Quantum memory with millisecond coherence in circuit QED

Abstract

Significant advances in coherence render superconducting quantum circuits a viable platform for fault-tolerant quantum computing. To further extend capabilities, highly coherent quantum systems could act as quantum memories for these circuits. A useful quantum memory must be rapidly addressable by Josephson junction-based artificial atoms, while maintaining superior coherence. We demonstrate a novel superconducting microwave cavity architecture that is highly robust against major sources of loss that are encountered in the engineering of circuit QED systems. The architecture allows for storage of quantum superpositions in a resonator on the millisecond scale, while strong coupling between the resonator and a transmon qubit enables control, encoding, and readout at MHz rates. This extends the maximum available coherence time attainable in superconducting circuits by almost an order of magnitude compared to earlier hardware. Our design is an ideal platform for studying coherent quantum optics and marks an important step towards hardware-efficient quantum computing in Josephson junction-based quantum circuits.

An architecture for integrating planar and 3D cQED devices

“…We characterize parameters of the complete system, including coupling and coherence values. Both the qubit and the high-Q cavity perform well, with qubit T 1 = 110 µs, qubit Ramsey decay time T * 2 = 40 µs, cavity T 1 = 2.8 ms (Figure 4b), and cavity T * 2 = 1.5 ms for the |1 Fock state 12 . These qubit lifetimes are among the best measured in 3D cavities, and the coaxial stub resonator T 1 does not decrease when a qubit is added.…”

“…To demonstrate an instance of a long-lived element in the presence of significant complexity, we combine a very high-Q 3D cavity with the coaxline architecture. In the resulting package (Figure 4a), the pads of a transmon qubit bridge two structures: the coax-line qubit-and-stripline system and a 3D coaxial stub cavity 12 . We characterize parameters of the complete system, including coupling and coherence values.…”

“…Transmon qubits in 3D cavities have shown exceptionally high coherence 10,11 . These cavities can be made seamless to limit dissipation while remaining integrable 12 . These features form the basis for a low-loss enclosure suited to expansion.…”

Quantum systems under frequency modulation

“…Just to mention a few, stochastic frequency fluctuations can be caused by changes in the interacting environment [136,204], fluctuations of spurious two-level systems [205], attachment and diffusion of molecules or atoms [170,172,175,206], or uncontrolled transitions in a coupled ancillary qubit [37,207]. Consider a quantum superposition of two Fock states |ψ = (|n + |n + 1 )/ √ 2 that evolves under the fluctuating Hamiltonian (137).…”

Mesoscopic Superposition States Generated by Synthetic Spin-Orbit Interaction in Fock-State Lattices

“…The coupling strength between the qubit and the resonators can be hundreds of MHz [49,50]. The single photon relaxation time of fixed frequency resonators can be milliseconds [48,51]. Tunable resonators based on kinetic inductance can also have a relaxation time as long as 6 µs [52].…”