Abstract
The following chapter is based on the publication “Real-time imaging of quantum entanglement” (Fickler et al., J Sci Rep 3:1914, 2013, [1]). As outlined in the previous section, the transfer setup is very flexible in creating two-dimensional transverse spatial mode entanglement. The investigation of such modes and their complex spatial patterns requires a detection scheme with a high spatial resolution. Masking the modal structure (as shown in the previous section) or scanning a single pixel detector enables the detection of spatial structures. However, these schemes come at a certain cost. Masking the beam and thereby measuring its transverse spatial structure needs to be adapted to the specific mode properties that are under investigation. Hence, it is not flexible for use with a larger variety of modes without adapting the shape of the mask (if possible at all). On the contrary, scanning a single pixel detector does not need to be adapted to the measured modal structure. However, the higher the resolution, the less efficient the detection is, which leads to a drastic increase in time consumption.
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Notes
- 1.
The quantum efficiencies (QE) given for CCD cameras should not be confused with the detection efficiencies of single-photon detectors, which can be as high as 60–70 % for avalanche photodiodes (APD). The QE for CCD chips is the ratio between incident photons and electrons generated due to the photon impact. It is often called the spectral response because of its wavelength dependence. Compared to APDs, the detection efficiency is lower because not all electrons might be detected. Other effects, such as fill factor of the chip or absorption of the photon at the gate electrodes, have to be considered as well. For more information see the web page of one of the biggest manufacturers of scientific CCD cameras, Andor (www.andor.com).
- 2.
In contrast, EMCCD cameras amplify the signal of the CCD chip afterwards by an electron multiplying process.
- 3.
All specifications of the ICCD camera have been taken from the manufacturer.
- 4.
The ICCD camera has a minimum insertion delay that is specified to be 19 ns. However, this is not adjustable via the electronics of the camera. Therefore, the slightly bigger but adjustable delay of 35 ns was chosen to find the best coincidence window (explained later). Moreover, an additional fiber easily delays the second photon by that time span.
- 5.
Instead of labeling the LG modes by their OAM content \(\pm l\), as was done in the last chapter, the abbreviation \(LG_{\pm l}\) will be used in this section. This is done to emphasize that the focus lies on the measurement of the transverse spatial mode rather than on its OAM content.
- 6.
Before the photon number per angular region was evaluated, the camera-induced readout noise mentioned earlier was subtracted. However, this subtraction should not be confused with the usual background subtraction. It is rather comparable to setting the threshold voltage in single-photon avalanche photodiode detectors, since the read-out noise is inherent to the read-out process of the ICCD. This can be adjusted as a threshold of whether a detected event is regarded as a photon or not.
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Fickler, R. (2016). Coincidence Imaging of Spatial Mode Entanglement. In: Quantum Entanglement of Complex Structures of Photons. Springer Theses. Springer, Cham. https://doi.org/10.1007/978-3-319-22231-8_4
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