Quantum computer calculates exact energy of molecular hydrogen

Jan 10, 2010
Image source: The Modern Green

In an important first for a promising new technology, scientists have used a quantum computer to calculate the precise energy of molecular hydrogen. This groundbreaking approach to molecular simulations could have profound implications not just for quantum chemistry, but also for a range of fields from cryptography to materials science.

"One of the most important problems for many theoretical chemists is how to execute exact simulations of chemical systems," says author Alán Aspuru-Guzik, assistant professor of chemistry and chemical biology at Harvard University. "This is the first time that a quantum computer has been built to provide these precise calculations."

The work, described this week in Nature Chemistry, comes from a partnership between Aspuru-Guzik's team of theoretical chemists at Harvard and a group of experimental physicists led by Andrew White at the University of Queensland in Brisbane, Australia. Aspuru-Guzik's team coordinated experimental design and performed key calculations, while his partners in Australia assembled the physical "computer" and ran the experiments.

"We were the software guys," says Aspuru-Guzik, "and they were the hardware guys."

While modern supercomputers can perform approximate simulations of simple molecular systems, increasing the size of the system results in an exponential increase in computation time. Quantum computing has been heralded for its potential to solve certain types of problems that are impossible for conventional computers to crack.

Rather than using binary bits labeled as "zero" and "one" to encode data, as in a conventional computer, stores information in qubits, which can represent both "zero" and "one" simultaneously. When a quantum computer is put to work on a problem, it considers all possible answers by simultaneously arranging its qubits into every combination of "zeroes" and "ones."

Since one sequence of qubits can represent many different numbers, a quantum computer would make far fewer computations than a conventional one in solving some problems. After the computer's work is done, a measurement of its qubits provides the answer.

"Because classical computers don't scale efficiently, if you simulate anything larger than four or five atoms -- for example, a chemical reaction, or even a moderately complex molecule -- it becomes an intractable problem very quickly," says author James Whitfield, research assistant in chemistry and chemical biology at Harvard. "Approximate computations of such systems are usually the best chemists can do."

Aspuru-Guzik and his colleagues confronted this problem with a conceptually elegant idea.

"If it is computationally too complex to simulate a quantum system using a classical computer," he says, "why not simulate quantum systems with another quantum system?"

Such an approach could, in theory, result in highly precise calculations while using a fraction the resources of conventional computing.

While a number of other physical systems could serve as a computer framework, Aspuru-Guzik's colleagues in Australia used the information encoded in two entangled photons to conduct their hydrogen molecule simulations. Each calculated energy level was the result of 20 such quantum measurements, resulting in a highly precise measurement of each geometric state of .

"This approach to computation represents an entirely new way of providing exact solutions to a range of problems for which the conventional wisdom is that approximation is the only possibility," says Aspuru-Guzik.

Ultimately, the same quantum computer that could transform Internet could also calculate the lowest energy conformations of molecules as complex as cholesterol.

Explore further: 3,000 atoms entangled with a single photon

More information: Nature Chemistry paper: dx.doi.org/10.1038/NCHEM.483

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Jan 10, 2010
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2.7 / 5 (3) Jan 10, 2010
Can it calculate the post synaptic potential energy in neural synapses? Maybe with cloud computing, the average guy can borrow that quantum computer?
2.3 / 5 (3) Jan 11, 2010
the information encoded in two entangled photons
A bit far-fetched for a "quantum computer".
But certainly attention arousing.
3 / 5 (5) Jan 11, 2010
A pity that the "information encoded in two entangled photons" is not more adequately described - the heuristic association between this entanglement and the derivation of analytical results is left in a condition that approaches mysticism, appears more like a belief system than a quantified methodological approach. Which photons these are, how they obtained their emission energy and the relation between their energy levels and any quantum-state medium they may have passed through, are associated with, or which has modulated their phase-states, is unaddressed. Presumably the qubit state information was programmed into these photons as amounting to a modelling approximation of the hydrogen molecule, is therefore axiomatically subject to (Bergson) falsification itself and has nothing to do with quantum measurement of actual molecular hydrogen. There is something simplistic about this, as a few hundred qubits could model the entire universe but take billions of years to derive analysis.
not rated yet Jan 11, 2010
Can it calculate the post synaptic potential energy in neural synapses? Maybe with cloud computing, the average guy can borrow that quantum computer?

Like timesharing on the old mainframes. Wouldn't that be nice? (until a PC version is built)

Thing is, QC has been "going to happen tomorrow" for so long, I am afraid to get hopeful that a general purpose Quantum Computer can even be prototyped in the next 20 years.
5 / 5 (1) Jan 11, 2010
Solving the energy levels for H2+ is a three-body problem. Solving H2 is a four-body problem. Since nobody has ever done an analytical solution of a three-body problem, let alone a four-body problem, I wonder if this is supposed to be an analytical solution, or just a very fancy approximation?
not rated yet Jan 11, 2010
I wonder if this is supposed to be an analytical solution, or just a very fancy approximation?

"This approach to computation represents an entirely new way of providing exact solutions to a range of problems for which the conventional wisdom is that approximation is the only possibility," says Aspuru-Guzik.
not rated yet Jan 12, 2010
One thing with quantum computing I still can't seem to figure out is if the CPU would actually use electrons (or photons) as the ACTUAL QUBITS? I would think it would have to use them to make the idea work since this whole idea relies on super position of particles, but how would the electrons be stored for any length of time and not dissipate into heat? Wouldn't quantum computers have to used super cooled conductors? Don't electrons still dissipate in super cooled conductors (although obviously at a much lesser rate)? Would a quantum computer need to be "fueled" with CPU particles after a while?

My terminology may be off, but I am hoping someone can make sense of what I am asking and maybe explain it to me. I read up on this a lot but I can't really find articles that are too specific, probably because the person writing the article doesn't understand it..

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