A conventional resonant detector is often subject to a trade-off between bandwidth and peak sensitivity. Anomalous dispersion has been shown to improve the bandwidth-sensitivity limit by signal amplification, but its unstable issue in operation needs additional feedback control. Here we propose a stable quantum amplifier enabled by two-mode non-degenerate parametric amplification. As the amplifier operates at threshold, one mode of the amplifier forms a parity-time(PT)-symmetric system with the mode of the original detector, while the other mode of the amplifier collects signal and gets extracted by a readout. Viewed more broadly in the context of coherent quantum control theory, we are attaching a controller that consists of a time reversal of the plant, which, by canceling the inertia of the plant, helps drive up the closed-loop signal gain. Examples of microwave cavity axion detectors and laser-interferometric gravitational-wave detectors will be discussed for this technique.
Progress in the emerging field of engineered quantum systems requires the development of devices that can act as quantum memories. The realisation of such devices by doping solid state cavities with paramagnetic ions imposes a trade-off between ion concentration and cavity coherence time. Here, we investigate an alternative approach involving interactions between photons and naturally occurring impurity ions in ultra-pure crystalline microwave cavities exhibiting exceptionally high quality factors. We implement a hybrid Whispering Gallery/Electron Spin Resonance method to perform rigorous spectroscopy of an undoped single-crystal sapphire resonator over the frequency range 8{19 GHz, and at external applied DC magnetic fields up to 0.9 T. Measurements of a high purity sapphire cooled close to 100 mK reveal the presence of Fe3+, Cr3+, and V2+ impurities. A host of electron transitions are measured and identified, including the two-photon classically forbidden quadrupole transition (Δms = 2) for Fe3+, as well as hyperfine transitions of V2+.
Optically detected magnetic resonance (ODMR) in nanodiamond nitrogen-vacancy (NV) centres is usually achieved by applying a microwave field delivered by micron-size wires, strips or antennas directly positioned in very close proximity (~ μm) of the nanodiamond crystals. The microwave field couples evanescently with the ground state spin transition of the NV centre (2.87 GHz at zero magnetic field), which results in a reduction of the centre photoluminescence. We propose an alternative approach based on the construction of a dielectric resonator. We show that such a resonator allows for the efficient detection of NV spins in nanodiamonds without the constraints associated to the laborious positioning of the microwave antenna next to the nanodiamonds, providing therefore improved flexibility. The resonator is based on a tunable Transverse Electric Mode in a dielectric-loaded cavity, and we demonstrate that the resonator can detect single NV centre spins in nanodiamonds using less microwave power than alternative techniques in a non-intrusive manner. This method can achieve higher precision measurement of ODMR of paramagnetic defects spin transition in the micro to millimetre-wave frequency domain. Our approach would permit the tracking of NV centres in biological solutions rather than simply on the surface, which is desirable in light of the recently proposed applications of using nanodiamonds containing NV centres for spin labelling in biological systems with single spin and single particle resolution.
Cryogenic sapphire resonators operating in Whispering Gallery Modes have very high Q-factors (> 109) at
microwave frequencies . Such a property makes them useful for a host of applications, which are only possible due
to the additional inclusion of residual paramagnetic impurities that annul the frequency-temperature dependence
of sapphire. More recently, residual Fe3+ impurities with parts-per-billion concentration within the lattice have
been shown to create a three level system corresponding to the spin states of the ion. By pumping at 31.3 GHz, a
stable 12.04 GHz maser signal (stability of parts in 1014) has been created without any stabilization circuitry. In
addition, we have observed the fundamental thermal noise limit near 4 K by operating such masers in a bimodal
conguration. Annealing one resonator in air has led to conversion of Fe2+ ions in the lattice to Fe3+, leading
to an orders of magnitude increase in active ion concentration. At the post-annealing Fe3+ concentration of 150
ppb , we observe nonlinear eects such as a degenerate four-wave mixing due to a χ(3) magnetic nonlinearity as
well as stable frequency comb generation.
The precise microwave characterization of dielectric materials is an important issue for emerging technologies of the 21st century. In this paper recent advances in resonant techniques for permittivity and dielectric loss tangent measurements of low and medium loss dielectrics at microwave frequencies are presented.
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