Hayden–Preskill thought experiment

In quantum information, the Hayden–Preskill thought experiment (also known as the Hayden–Preskill protocol) is a thought experiment that investigates the black hole information paradox by hypothesizing on how long it takes to decode information thrown in a black hole from its Hawking radiation.

The thought experiment concerning Alice and Bob is as follows: Alice throws a k qubit quantum state into a black hole that is entangled with Bob's quantum computer. Bob collects the Hawking radiation emitted by the black hole and feeds it into his quantum computer where he applies the appropriate quantum gates that will decode Alice's state. Bob only needs at least k qubits from the black hole's Hawking radiation to decode Alice's quantum state.

The black hole can be thought of as a quantum information mirror, because it returns scrambled information almost instantly, with a delay that can be accounted for by the scrambling time and the time it takes for the black hole to radiate the qubits. This decoding method, known as the Yoshida-Kitaev decoding scheme, can theoretically be applied to a small system thermalized with a large system. This opens up the possibility of testing the Hayden–Preskill thought experiment in real life.