There have been many new developments in quantum computing during the last few years, but last Sunday a paper appeared in *Nature Chemistry* that shows how far the area really has come. It seems that now things are getting really interesting: American and Australian scientists just built a quantum circuit that calculated the energy Eigenvalues of molecular hydrogen to an accuracy of 1 KJ/mol.

Of course we knew these Eigenvalues before, they can be calculated e.g. with DFT calculations. The amazing thing is the quantum circuit part. Quantum computers existed only in theory. Until now.

A Quantum computer is a device that uses Qubits, which don’t have either the values 0 or 1 but a superposition of both, represented by a vector. Hence, the logic gates in a quantum computer don’t simply switch those values, but they rotate the vector. Because of this fact quantum algorithms are a lot more efficient than their classical equivalents. The probably best-known example is the Shor-Algorithm for integer factorization.

Chemists, however, will most probably be among the first to use this brand-new technology, for a very simple reason. Simulating molecules apparently is what quantum computers do best and easiest. Every conceivable Hamiltonian can be represented as an – often very simple – combination of quantum logic gates, the building blocks of a quantum computer.

And that’s what the Authors did: They built the experiment, encoded the state of the hydrogen wave function in 20 Qubit and fed the data through the circuit. The photons of the experiments simulate the exact evolution of the wave function, so from the output bits the Eigenvalue could be directly calculated.

Of course this is only the first step, since a part of the quantum calculation still had to be done with normal computers and the overall setup is not scalable, but that is probably just a matter of time.

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Lanyon, B., Whitfield, J., Gillett, G., Goggin, M., Almeida, M., Kassal, I., Biamonte, J., Mohseni, M., Powell, B., Barbieri, M., Aspuru-Guzik, A., & White, A. (2010). Towards quantum chemistry on a quantum computer Nature Chemistry, 2 (2), 106-111 DOI: 10.1038/nchem.483

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