High-fidelity two-qubit gates via dynamical decoupling of local 1/f noise at the optimal point

We investigate the possibility of achieving high-fidelity universal two-qubit gates by supplementing optimal tuning of individual qubits with dynamical decoupling (DD) of local 1/f noise. We consider simultaneous local pulse sequences applied during the gate operation and compare the efficiencies of periodic, Carr-Purcell, and Uhrig DD with hard pi pulses along two directions (pi(z/y) pulses). We present analytical perturbative results (Magnus expansion) in the quasistatic noise approximation combined with numerical simulations for realistic 1/f noise spectra. The gate efficiency is studied as a function of the gate duration, of the number n of pulses, and of the high-frequency roll-off. We find that the gate error is nonmonotonic in n, decreasing as n(-alpha) in the asymptotic limit, alpha >= 2, depending on the DD sequence. In this limit pi(z)-Urhig is the most efficient scheme for quasistatic 1/f noise, but it is highly sensitive to the soft UV cutoff. For small number of pulses, pi(z) control yields anti-Zeno behavior, whereas pi(y) pulses minimize the error for a finite n. For the current noise figures in superconducting qubits, two-qubit gate errors similar to 10(-6), meeting the requirements for fault-tolerant quantum computation, can be achieved. The Carr-Purcell-Meiboom-Gill sequence is the most efficient procedure, stable for 1/f noise with UV cutoff up to gigahertz.