useful ) task — a milestone called quantum supremacy . Some chemists suspect that solving important chemistry problems will require hundreds of thousands or millions of qubits , in order to correct errors that arise from noise . Others — including those working at IBM and Google — believe it will be possible to do useful chemistry with fewer than 100 qubits . As they try to build larger systems , the big question for these companies is “ how do you get value from a quantum
computer in the next few years ?” says Jay Gambetta , who works on quantum computing and information theory at IBM Research . The IBM team hopes the chemistry community will help them find out . The company has made a 16-qubit quantum computer accessible over the cloud , and has posted quantum chemistry algorithms researchers can use to simulate small molecules . Here are some of the chemistry problems researchers want to tackle with quantum computers : Making greener fertilizer
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