# A compressed classical description of quantum states

## Abstract

We show how to approximately represent a quantum state using the square root of the usual amount of classical memory. The classical representation of an n-qubit state ψ consists of its inner products with O(√2n) stabilizer states. A quantum state initially specified by its 2n entries in the computational basis can be compressed to this form in time O(2npoly(n)), and, subsequently, the compressed description can be used to additively approximate the expectation value of an arbitrary observable. Our compression scheme directly gives a new protocol for the vector in subspace problem with randomized one-way communication complexity that matches (up to polylogarithmic factors) the optimal upper bound, due to Raz. We obtain an exponential improvement over Raz’s protocol in terms of computational efficiency.