Methods towards photonic quantum error detection and correction

What would computer pioneers Mauchly and Eckert's reaction be if they could take a glimpse at life in 2021? In 1945 they built ENIAC, the first general-purpose digital computer-which took up an entire room. That processing power is now completely eclipsed by devices that can fit on our wrist! Despite tremendous progress over the past 50 years, a new paradigm is on the horizon which will shatter the way we think about information and deliver new computing technologies limited only by nature herself. Laptops, tablets, and phones-devices we rely on every day-consist of billions of logic gates forming the building blocks of processors. They process binary data (0s and 1s) in a meaningful way. While serving us very well so far, certain tasks easily push current classical technology to its absolute limit. Examples include solving hard equations, big-data analysis, simulating complex molecules for drug discovery and predicting real-time changes in weather and financial markets. These all require serious computational grunt and in some cases are impossible to complete even with the fastest classical supercomputers but are made possible with quantum computers. The first quantum revolution occurred when physicists discovered that the laws of nature affect atomic-scale objects differently compared to everyday objects we interact with. This led scientists to propose the coming of a second quantum revolution, one which harnesses the properties of electrons, atoms, and photons-single particles of light-to create powerful new technologies. The allure of quantum computers is the unparalleled processing power that they could provide. They use quirky effects such as superposition, where a particle can be in two places at once, and entanglement where observing one particle can provide information about another, no matter how far apart they are. We can see these counterintuitive but very real effects in the properties of photons such as their position, time of arrival, frequency, or polarisation and define special quantum bits or qubits. With their ability to travel fast and far, and well-established techniques to create, manipulate and detect them, photons make a favourable choice for qubits. Qubits are however much more fragile than their classical counterparts and quantum information can easily be corrupted by loss or by improper preparation or manipulation of photons. Qubits are manipulated by circuits-much like our everyday computer. These errors can quickly stack up for large numbers of qubits and operations which are often required for tackling difficult computational tasks. I will investigate strategies to detect and correct errors in photonic quantum processors. Much like electrical circuits in classical computers, I will use chips containing circuits, only these circuits carry optical signals in the form of qubits encoded onto pulses of light. I will use chips designed to limit loss together with a new novel device designed to improve the collection of the light. Errors arising from faults in the circuitry can be detected using multiple copies of the circuit with the effect of routing these errors to specific parts of the chip where they can be discarded. Additionally, I will engineer exotic states of light called 'GKP' states (after inventors Gottesman, Kitaev and Preskill) which can inherently cope with a broad class of errors and serve as a key building block towards creating scalable universal fault-tolerant quantum computers. While GKP states have been known for almost 20 years, limitations in photonic hardware have prohibited their creation in the lab. However, recent innovations in light sources, circuits, and detectors by my group together with new theoretical insight by physicists around the world bring us within grasp of these important quantum states of light. These routes to handling errors will be vital to realising powerful photonic quantum technologies and will spur further innovations in the near future.