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

Abstract

Defects in Josephson junctions are considered a nuisance when it comes to using superconducting circuits as building blocks for a quantum-information processor. But if the interaction between the circuit and defects is accurately controlled—as has been demonstrated now—the imperfections might be useful, serving as memory elements. A quantum computer will require quantum bits (qubits) with good coherence that can be coupled together to form logic gates1,2. Superconducting circuits offer a novel solution3,4,5,6,7,8,9 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 storage10,11. Here, we demonstrate the first quantum memory operation12 on a Josephson-phase qubit by transferring an arbitrary quantum state to a two-level state13 (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 tomography2,14 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.

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

Abstract

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