Quantum methods for clock synchronization: Beating the standard quantum limit without entanglement

We introduce methods for clock synchronization that make use of the adiabatic exchange of nondegenerate two-level quantum systems: ticking qubits. Schemes involving the exchange of N independent qubits with frequency ω give a synchronization accuracy that scales as (ωsqrt[N])⁻¹ —i.e., as the standard quantum limit. We introduce a protocol that makes use of N_c coherent exchanges of a single qubit at frequency ω , leading to an accuracy that scales as (ωN_c)⁻¹  ln  N_c . This protocol beats the standard quantum limit without the use of entanglement, and we argue that this scaling is the fundamental limit for clock synchronization allowed by quantum mechanics. We analyze the performance of these protocols when used with a lossy channel.