Shaped microwave pulses for measuring hybrid quantum devices

Research into hybrid quantum devices is motivated by the potential to combine the long coherence times of natural spin systems with the scalability of circuit quantum electrodynamics for applications in solid state quantum information processing [1]. Electron Spin Resonance (ESR) is a standard technique for the measurement of a spin system, using an inductively-coupled microwave resonator to excite the spins with pulsed radiation and detect their characteristic magnetic field response [2]. It has further been used to probe the spins present on the surface of quantum devices, from which it is possible to attribute chemical fingerprints to sources of decoherence [3]. All else being equal, resonators with high-Q factors yield better ESR sensitivity (which scales with Q squared).
However, the ringdown time taken for the energy to dissipate from the resonator might then exceed the characteristic decay time of the spins, concealing their signal [2]. The high Q also distorts the excitation signal applied to the spins. To try to overcome these issues, and develop practical ESR which benefits from high-Q resonators, we combine ringdown suppression and control theory to apply shaped microwave pulses, to enable efficient manipulation and detection of spin ensembles on short timescales. Ringdown suppression uses a phase-inverted pulse of sucient energy to drive the resonator oscillations to zero [4]. Preliminary results from a high-Q dielectric resonator of resonant frequency 8.7 GHz show the ringdown suppressed by 65 percent through the use of a square-shaped compensation pulse. To alter the pulse distribution in the frequency domain, frequency-swept pulses and pulse shaping around the point of phase modulation are also considered. We further discuss the application of shaped pulses and optimal control theory to high-Q superconducting planar micro-resonators, which are natural candidates for extending practical ESR to small numbers of spins.

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