Quantum states can exhibit bizarre but powerful properties, such as being in a superposition or containing correlations not possible in classical physics. If these properties can be controlled, then they can be exploited in quantum technologies to dramatically transform computing, enable secure cryptography, and unlock new ways of observing the universe. Quantum optics is a particularly fertile field for testing and developing these technologies – but how exactly can we design a quantum optics experiment to produce useful quantum states of light that can be put to good use? The usual methods involve painstaking calculations, clever insights, and utilising knowledge built up from years of experience and careful reading of previous researchers’ work. But the counter-intuitive nature of the quantum world, whilst enabling disruptive new technologies, can make it particularly challenging to design quantum experiments that can engineer useful states – our usual intuitions can fail us here. Indeed, while the current techniques used by researchers have led to a host of impressive and exciting results, we are far from finding the optimal methods to manipulate and control quantum states.
To overcome this I developed a new technique that instead employs computers algorithms to design quantum optics experiments for us1. While computers are not yet creative, and in many tasks can be outsmarted by children, they do have the unique ability to perform millions of calculations per second, and it is this powerful feature of computers that I exploit. Specifically, my algorithm shuffles through different combinations of experimental equipment – such as beam splitters, phase shifters, and non-linear crystals (that “squeeze” the light) – to find arrangements that can produce quantum states of light with specific properties, which can be used for a given task. As with a recent independent project using related techniques by Mario Krenn and Anton Zeilinger2, my computer algorithm found numerous solutions that surpass the previous results in the literature whilst involving surprising experimental arrangements quite different from the human designs.
The picture above is an artist’s impression of the algorithm, named “Tachikoma”. While my first work only found quantum states for making high-precision measurements, future work will find states for a wide range of tasks: highly entangled states, states with a large quantum Fisher information, and the preposterously named zombie cat states and three-headed cat states!
Artwork by Joseph Namara Hollis, josephhollis.com.
- P. A. Knott, New Journal of Physics 18, 073033 (2016) http://iopscience.iop.org/article/10.1088/1367-2630/18/7/073033/meta
- Krenn, Malik, Fickler, Lapkiewicz & Zeilinger, Phys. Rev. Lett. 116 (2016) https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.090405