Quantum information science promises to fundamentally change the way we do things, not unlike how classical information science continues to change every aspect of our daily lives. Classical information science teaches us how difficult it is to break a cipher, or how long it will take a computer to do a calculation; quantum information science predicts fundamentally secure cryptography, and computers that solve certain problems faster than any conceivable classical machine.
At first glance, it is surprising to think that randomness can actually help perform information processing tasks, and yet it can: for example, a random cryptographic key is known to be the best way to hide messages; more surprisingly, there exist problems where, rather than execute a deterministic algorithm as classical computers normally do, it is better to guess -- that is, invoke randomness -- while computing a solution. Thus we say that randomness is a resource in classical information theory; having a coin at hand that one can flip is a tangible asset. This is especially true when one wants to test a complex device or process: send it random inputs, and investigate how the outputs behave.
We can also purposely introduce randomness into quantum information protocols and ask if this can make certain tasks easier. It turns out the answer is also yes, giving rise to the study, for example, of random quantum circuits, or random quantum error correcting codes. In the formalism of quantum mechanics these are expressed as random operators, rather than simple random numbers, but they can be thought of as resources for quantum information science in much the same way as in the classical case.
However, both classically and quantumly, generating truly random resources is very difficult; one can imagine trying to encrypt terabytes of information by flipping a coin billions of times. In practice we rely on so-called pseudorandom resources that, given a finite amount of time or computing power, can never be distinguished from truly random. If we think of increasingly complex tests one might do to check for randomness, a pseudorandom resource will pass these tests up to a certain level of complexity (and fail beyond that). Such resources are much easier to create than truly random ones, and pseudorandom number generators are a cornerstone of today's information technologies.
This research project aims to make pseudorandom resources available to quantum information technologies. In the quantum realm, the notion of pseudorandomness is captured by what are called quantum 't-designs'. These are resources -- ensembles of quantum operators -- that pass randomness tests up to some level of complexity (more precisely, t corresponds to the degree of a statistical moment). The project has two main components; the first will be a systematic study of the mathematical structure of t-designs, finding new ones along the way, and then optimising these resources for specific quantum technologies; at the University of Bristol a technology we focus on is integrated quantum photonics, and so the second part of this project will be to use our theoretical work to propose and perform quantum photonic experiments that demonstrate quantum pseudorandomness.
Quantum technology is in its infancy, and this research will be an important early step in understanding and solving the problem of efficiently producing the randomness that is crucial to information science. In the short term, the results will be used to tackle challenging problems such as finding the best way to characterise increasingly complex quantum devices, like the ones being developed by hundreds of partners in the UK Quantum Technology Network. In the longer term, it will enable customised, plug-in pseudorandom resources for any quantum platform, which will be used in a multitude of future quantum information applications.
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