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Bulletin of the American Physical Society

High Fidelity Measurement of Single and Multi-qubit Superconducting Quantum Circuits

S. Kundu, Sumeru Hazra, Tanay Roy, Madhavi Chand, Kishor V. Salunkhe, Meghan P. Patankar, Rajamani Vijayaraghavan

Published Mar 5, 2018• 0 citations
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Quantum processors with native multi-qubit gates may offer very efficient implementations of near-term quantum algorithms on noisy hardware. Here, we introduce the multiunimon, a superconducting multimode circuit that encodes multiple qubits and enables native multi-qubit gates in a device consisting of a single Josephson junction embedded in a coplanar waveguide structure. Closely related to the unimon qubit, it inherits properties such as high anharmonicity, full protection against low-frequency charge noise, and partial protection against flux noise. By designing such a three-qubit device with Josephson-to-inductive energy ratio above unity and using a leakage-aware encoding scheme for the computational states, we simulate all twelve different controlled-controlled-NOT gates with a mean fidelity of 99.5% with simple sine-squared pulses of comparable length to single-qubit gates. The performance is limited by incoherent errors dominated by dielectric loss. With improvements in noise protection, design, and pulse shaping, the simulations suggest that fidelities approaching 99.99% are within reach. Our results demonstrate the potential of the multiunimon as a highly connected multi-qubit unit for larger superconducting quantum processors.
It is a challenge to identify the quantum state with high fidelity as the quantum system size becomes large. The larger system size requires more parameters to be calculated and hence more measurement and more time. For example, 16 and 64 parameters are needed to express two and three qubit state in density matrix, respectively. Here each parameter is determined by the quantum state tomography (QST) after the initial state is prepared by performing simple single gate operations and/or two qubit CNOT gate. This enables us to determine the state fidelity of the given state preparation.