Process tomography of quantum memory in a Josephson-phase qubit coupled to a two-level state

A quantum computer will require quantum bits (qubits) with good coherence that can be coupled together to form logic gates(1,2). Superconducting circuits offer a novel solution(3-9) because qubits can be connected in elaborate ways through simple wiring, much like that of conventional integrated circuits. However, this ease of coupling is offset by coherence times shorter than those observed in molecular and atomic systems. Hybrid architectures could help skirt this fundamental trade-off between coupling and coherence by using macroscopic qubits for coupling and atom-based qubits for coherent storage(10,11). Here, we demonstrate the first quantum memory operation(12) on a Josephson-phase qubit by transferring an arbitrary quantum state to a two-level state(13) (TLS), storing it there for some time, and later retrieving it. The qubit is used to probe the coherence of the TLS by measuring its energy relaxation and dephasing times. Quantum process tomography(2,14) completely characterizes the memory operation, yielding an overall process fidelity of 79%. Although the uncontrolled distribution of TLSs precludes their direct use in a scalable architecture, the ability to coherently couple a macroscopic device with an atomic-sized system motivates a search for designer molecules that could replace the TLS in future hybrid qubits.