High Fidelity Single Qubit Operations Using Pulsed Electron Paramagnetic Resonance

Systematic errors in spin rotation operations using simple rf pulses place severe limitations on the usefulness of the pulsed magnetic resonance methods in quantum computing applications. In particular, the fidelity of quantum logic operations performed on electron spin qubits falls well below the threshold for the application of quantum algorithms. Using three independent techniques, we demonstrate the use of composite pulses to improve this fidelity by several orders of magnitude. The observed high-fidelity operations are limited by pulse phase errors, but nevertheless fall within the limits required for the application of quantum error correction.

Figure 1

Rabi oscillations for $i\mathrm{\text{−}}{\mathrm{NC}}_{60}$ in ${\mathrm{CS}}_2$ at 190 K (solid curve). BB1-Rabi oscillations exploiting BB1 composite pulses to remove the decay caused by pulsed field inhomogeneity (dashed curve).

Figure 2

The Fourier transform of ESEEM in $i\mathrm{\text{−}}{\mathrm{NC}}_{60}$ shows two frequency components of 26 and 52 kHz. For simple $\pi$ refocusing pulses, the 26 kHz component is attributed to pulse error (solid line) and is removed when a BB1 composite $\pi$ pulse is used (dashed line). For a refocusing pulse of approximately $0.61\pi$, the 52 kHz component, which would be present due to pulse error, can also be removed by using a BB1 composite $0.61\pi$ pulse (dotted line).

Figure 3

Comparison of the echo signal decays in the reference CPMG sequence (dots) with that of the error-sensitive CP (crosses) sequence provides a measure of rotation angle errors. CP echoes generated with simple $\pi$ pulses decay more rapidly owing to rotation angle errors. When the simple $\pi$ pulses are replaced with BB1 composite $\pi$ pulses (open circles), the decay rate is very close to that in CPMG.