Procedure for systematically tuning up crosstalk in the cross resonance gate (1603.04821v1)

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 paper introduces a novel calibration procedure that employs an active cancellation tone to suppress unwanted CR Hamiltonian interactions.
• The method achieves interleaved randomized benchmarking fidelities over 99% while reducing gate times to as low as 160 ns.
• This advancement refines two-qubit operations in fixed-frequency transmons, enhancing scalability and reliability in superconducting quantum architectures.

Systematic Tuning of Crosstalk in the Cross Resonance Gate

The paper under consideration presents significant advances in the field of quantum computing, particularly focusing on the implementation of the cross resonance (CR) gate for superconducting qubits. Conducted by researchers from the IBM T.J. Watson Research Center, the paper enhances both theoretical understanding and practical execution of the CR gate, yielding substantial improvements in two-qubit gate performance—a critical aspect for quantum algorithms and error correction protocols.

The CR gate, distinct in its utilization of an all-microwave control mechanism, holds importance due to its compatibility with 2D superconducting qubit architectures. Unlike gates requiring magnetic flux tuning, which involve higher complexities and susceptibility to noise, the CR gate operates efficiently with fixed-frequency transmons, the qubit of choice due to their extended coherence times and robust single-qubit gate fidelities. Historically, the fidelity of CR gates has lingered around 94-96% with gate times in excess of 300 ns. The researchers overcome these limitations, achieving interleaved randomized benchmarking fidelities over 99%, with gate times reduced by half.

Central to these enhancements is an innovative calibration procedure for the CR gate, characterized by a precise measurement of the full CR Hamiltonian. This advancement refines the control over error terms that previously constrained gate fidelity. The method leverages an active cancellation tone on the target qubit to suppress unwanted interactions within the CR Hamiltonian, facilitating a more refined gate operation.

The research utilizes two fixed-frequency transmon qubits interconnected through a bus resonator and divided into categories such as $IX$, $IY$, $IZ$, $ZX$, $ZY$, and $ZZ$ Hamiltonian terms, as derived from CR Rabi oscillations. The paper identifies negligible $IZ$ and $ZZ$ contributions, uncovering that phase differences between conditional and single-qubit terms suggest an intricate classical crosstalk, which impacts calibration. The employed Hamiltonian tomography procedure effectively measures error terms, specifically addressing these issues through phase and amplitude modulation to optimize gate performance.

These methodological improvements allow for a CR gate time as fast as 160 ns with a fidelity of 0.991. The practical demonstration of entangling qubit capabilities, as illustrated in Fig. 4 of the original paper through Bloch sphere visualization, reveals significantly coherent paths compared to uncontrolled models, attesting to the efficacy of the cancellation tone.

Future theoretical and practical considerations are implied within the research, specifically the aspiration to calibrate non-echoed CR gates and the refinement of calibration protocols. The paper's contribution delineates a pathway for enhanced fidelity and reduced gate times, reinforcing the viability of microwave-controlled gates in emerging quantum computing architectures.

In conclusion, this work provides a profound contribution to the operational efficiency of quantum gates, notably the CR gate's application in an all-microwave paradigm. The elucidation of a meticulous calibration scheme and deeper understanding of crosstalk effects set a precedent for future enhancements, potentially influencing the scalability and reliability of quantum devices as they progress toward practical deployment.