We present a proposal to search for QCD axions with mass in the 200 µeV range, assuming that they make a dominant component of dark matter. Due to the axion-electron spin coupling, their effect is equivalent to the application of an oscillating rf field with frequency and amplitude fixed by the axion mass and coupling respectively. This equivalent magnetic field would produce spin flips in a magnetic sample placed inside a static magnetic field, which determines the resonant interaction at the Larmor frequency. Spin flips would subsequently emit radio frequency photons that can be detected by a suitable quantum counter in an ultra-cryogenic environment. This new detection technique is crucial to keep under control the thermal photon background which would otherwise produce a too large noise.

The Casimir effect is a well-known macroscopic consequence of quantum vacuum fluctuations, but whereas the static effect (Casimir force) has long been observed experimentally, the dynamic Casimir effect is up to now undetected. From an experimental viewpoint a possible detection would imply the vibration of a mirror at gigahertz frequencies. Mechanical motions at such frequencies turn out to be technically unfeasible. Here we present a different experimental scheme where mechanical motions are avoided, and the results of laboratory tests showing that the scheme is practically feasible. We think that at present this approach gives the only possibility of detecting this phenomenon. PACS numbers: 12.20.Fv, 42.50.Dv, For any quantum field, the vacuum is defined as its ground state. Differently than in the classic case, this ground state, due to the uncertainty principle, is not empty, but filled with field fluctuations around a zero mean value. Moreover this vacuum state depends on the field boundary conditions : if they change, there will be a correspondingly different vacuum (whose fluctuations will have a different wavelength spectrum). Thus a quantum vacuum state may be equivalent to real particles of a new vacuum after a change in boundary conditions.If we consider the electromagnetic field, the peculiar nature of the quantum vacuum has experimentally observable consequences in the realm of microscopic physics, such as natural widths of spectral lines, Lamb shift, anomalous magnetic moment of the electron and many more. It is perhaps even more striking that there exist also observable effects at a macroscopic level. The Casimir force (static Casimir effect [1,2]) is one of these macroscopic effects which has been observed experimentally. A dynamic Casimir effect is also predicted to occur when one boundary is accelerated in a nonuniform way, as for instance when a metal surface undergoes harmonic oscillations. In this case a number of virtual photons from the vacuum are converted into real photons ("Casimir radiation"), while the moving metal surface loses energy [3,4,5].It is worth notice that, whereas the static Casimir effect has been observed by several experiments [6], the Casimir radiation is to date unobserved, in spite of the abundant theoretical work done in this field [7,8,9,10,11] (see [8] for a historical review and a bibliography of the relevant studies). We argue that this lack of experimental activity stems from the rooted idea of using mechanical oscillations. We shall show that this is unfeasible with present-day techniques.Here we shall present a new experimental approach where an effective motion is generated by the excitation of a plasma in a semiconductor. In terms of power this effective motion is much more convenient than a mechanical motion, since in a metal mirror only the conduction electrons reflect the electromagnetic waves, whereas a great amount of power would be wasted in the acceleration of the much heavier nuclei. Some authors [12,13,14] have made use of our idea in order to cons...

To account for the dark matter content in our Universe, post-inflationary scenarios predict for the QCD axion a mass in the range (10 − 10 3 ) µeV. Searches with haloscope experiments in this mass range require the monitoring of resonant cavity modes with frequency above 5 GHz, where several experimental limitations occur due to linear amplifiers, small volumes, and low quality factors of Cu resonant cavities. In this paper we deal with the last issue, presenting the result of a search for galactic axions using a haloscope based on a 36 cm 3 NbTi superconducting cavity. The cavity worked at T = 4 K in a 2 T magnetic field and exhibited a quality factor Q0 = 4.5 × 10 5 for the TM010 mode at 9 GHz. With such values of Q the axion signal is significantly increased with respect to copper cavity haloscopes. Operating this setup we set the limit gaγγ < 1.03 × 10 −12 GeV −1 on the axion photon coupling for a mass of about 37 µeV. A comprehensive study of the NbTi cavity at different magnetic fields, temperatures, and frequencies is also presented.

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