Getting almost all the bits from a quantum random access code

Abstract

A quantum random access code (QRAC) is a map $x\mapsto {\rho }_x$ that encodes $n$-bit strings $x$ into $m$-qubit quantum states ${\rho }_x$, in a way that allows us to recover any one bit of $x$ with success probability $\ge p$. The measurement on ${\rho }_x$ that is used to recover, say, $x_1$ may destroy all the information about the other bits; this is in fact what happens in the well-known QRAC that encodes $n=2$ bits into $m=1$ qubits. Does this generalize to large $n$, i.e., could there exist QRACs that are so "obfuscated" that one cannot get much more than one bit out of them? Here we show that this is not the case: for every QRAC there exists a measurement that (with high probability) recovers the full $n$-bit string $x$ up to small Hamming distance, even for the worst-case $x$.