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### DDBlock

## Quantum Information Theory

The first quantum revolution in physics (1900-1917) was when quantum principles were first postulated and applied to physical systems in thermal equilibrium, including electromagnetic, atomic, and crystalline systems.

The second quantum revolution (1925-1935) was when the full mathematical formalism was discovered and applied to all of physics (bar gravity), and when many of its weird implications were found, including entanglement, the EPR-steering phenomenon, and the measurement problem.

The years since 1995 have seen a third quantum revolution in physics (and beyond), based on the idea of quantum information. In this period, quantum information theory has predicted that the world will be able to do useful things for us that we never imagined ¬– it allows certain computations to be performed faster than was believed possible, and it allows distribution of a secret key between two parties even when their equipment could have been built by an adversary (as long as their labs are secure).

These startling advances in computing and cryptography have driven rapid experimental progress in quantum information processing, and the generation, storage, and transmission of entanglement. CQC^{2}T is a world-leader in these technologies in optics and silicon, and in the associated theory. In addition, like the first and second quantum revolutions, the third quantum revolution has changed the way we think about the physical world. Specifically, it has led to a new perspective on the quantum state, that of being a state of information. It is a perspective that has been highly productive in quantum communication, quantum metrology, and quantum control, and that offers new ways to understand nature.

The Quantum Information Theory Program in CQC^{2}T aims to sustain the progress arising from the third quantum revolution in its many aspects, from underpinning and motivating the technological development in both quantum communication and quantum computation, to uncovering new implications and applications of the quantum information perspective. Amongst its specific aims are the following:

(1) To provide quantum information theory support for the design of the quantum information network using quantum repeater nodes, which enable “flying qubits” (photons) to be converted to stationary qubits for storage and potentially for deterministic multi-qubit processing.

(2) To provide quantum optical theory support for the design of the interface between photons and stationary qubits (e.g. solid-state qubits). This has applications both for quantum repeaters and for single-photon sources, which are necessary for scalable quantum computing using linear optics.

(3) To develop innovative forms of quantum measurements using adaptivity and other recent developments in informational control theory. These will have applications in metrology at the ultimate quantum limit (see top figure) and in photonic quantum computing.

(4) To invent, and develop implementations for, novel measurement-based control protocols for registers and other quantum systems, including rapid purification (extracting information about the final state), rapid read-out (extracting information about the initial state), and efficient tracking (storing the present state using the least information) (see middle figure).

(5) To find medium-scale quantum computer algorithms that perform a convincingly non-trivial calculation, suitable for demonstration on the Centre’s quantum information processing hardware.

(6) To discover distributed quantum information tasks based on nonlocal quantum effects such as EPR-steering (see bottom figure), and to develop these into practical long-distance quantum communication protocols.