www.qutools.com quED Manual 15
d) If the displayed coincidence count rate is zero and single count rates in both arms are at
the level of dark count rates, please follow the steps of the full alignment procedure
described in section 3.4.3.
Improving the Coupling
3.4.1 Operation of mirror mounts
The setup features a total of four mirror mounts to hold the two folding mirrors as well as the
two fiber couplers. They allow for a fine angular alignment of the respective component. The
movements of both the mirror and the coupler are controlled using a pair of adjustment
screws, one, on the top, providing the control in vertical and the other, close to the breadboard,
in horizontal direction.
Please note that all manipulations have to be performed in very small steps. Especially when
finalizing the alignment, tiny steps of only a view degree are required.
In any case, you should avoid by all means any adjustments without signal feedback. This
means you should never turn any of the screws so far as to loose the single count rate
completely. Furthermore, make sure that the count rates you observe are actual down-
conversion signal before you make any adjustments. If the laser is accidentally switched off or if
the polarizers are set disadvantageous, the dark count rates of the detectors can easily be
mistaken for signal.
When improving the coupling and later the entanglement two fundamentally different actions
have to be performed:
Pointing alignment: This first type of adjustment aims to correct for a simple mispointing. In
the quED case, this means that one (or both) of the fiber couplers does not point exactly into
the pump center within the BBO crystal while it is assumed, that the angle of the coupler axis
relative to the pump beam is fine. This is the most common misalignment to occur with the
quED as the overlap of the two coupler modes with the pump center (and each other) is quite
sensitive. This is a 2 degree of freedom (DOF) problem which can be addressed using the
adjusters of the respective fiber coupler only. (We strongly advice not to change the orientation
of the mirrors in this situation despite being theoretically possible).
To Next Page
Loading...
