Programming the Universe: A Quantum Computer Scientist Takes on the Cosmos

by Seth Lloyd

Imagine the quantum computation as embedded in space and time. Each logic gate now sits at a point in space and time, and the wires represent physical paths along which quantum bits flow from one point to another. The first feature to note is that there are many ways to embed the quantum computation in space and time. Each quantum logic gate can be put down at any point where there is not another quantum logic gate, and the wires can squiggle all over the place to connect the logic gates. What happens to quantum information in the computation is independent of how the quantum computation is embedded in the spacetime. In the language of general relativity, the dynamical content of the quantum computation is “generally covariant”—that is, the quantum computation “doesn’t care” how it is embedded in space and time as long as the qubits interact with one another in the right sequence. The fact that a quantum computation doesn’t care how it is embedded in spacetime means that the spacetime derived from the quantum computation obeys the laws of general relativity. Why? Because Einstein derived the laws of general relativity by requiring that those laws don’t care how the underlying physical dynamics of matter is embedded in spacetime. Under the proper assumptions, general relativity is the only theory of gravity that is generally covariant.