  Macroscopic quantum tunneling (MQT) is a fundamental phenomenon of quantum
mechanics related to the actively debated topic of quantum-to-classical
transition. The ability to realize MQT affects implementation of qubit-based
quantum computing schemes and their protection against decoherence. Decoherence
in qubits can be reduced by means of topological protection, e.g. by exploiting
various parity effects. In particular, paired phase slips can provide such
protection for superconducting qubits. Here, we report on the direct
observation of quantum paired phase slips in thin-wire superconducting loops.
We show that in addition to conventional single phase slips that change
superconducting order parameter phase by $2\pi$, there are quantum transitions
changing the phase by $4\pi$. Quantum paired phase slips represent a
synchronized occurrence of two macroscopic quantum tunneling events, i.e.
cotunneling. We demonstrate the existence of a remarkable regime in which
paired phase slips are exponentially more probable than single ones.
