Undoing quantum operations using integrated photonics technologies

University of Bristol

Among the most timeless science-fiction devices are machines capable of reversing time. In reality, we consider such machines impossible due to the "irreversibility" of many physical phenomena (e.g. a frying egg). However, if a physical phenomenon is "reversible" (e.g. a pendulum), nothing prevents us from reversing it temporally. Or does it?

This assumption is only true if the phenomenon is both reversible and known. In other words, if we know nothing about it except that it is reversible, we cannot do much with this information. This may seem like a minor issue, but it is not in the case of quantum mechanics as quantum systems often undergo unknown evolutions.

Why is it important to be able to time-reverse the evolution of a quantum system? One of the greatest challenges in modern science is to build quantum computers. These are machines that can outperform even today's most powerful supercomputers using the laws of quantum physics to store data and perform calculations. Applications of such devices will span a variety of disciplines, from artificial intelligence to computer security, better batteries, cleaner fertilisers and drug development. Learning how to control the evolution of a quantum system will be crucial to the success of this technology (just think how often, in everyday computer use, we use the 'Undo button' to revert an operation we have just performed).

The goal of my project is to provide the first experimental realisation of quantum circuits that can efficiently reverse an arbitrary unknown quantum operation. To achieve this, I will implement highly complex schemes that rely on a unique and demonstrated feature of the quantum world: quantum teleportation. These schemes will require cutting-edge performance in the generation, processing and detection of quantum information. To this end, I will capitalise on the potential of integrated photonics, one of today’s most promising platforms for developing quantum technologies. This study promises to broaden the wide range of prospective commercial applications of quantum computing, enabling us, for instance, to minimise noise in quantum communication or to reset operations performed by a server whose computation is unknown to the user.