A Small Qubit Leak Becomes A Big Problem After Many Pulses

The vulnerability of quantum sensors to even minor qubit leaks is amplified by the thousands of pulses now routinely applied to extend sensing duration. A new analysis reveals that certain dynamical decoupling protocols, commonly used to achieve long qubit coherence, can degrade sensing performance far more than they affect coherence itself, challenging the assumption that long coherence guarantees good sensing.

Researchers found that differing pulse protocols cause coherence times to vary by orders of magnitude due to qubit leakage, where pulses couple to states outside the two-level qubit subspace. Even a very small leak can become problematic after a large number of pulses.

Carr-Purcell Sequence Sensitivity Reveals Pulse Error Impact

Pulse imperfections during quantum sensing can dramatically alter results, with coherence times differing by orders of magnitude depending on the specific pulse protocol employed, according to new simulations. The work focuses on the Carr-Purcell (CP) pulse sequence and its variants, revealing that seemingly minor deviations from ideal pulses accumulate over the thousands of pulses now common in sensing applications. Researchers tested these sequences through both experimental measurements and simulations of nuclear magnetic resonance spectroscopy using unpolarized nuclear spins.