Procedure for systematically tuning up cross-talk in the cross-resonance gate

Abstract

We present improvements in both theoretical understanding and experimental implementation of the cross resonance (CR) gate that have led to shorter two-qubit gate times and interleaved randomized benchmarking fidelities exceeding 99%. The CR gate is an all-microwave two-qubit gate offers that does not require tunability and is therefore well suited to quantum computing architectures based on 2D superconducting qubits. The performance of the gate has previously been hindered by long gate times and fidelities averaging 94-96%. We have developed a calibration procedure that accurately measures the full CR Hamiltonian. The resulting measurements agree with theoretical analysis of the gate and also elucidate the error terms that have previously limited the gate fidelity. The increase in fidelity that we have achieved was accomplished by introducing a second microwave drive tone on the target qubit to cancel unwanted components of the CR Hamiltonian.The cross resonance (CR) gate is an entangling gate for superconducting qubits that uses only microwave control [1,2] and has been the standard for multiqubit experiments in superconducting architectures using fixed-frequency transmon qubits [3,4]. Superconducting qubits arranged with shared quantum buses [5] allow qubit networks to be designed with any desired connectivity. This flexiblity of design also translates into a flexibility of control and many choices in entangling gate implementations. The CR gate is one choice of two-qubit gate that uses only microwave control, as opposed to using magnetic flux drives to tune two qubits into a specific resonance condition to entangle, as in the controlledPhase gate [6,7], or to tune a coupler directly [8][9][10][11]. The CR gate requires a small static coupling of the qubit pair that slightly hybridizes the combined system and one additional microwave drive. The relatively low overhead of the CR scheme (the additional control line is combined with a single-qubit drive at room temperature) makes it an attractive gate for use in quantum computing architectures based on planar superconducting qubits. Additionally, the CR gate is well-suited to transmon qubits [12], which have become the superconducting of choice due to promising long coherence and lifetimes [13,14], limited charge noise [15], and high single-qubit gate fidelities [16]. The microwave-only control allows the use of fixed-frequency transmons, further reducing the sources of possible noise [17]. Given all of these qualities, the CR gate has been a useful tool for application in multiqubit experiments, including demonstrations of parity measurements required for the surface code [3].Despite the appeal of the CR gate, its implementation has been hindered by slow gate times. The CR gate relies on an always-on qubit-qubit coupling, but large couplings can lead to crosstalk between qubits. This leads to a trade-off between fast, high-fidelity two-qubit gates and high-fidelity simultaneous single-qubit gates. As a result, typical CR gates between transmon devices have resu...

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Overlap junctions for high coherence superconducting qubits

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