Suppression of photon shot noise dephasing in a tunable coupling superconducting qubit

Abstract

We demonstrate the suppression of photon shot noise dephasing in a superconducting qubit by eliminating its dispersive coupling to the readout cavity. This is achieved in a tunable coupling qubit, where the qubit frequency and coupling rate can be controlled independently. We observe that the coherence time approaches twice the relaxation time and becomes less sensitive to thermal photon noise when the dispersive coupling rate is tuned from several MHz to 22 kHz. This work provides a promising building block in circuit quantum electrodynamics that can hold high coherence and be integrated into larger systems.npj Quantum Information (2017) 3:1 ; doi:10.1038/s41534-016-0002-2 INTRODUCTIONSuperconducting quantum circuits are a strong candidate for quantum computing, 1-3 and a convenient platform for quantum optics 4-6 and quantum simulation. 7,8 Extensive efforts have been made in the last decade to isolate these quantum systems from various decay channels and noise sources in the environment, leading to an increase of several orders of magnitude in energy relaxation time T 1 and phase coherence time T 2 .9 State-of-the-art devices have achieved T 1 and T 2 in the millisecond regime 10,11 and pushed gate fidelity close to the threshold for fault-tolerant quantum computing.12 However, the progress in T 2 is slower than that in T 1 and T 2 /T 1 ratios in these devices fall in the range between 0.5 and 1.5. 10,13,14 Deviation from the theoretical limit of T 2 = 2T 1 indicates dephasing mechanisms that need to be understood and circumvented.In circuit quantum electrodynamics (cQED), 15,16 manipulation and readout of a superconducting qubit are mediated by its coupling to a transmission line cavity. When the coupling is dispersive, photons in the cavity can be utilized to measure the qubit if their phase is shifted by a distinguishable amount depending on the qubit state. On the other hand, changes in cavity photon number will shift the qubit frequency due to the same coupling mechanism. When the amount of the frequency shift is large enough, thermal or quantum fluctuations of cavity photons lead to dephasing of the qubit. This photon shot noise dephasing mechanism has been studied theoretically 17 and experimentally 14 and has become a dominant factor that limits the coherence time of superconducting qubits. The qualitative discussion above indicates that the dephasing can be suppressed by reducing (1) the photon number fluctuation, characterized by cavity decay rate κ, (2) the thermal photon population n th , and (3) the frequency shift caused by each photon, characterized by the dispersive coupling rate χ. Most work in the past has adopted the first two strategies and used high-Q (>10 6 ) 3D cavities 10,18 and careful filtering and thermal anchoring to reduce κ and n th .

Enhanced qubit readout using locally generated squeezing and inbuilt Purcell-decay suppression

“…Weakly coupled qubit-cavity systems are an emerging topic of current research, with recent developments in the coupling of a single-electron spin to a microwave cavity [25][26][27] having projected best-case coupling strengths that are much weaker than typical cavity dissipation rates, putting them firmly in the weak coupling regime. In addition, new circuit QED designs have been proposed that leverage weak coupling to suppress photon shot noise dephasing [28]. Our in situ two-mode squeezing setup (ISTMS) would allow an exponential enhancement of measurement rates in such systems, in a way that would be impossible even if one were able to inject external squeezing perfectly.…”

Autonomous quantum state transfer by dissipation engineering

“…Leakage error out of the 12-dimension Hilbert space of H A ⊗H B ⊗H R is not included, but its leading contribution from spurious transition of R to its second excited state is estimated to be less than 0.2%. Further improvement beyond these numerical results is possible if Purcell filters [43], advanced thermalization techniques [44], or active/passive methods to cancel χ b [45,46] are employed.…”

Three-qubit direct dispersive parity measurement with tunable coupling qubits

“…As one can readily check from Eqs. (30), this condition is never satisfied for a transmon and consequently it cannot be employed directly for the dispersive three-qubit parity measurement described in Sec. II.…”