Hamburg 2009 – scientific programme

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Q: Fachverband Quantenoptik und Photonik

Q 63: Quanteninformation: Quantencomputer

Q 63.8: Talk

Friday, March 6, 2009, 15:45–16:00, VMP 6 HS-A

Most quantum states are too entangled to be useful as computational resources — •David Gross¹, Steve Flammia², and Jens Eisert³ — ¹Technical University of Braunschweig — ²Perimeter Institute for Theoretical Physics, Waterloo — ³University of Potsdam

It is often argued that entanglement is at the root of the speedup for quantum compared to classical computation, and that one needs a sufficient amount of entanglement for this speedup to be manifest. In measurement-based quantum computing (MBQC), the need for a highly entangled initial state is particularly obvious. Defying this intuition, we show that quantum states can be too entangled to be useful for the purpose of computation. We prove that this phenomenon occurs for a dramatic majority of all states: the fraction of useful n-qubit pure states is less than exp(−n²). Computational universality is hence a rare property in quantum states. This work highlights a new aspect of the question concerning the role entanglement plays for quantum computational speed-ups. The statements remain true if one allows for certain forms of post-selection and also cover the notion of CQ-universality. We identify scale-invariant states resulting from a MERA constructrion as likely candidates for physically relevant states subject to this effect.