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Since its discovery in the early part of the 20th century, quantum entanglement has inspired puzzlement, and Einstein, Podolsky and Rosen famously described some consequences of entanglement which seemed at odds with natural assumptions about physical laws. This intuition was made more explicit by the seminal work of John Bell, who showed that the correlations between measurements upon spatially separated systems, should, based on very natural assumptions, fulfil certain bounds, while quantum mechanics violates this bound. More recently, Greenberger, Horne, Zeilinger (GHZ) and Mermin gave an example of a correlation present in measurements upon a three-body entangled state, where classical physics would predict the opposite.

The development of quantum information has presented us with an array of new examples of the remarkable properties of quantum systems. For example, measurement-based quantum computation (MBQC) has shown us that there exist entangled states which can encode any computation, even a quantum computation, in the correlations between measurements on single qubits.

Our work shows that there is a direct link between these famous puzzles of quantum mechanics and measurement-based quantum computation. More explicitly, we have shown that all of these “paradoxes”, from John Bell through to GHZ, can be thought of as examples of measurement-based quantum computation. This has allowed us to generalise these quantum puzzles much more broadly, which will give us new insights into the fundamental physics behind these properties, and also allow us to develop new applications and protocols based on multi-party entanglement.

This also further emphasises the important close relation between the foundations of quantum mechanics and quantum information - in improving our understanding of one, we improve our understanding of the other, and demonstrates that experimental progress toward building a quantum computer is also giving us new tests of the fundamental nature of the world.

Ancilla-driven quantum computation

While measurement-based quantum computation has provided fertile ground for new experimental proposals for quantum computation, there exist many systems which are difficult to efficiently measure, such as single spins. These systems often have a natural coupling to another degree-of-freedom, such a charge, which may be more readily measured. Ancilla-driven quantum computation is an attempt to exploit such asymmetries in the physical systems at our disposal. We have shown that one can achieve universal quantum computation is a system where only a subset of the qubits are fully controllable and measureable, and the remainder evolving naturally under a Hamiltonian, cannot be measured. We hope that this new formulation of quantum computation may provide new experimental proposals for quantum information processing which may better exploit the natural structures of these systems.