We have recently developed an in-situ technique we call Cryoscope, which uses a qubit to accurately sample the flux pulses used to dynamically control its frequency. This measurement is key for determining the linear-dynamical distortion on the flux control line and later correcting it, as needed for high-fidelity two-qubit gates. We invite you to check out our manuscript here!
The Net-Zero two-qubit gate
We have recently developed a new type of conditional-phase gate for transmon qubits providing several key improvements over standard flux-pulsing-based versions. The Net-Zero gate uses “leakage interference” to minimize leakage to non-computational states. The zero-average, bipolar shape of the pulse makes the gate robust to long timescale distortions in the flux control line and additionally provides an echo effect. We demonstrate a state-of-the-art conditional-phase gate of duration 40 ns achieving 99.1% fidelity and 0.1% leakage.
Variational eigensolver with error mitigation
Running accurate algorithms without the need for fully-fledged quantum error correction is a key challenge for quantum computing in the NISQ era. This challenge calls on us to develop techniques that mitigate errors with small overhead .We have recently introduced and demonstrated our own technique for error mitigation, termed symmetry verification, in the context of solving molecular spectra with a quantum processor. In our recent paper, we demonstrate the use SV to reduce the energy error and state infidelity by one order of magnitude in a variational quantum eigensolver for the ground state of the hydrogen molecule.
Entangling qubits on separate chips!
While the processing power in circuit QED chips is growing rapidly, it remains an open challenge to establish high-fidelity quantum links between qubits on different chips. We have achieved entanglement between transmon qubits on different cQED chips with 49% concurrence and 73% Bell-state fidelity. We engineer a half-parity measurement by successively reflecting a coherent microwave field from two nearly-identical transmon-resonator systems. By ensuring that the measured output field does not distinguish |01> from |10>, unentangled superposition states are probabilistically projected onto entangled states in the odd-parity subspace. We use in-situ tunability and an additional weakly coupled driving field on the second resonator to overcome imperfect matching due to fabrication variations. Please check out our manuscript here!