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Qubit Calibration by Remote Control
September 9, 2020 (HP142). Scientists at IBM have demonstrated a more general randomized benchmarking system by including an additional two-qubit primitive: the controlled-S phase (CS) gate. In their paper, “Experimental implementation of non-Clifford interleaved randomized benchmarking with a controlled-S gate,”  Shelly Garion of the IBM Research Haifa, Israel and Naoki Kanazawa of IBM Research Tokyo, Japan present a demonstration of a low-error non-Clifford CS gate. The CS is similar to a CZ in that it is a two-qubit gate that conditionally performs a phase rotation on a target qubit based on the state of a control qubit; however, the phase imparted is π/2 in the CS, whereas in the CZ it is π. Figure 1a shows the microwave pulse sequence used to perform the CS where the two-qubit interaction is accomplished via two cross-resonance (CR) pulses . This non-Clifford two-qubit entangling gate is universal when combined with the Clifford group . They incorporated the CS gate into their circuit construction and performed non-Clifford CNOT-Dihedral interleaved randomized benchmarking to measure the gate error at 5.9(7)×10-3 for the CS with a gate time of 263 ns. Figure 1b shows that the CS gate error rates measured with both interleaved RB and quantum process tomography (QPT) approach the coherence limit.
This is an impressive result, made more so by the fact that it was accomplished by scientists who were not even on the same continent as the qubit itself. The experiment was performed through a remote Python interface on one of the quantum processors available through the IBM Quantum Experience platform. Customized control pulses were constructed using the open-source Qiskit Pulse framework by Kanazawa-san, and the calibration and refinement of the CS gate were accomplished without significant intervention during the experiment from lab personnel at IBM’s T.J. Watson Research Center in Yorktown Heights, NY, USA where IBM’s quantum computing systems are maintained.
Contributed by Tony Przybysz, SNF Co-editor Electronics (digital & quantum computing)