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the concept of an optical light shining through the wall ( lsw ) experiment has been adapted to microwaves as described in @xcite . a block diagram of the setup at cern as it has been used to search for axion like particles ( alps ) with microwaves is shown in fig .
[ fig : ovrblock ] , it consists of two identical microwave cavities with a loaded quality factor of @xmath0 and a spacing between them of less than 20  mm .
one serves as alp emitter and is excited by 50  w of rf power on its resonant frequency @xmath1  ghz .
it develops a strong electromagnetic ( em ) field , corresponding to a large number of microwave photons @xmath2 .
these can convert to alps by the primakoff effect @xcite .
alps do not interact with matter and propagate towards the detection cavity , which is connected to a very sensitive microwave receiver .
the reciprocal conversion process transforms alps back to microwave photons , which can be observed as an excitation of the seemingly empty and well shielded detection cavity .
since there is no energy loss associated with the alp conversion process , the signal from the detection cavity would be observable at exactly the frequency @xmath3 , making a narrowband receiving concept feasible .
in this section , we give an overview of the critical engineering challenges , which were encountered during the realization of the microwave based lsw experiment :    electromagnetic shielding : :    shielding is required around the detecting cavity and the microwave    receiver to eliminate ambient electromagnetic interference ( emi ) and    to mitigate coupling to the emitting cavity by electromagnetic    leakage .
this would generate false positive results , as a signal    originating from leakage can not be discriminated against an alp    signal . within 2 cm , the field strength must be reduced by at least a    factor of @xmath4 to obtain    meaningful results and avoid degradation of alp sensitivity .
most    microwave components used in the setup like sma connectors or    semi - rigid coaxial cables provide less than 120 db of shielding ,    making an external shielding enclosure and strategic use of optical    fibres for signal transmission necessary . with the current setup , over    300
db shielding effectiveness has been achieved @xcite .
detection of weak signals : :    the smallest detectable signal power during the alps run in june 2013    is    @xmath5  dbm  @xmath6
w  @xmath7  photons / s .    as the experiment is carried out at room temperature , the thermal    noise * density * is @xmath8  dbm / hz . a very    narrow band filter can be implemented to detect the sinusoidal signal    within the background noise .
it is realized by a discrete fourier    transformation over the entire recorded time trace of length    @xmath9  h. each resulting spectral bin will respond to    signals within its resolution bandwidth , given by    @xmath10 . as noise power scales    linearly with resolution bandwidth and
signal power stays constant , we    achieve a signal to noise ratio improvement proportional to    measurement time . to keep the filter on the right center frequency
, a    global 10  mhz reference clock is used to phase lock all oscillators    involved in the downmixing chain .
this detection method has been    successfully demonstrated with resolution bandwidths down to    10  @xmath11hz in @xcite . keeping the cavities on tune
: :    for an exclusion result , it is necessary to prove that the detector is    working and that a potential alp signal could not have been concealed .
detection sensitivity will be limited if the resonant frequency of any    of the cavities does not equal the system frequency    @xmath12 within its 3  db bandwidth .
the resonant    frequency of the cavities can drift due to thermal expansion . for the    emitting cavity ,
the reflected rf power was monitored to ensure it is    on resonance during the whole measurement run . for the detecting    cavity ,
its thermal noise density was measured before and after the    recording of experimental data , indicating its resonant frequency .
compatibility with magnetic fields : :    as the shielding enclosure and the cavities have to be placed in a    strong magnetic field for alp measurements , they need to be    constructed from non - ferromagnetic materials like aluminium or brass .
this is to prevent field distortion and to avoid strong attractive    forces during insertion of the setup into the magnet .
some electronic    components like the low noise amplifier and the analog optical    transmitter need to be placed as close as possible to the detection    cavity .
therefore special precautions had to be taken to ensure they    operate reliably within the magnetic field .
for example , ferrite or    iron cored inductors or transformers had to be avoided as the material    saturates and changes its magnetic properties in strong magnetic    fields .
r0.5 [ fig : exclsignal ]        the most sensitive measurement run for alps has been carried out in june 2013 in cooperation with the brain & behaviour laboratory of geneva university .
we were able to operate the setup within the bore of a superconducting magnet , which is part of an mri scanner .
it provides a solenoid - like field of @xmath13  t. over the course of one weekend , 2  x  10  h of experimental data were recorded . as no alps were detected , the corresponding exclusion limits in comparison to other experiments are shown in fig .  2 .
for diagnostic purposes , a sinusoidal signal of known frequency is emitted within the shielding enclosure .
this `` test tone '' of relatively low and constant power ( @xmath14  dbm ) couples from a @xmath15 antenna to the detection cavity and to the components of the receiver frontend . by identifying the signal in the recorded spectrum ,
we demonstrate that the entire signal processing chain was operational during the measurement .
this also allows to evaluate unwanted frequency offsets , frequency drifts , or phase noise by comparing shape and position of the measured signal peak to the expected one .
furthermore , as long as the test tone signal is observed with constant power during the measurement run , it qualifies that the em shielding performance has not degraded .
figure  3 shows the resulting power spectrum from the measurement run .
the test tone signal is visible as a single peak , clearly above the noise floor , spanning only one single bin .
a frequency window ( shaded green ) has been defined with a width of @xmath16 , around the frequency where an alp signal would be expected .
the peaks within this window do not exceed the detection threshold of @xmath17  w , allowing us to set an exclusion limit for alps .    in order to define the detection threshold , the histogram of @xmath18 frequency bins  containing exclusively spectral background noise 
was evaluated .
@xmath19 was set such , that only in 1% of all measurement runs , a single peak signal above the detection threshold would appear within the wisp window , resulting in a false positive outcome of the experiment .
[ fig : resultspect ]    , no alp signal is visible above the detection threshold.,scaledwidth=100.0% ]
profiting from the high mobility and ruggedness of a microwave based lsw experiment ( compared to optical setups ) , we were able to successfully deploy and operate the apparatus within a 3  t mri magnet .
over the course of one weekend sufficient experimental data was recorded to achieve comparable sensitivity to first generation optical lsw experiments ( e.g. , alps-1 at desy ) .
sensitivity could be further enhanced with a stronger magnet .
there is a recent trend towards 7  t mri systems in medical research , which might provide us an opportunity for a follow up alps measurement .
furthermore , sensitivity can be enhanced with larger and thus lower frequency cavities .
one might think of a lsw setup consisting of 200  mhz sps standing wave cavities @xcite within the 3  t m1 magnet @xcite at cern . +
special thanks to s.  w.  rieger and the brain & behaviour laboratory of geneva university , for making the mri magnet accessible to us on weekends .
the authors would like to thank r.  jones , e.  jensen and the be department management for encouragement and support .
thanks to the organizers of the patras workshop for a very enjoyable and inspiring conference . supported by the wolfgang - gentner - programme of the bundesministerium fr bildung und forschung ( bmbf ) .
f. caspers et al .
, _ demonstration of @xmath20 w signal detection methods in the microwave range at ambient temperature _ , cern - be - note-2009 - 026 p.e .
faugeras et al . , the new rf system for lepton acceleration in the cern sps , proc . of pac 1987
`` light shining through the wall '' experiments can probe the existence of `` axion like particles '' through their weak coupling to photons . 
 we have adapted such an experiment to the microwave regime and constructed the table top apparatus . 
 this work presents an overview of the experimental setup and then focuses on our latest measurement run and its results . by operating the apparatus within a superconducting mri magnet , 
 competitive exclusion limits for axion like particles to the first generation optical light shining through the wall experiments have been achieved .