Upper-Bounds on Qubit Coherence Set by Master Clock Instabilities
Journal Article - Open Access
MIT Lincoln Laboratory Lexington United States
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Experimentalists seeking to improve the coherent lifetimes of quantum bits have generally focused to date on addressing their respective leading decoherence mechanisms through, for example, improvements to qubit designs, materials, and system isolation from extrinsic perturbations. In the case of the phase degree of freedom in a quantum superposition, however, the coherence that must be preserved ultimately includes that of the qubit relative to the system clock. In this manuscript we clarify the impact of clock instability on qubit dephasing and provide quantitative estimates of fidelity upper-bounds set by noisy phase fluctuations in the clock. We first demonstrate analytically that phase fluctuations in the clock - a master oscillator typically implemented and referred to as the local oscillator LO - is indistinguishable from a pure dephasing field arising from other environmental mechanisms. Using these results, we then apply commonly quoted LO phase-noise specifications to rigorously calculate performance bounds on qubit operational fidelities due to LO phase fluctuations. We find that for state-of-the-art performance specifications, phase fluctuations at frequencies far from the carrier dominate operational error rates. Furthermore, the control bandwidth impacts the degree to which thermal noise forms an effective phase-error floor. Our results motivate additional research directions needed in support of quantum information systems and highlight challenges and advantages that particular qubit technologies possess in the context of phase-noise induced errors.
- Quantum Theory and Relativity