Compression for Qubit Clocks

Two-Ievel (qubit) clock systems are often used to perform precise measurement of time. In this work, we propose a compression protocol for $n$ identically prepared states of qubit clocks. The protocol faithfully encodes the states into $(1\mathrm{/}2)\log n$ qubits and $(1\mathrm{/}2)\log n$ classical bits and works even in the presence of noise. If the purity of the clock states is fixed, $(1\mathrm{/}2)\log n$ qubits are sufficient. We also prove that this protocol requires the minimum amount of total memory among all protocols with vanishing error in the large $n$ limit.