“Quantum information is the lovechild of quantum physics and computer science. In this talk we look at the science of quantum information: what it is and isn’t; why biologists, chemists, mathematicians and physicists might be interested in it; and where we are today.” - Prof. Andrew White, 1st August 2014
In principle, quantum mechanics can exactly describe *any* system of quantum particles—from simple molecules to unwieldy proteins—but in practice this is impossible as the number of equations grows exponentially with the number of particles. For example, the fundamental problem faced in quantum chemistry is the calculation of molecular properties, such as total energy of the molecule, which can be calculated by solving the Schrödinger equation. However, the computational resources required increase exponentially with the number of atoms involved. Recognising this, in 1982 Richard Feynman suggested using quantum components for the calculations but it wasn’t until the 1990′s than a quantum algorithm was proposed where the computational resources increased only polynomially in the molecular size. Despite the many different physical architectures that have been explored experimentally since that time—including ions, atoms, superconducting circuits, and photons—this appealing algorithm was not demonstrated until 2010. I will discuss how we have taken advantage of recent advances in photonic quantum computing to present an optical implementation of the smallest quantum chemistry problem: obtaining the energies of H2, the hydrogen molecule, in a minimal basis at up to 47 bits of precision .
The extended Church-Turing thesis posits that any computable function can be calculated efficiently by a probabilistic Turing machine. If this thesis held true, the global effort to build quantum computers might ultimately be unnecessary. The thesis would however be strongly contradicted by a physical device that efficiently performs a task believed to be intractable for classical computers. BosonSampling—the sampling from a distribution of n photons undergoing some linear-optical process—is a recently developed, and experimentally accessible example of such a task . Here we report an experimental verification of one key assumption of BosonSampling: that multi-photon interference amplitudes are given by the permanents of submatrices of a larger unitary describing the photonic circuit. If you don’t understand what that last sentence means, come along to the talk, it’ll be much clearer!
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. M. A. Broome, et al., Science 339, 794 (2013).