# Director's Colloquium

## Intriguing chemists and upsetting computer scientists using light and mirrors

**Professor Andrew White**

**Thursday 5 June 2014 12–1pm**

**Leonard Huxley Lecture Theatre**

**Abstract: Lift**

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.

**Abstract: Stuck-in-the-lift**

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 [1].

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 [2]. 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!

[1]. B. P. Lanyon et al., Nature Chemistry 2, 106 (2010).

[2]. M. A. Broome, et al., Science 339, 794 (2013).

**Andrew White** was raised in a Queensland dairy town, before heading south to the big smoke of Brisbane to study chemistry, maths, physics and—during World Expo 88—the effects of alcohol on uni students from around the world. Deciding he wanted to know what the cold felt like, he first moved to Canberra, then Germany—completing his PhD in quantum physics—before moving on to Los Alamos National Labs in New Mexico where he quickly discovered that there is more than enough snow to hide a cactus, but not nearly enough to prevent amusing your friends when you sit down. Over the years he has conducted research on various topics including shrimp eyes, nuclear physics, optical vortices, and quantum computers. He likes quantum weirdness for its own sake, but his current research aims to explore and exploit the full range of quantum behaviours—notably entanglement—with an eye to engineering new technologies and scientific applications

*Please join us for snacks prioir to the Colloquium, 11:30am, in the Oliphant tearoom.*