YQI Colloquium - Mohamed Bourennane - Stockholm University

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Title: Quantum Random Access Codes: From Nonclassicality to Applications

Abstract:

A random-access code is a simple communication game. A sender encodes several pieces of information into a single message, and a receiver must decode whichever piece is requested, without the sender knowing which one in advance. Using a quantum system as the message gives a success rate that no classical encoding of the same size can match, even though Holevo’s bound limits how much information the system can carry. This gap makes quantum random access codes (QRACs) both a resource for communication tasks and a sensitive probe of what is genuinely quantum. In this colloquium, I will introduce QRACs and follow them through a series of photonic experiments. We first extend them from qubits to single d-level systems, where the quantum advantage persists in high dimensions. We then compare quantum communication with shared entanglement as resources for random access coding and find that the two are complementary rather than interchangeable. Turning to foundations, I will show how QRAC-type games reveal preparation contextuality, characterise unsharp qubit measurements, and certify measurement incompatibility when several receivers measure in sequence. On the practical side, using more inputs and better post-processing increases the amount of semi-device-independent randomness that can be certified. I will end at the card table. In the bidding phase, partners must convey as much as possible about their hands under strict communication rules. I will conclude with an outlook on where QRACs are heading: as building blocks for quantum networks and communication complexity protocols, as tools for certifying devices and randomness with minimal assumptions, and as a continuing source of insight into the boundary between classical and quantum information.