Harmonic mixing in two coupled qubits: Quantum synchronization via ac drives

Simulating a system of two driven coupled qubits, we show that the time-averaged probability to find one driven qubit in its ground or excited state can be controlled by an ac drive in the second qubit. Moreover, off-diagonal elements of the density matrix responsible for quantum coherence can also be controlled via driving the second qubit; that is, quantum coherence can be enhanced by appropriate choice of the biharmonic signal. Such a dynamic synchronization of two differently driven qubits has an analogy with harmonic mixing of Brownian particles forced by two signals through a substrate. Nevertheless, the quantum synchronization in two qubits occurs due to multiplicative coupling of signals in the qubits rather than via a nonlinear harmonic mixing for a classical nanoparticle. Quantum harmonic mixing proposed here can be used to manipulate one driven qubit by applying an additional ac signal to the other qubit coupled with the one we have to control.