AAWSAP DIRD, Quantum Tomography of Negative Energy States in the Vacuum, January 2011

Text

This text came with the file. It may have mistakes. When the text and the image disagree, the image is right.

UNCLASSIFIED//FOR OFFICIAL 03E 014L I
Photodiodes
Most photodetectors apply a version of the photoelectric effect to operate in which
incident light radiation ionizes a piece of photosensitive material in the detector and
produces freely moving electrons, i.e., an electric current is created that can be
amplified and handled by electronic means. A commonly used type of detector is the
linear-response photodiode. In most cases, the photosensitive part of the detector is a
P-1-N structure, a sandwich of Positively doped, Intrinsic, and Negatively doped
semiconductor material. Commonly, silicon (Si) or indium gallium arsenide (lnGaAs)
are used where Si detects light out to a 1 µm wavelength and InGaAs operates in the
range 0.19 µm to 2.6 µm. A bias voltage of about 10 Volts is applied to drain the
majority carriers (electrons in N and holes in P) out of the intrinsic zone. In this
depletion region an unstable situation is created for the minority carriers. As soon as
electron-hole pairs are present in the intrinsic zone, the bias voltage produces a current
that is proportional to the number of carriers. Electrons in the valence band are lifted
into the conduction band by the absorption of light radiation, i.e., the absorption of a
single photon lifts one electron into the conduction band, which creates electron-hole
pairs in the depletion zone. This process can be made highly efficient because the
applied voltage is very low so that no avalanche of charge carriers into the conduction
band (via collisions) is formed. The current response of the detector is linear in the
intensity of the detected light. However, thermal fluctuations cause Nyquist noise in
the photocurrent. Thermal effects also create electron-hole pairs in the depletion zone
thus producing dark current, which is electronic noise. Because of this electronic noise,
linear-response photodiodes do not reach single-photon resolution. They are suitable
for relatively high intensities, greater than about 100 photons per microsecond.
There are inefficiencies and noise associated with realistic photodetection. A convenient
model to understand the effect these have on experiments is provided by imagining a
fictitious beam splitter placed in front of an ideal detector. See Figure 9. Only the
transmitted photons are counted, so that the transmissivity of the fictitious beam
splitter corresponds to the detection efficiency. Dissipation is always accompanied by
fluctuations. These degrade the quantum noise properties of the detected light. The
fluctuations are modeled by a vacuum entering the unused port of the fictitious beam
splitter. This analysis shows how the nonclassical features of light are lost when the
detectors are inefficient.
Balanced Homodyne Detection
Under idealized conditions the photon number is measured in direct photodetection.
However, another method of detection exists, in which the light field amplitudes (the
quadrature components) are measured instead of the quantized light intensity.
Intensity (photon number) and field amplitude (quadrature) are distinct quantities.
There is no simple relationship between the photon statistics and the quadrature
distributions in the quantum regime, but the two are shown to be related via the
mathematics and procedures of quantum state sampling [38]. Furthermore, the field
amplitudes contain phase information, and so they are dependent on phase.
UNCLASSIFIED//FOA &FFI@IAL U.!E ONE I
27