Characterizing conditional quantum dynamics on superconducting quantum processors

Abstract The pursuit of practical quantum computation relies on the precise implementation and characterization of high-fidelity entangling gates. We present a detailed experimental characterization of the controlled-NOT (CX) and controlled-phase (CZ) gates on a noisy intermediate-scale quantum (NISQ) superconducting processor using full quantum process tomography and direct state measurements. The gates were benchmarked against a noise-free quantum simulator to isolate hardware-induced errors. The CX gate, implemented as a native gate, achieved an average process fidelity of $$\mathcal {F}_p^{\text {CX}} = 93.02\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mi>F</mml:mi> <mml:mi>p</mml:mi> <mml:mtext>CX</mml:mtext> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mn>93.02</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> and correctly prepared the target state with $$P_{00}^{\text {CX}} = 92.40\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mn>00</mml:mn> </mml:mrow> <mml:mtext>CX</mml:mtext> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mn>92.40</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> probability. The CZ gate, decomposed into a sequence of single-qubit and CX gates, achieved $$\mathcal {F}_p^{\text {CZ}} = 92.59\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mi>F</mml:mi> <mml:mi>p</mml:mi> <mml:mtext>CZ</mml:mtext> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mn>92.59</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> with superior state-preparation fidelity of $$P_{00}^{\text {CZ}} = 97.08\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mn>00</mml:mn> </mml:mrow> <mml:mtext>CZ</mml:mtext> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mn>97.08</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> . Remarkably, the compiled CZ gate outperformed the native CX gate in state preservation by 4.68-percentage points, demonstrating the effectiveness of hardware-aware compilation. Our analysis provides a comprehensive benchmark of these essential gates, revealing that compilation strategy can be as crucial as native hardware performance for reliable quantum circuit execution in the NISQ era.