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Register a new userUltra-Broadband Microwave Frequency-Comb Generation in Superconducting Resonators
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We have generated frequency combs spanning 0.5 to 20 GHz in superconducting half wave resonators at T=3 K. Thin films of niobium-titanium nitride enabled this development due to their low loss, high nonlinearity, low frequency dispersion, and high critical temperature. The combs nucleate as sidebands around multiples of the pump frequency. Selection rules for the allowed frequency emission are calculated using perturbation theory and the measured spectrum is shown to agree with the theory. The sideband spacing is measured to be accurate to 1 part in 10 million. The sidebands coalesce into a continuous comb structure that has been observed to cover at least 6 octaves in frequency.
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In many hybrid quantum systems, a superconducting circuit is required that combines DC-control with a coplanar waveguide (CPW) microwave resonator. The strategy thus far for applying a DC voltage or current bias to microwave resonators has been to apply the bias through a symmetry point in such a way that it appears as an open circuit for certain frequencies. Here, we introduce a microwave coupler for superconducting CPW cavities in the form of a large shunt capacitance to ground. Such a coupler acts as a broadband mirror for microwaves while providing galvanic connection to the center conductor of the resonator. We demonstrate this approach with a two-port $lambda/4$-transmission resonator with linewidths in the MHz regime ($Qsim10^3$) that shows no spurious resonances and apply a voltage bias up to $80$ V without affecting the quality factor of the resonator. This resonator coupling architecture, which is simple to engineer, fabricate and analyse, could have many potential applications in experiments involving superconducting hybrid circuits.
We have measured noise in thin-film superconducting coplanar waveguide resonators. This noise appears entirely as phase noise, equivalent to a jitter of the resonance frequency. In contrast, amplitude fluctuations are not observed at the sensitivity of our measurement. The ratio between the noise power in the phase and amplitude directions is large, in excess of 30 dB. These results have important implications for resonant readouts of various devices such as detectors, amplifiers, and qubits. We suggest that the phase noise is due to two-level systems in dielectric materials.
We describe an experimental protocol to characterize magnetic field dependent microwave losses in superconducting niobium microstrip resonators. Our approach provides a unified view that covers two well-known magnetic field dependent loss mechanisms: quasiparticle generation and vortex motion. We find that quasiparticle generation is the dominant loss mechanism for parallel magnetic fields. For perpendicular fields, the dominant loss mechanism is vortex motion or switches from quasiparticle generation to vortex motion, depending on cooling procedures. In particular, we introduce a plot of the quality factor versus the resonance frequency as a general method for identifying the dominant loss mechanism. We calculate the expected resonance frequency and the quality factor as a function of the magnetic field by modeling the complex resistivity. Key parameters characterizing microwave loss are estimated from comparisons of the observed and expected resonator properties. Based on these key parameters, we find a niobium resonator whose thickness is similar to its penetration depth is the best choice for X-band electron spin resonance applications. Finally, we detect partial release of the Meissner current at the vortex penetration field, suggesting that the interaction between vortices and the Meissner current near the edges is essential to understand the magnetic field dependence of the resonator properties.
Development of chip-scale optical frequency comb with the coverage from ultra-violet (UV) to mid-infrared (MIR) wavelength is of great significance. To expand the comb spectrum into the challenging UV region, a material platform with high UV transparency is crucial. In this paper, crystalline aluminum nitride (AlN)-onsapphire film is demonstrated for efficient Kerr frequency comb generation. Near-infrared (NIR) comb with nearly octave-spanning coverage and low parametric threshold is achieved in continuous-wave pumped high-quality-factor AlN microring resonators. The competition between stimulated Raman scattering (SRS) and hyperparametric oscillation is investigated, along with broadband comb generation via Raman-assisted four-wave mixing (FWM). Thanks to its wide bandgap, excellent crystalline quality as well as intrinsic quadratic and cubic susceptibilities, AlN-on-sapphire platform should be appealing for integrated nonlinear optics from MIR to UV region.
Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communications and for precision metrology, timing and spectroscopy. To date, wide-spanning combs are most often generated by mode-locked lasers or dispersion-engineered resonators with third-order Kerr nonlinearity. An alternative comb generation method uses electro-optic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability. Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. In this work, we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large electro-optic response, ultra-low optical loss and highly co-localized microwave and optical felds, while enabling dispersion engineering. Our measured EO frequency comb spans more than the entire telecommunications L-band (over 900 comb lines spaced at ~ 10 GHz), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 Hz to 100 MHz), and utilize this feature to generate dual frequency combs in a single resonator. Our results show that integrated EO comb generators, capable of generating wide and stable comb spectra, are a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy to optical communications.
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