Quantum noise limit

In plain English, we must predict the outcomes A2 and B2 with a precision better than the statistical spreading of the outcomes A and B with the additional constraint that A and l> are outcomes of quantum noise limited measurements. 

If the modulation frequency is chosen sufficiently high (f2 > 1000 MHz), the technical noise may drop below the quantum-noise limit set by the statistical... 

Fig. 9.98 Schematic diagram of squeezing experiments with a nonlinear medium in a Mach-Zehnder interferometer (a) experimental arrangement (b) noise power density p (j)) and quantum-noise limit po which is independent of the phase 0...

The best squeezing results with a noise suppression of 60 % ( -4 dB) below the quantum noise limit po were obtained by Kimbel and coworkers [1336] with an optical parametric oscillator, where the parametric interaction in a MgOiLiNbOs crystal was used for squeezing. 

The observed signal-to-noise power ratio for these experiments was found to behave in accordance with the theoretical expression obtained for parallel, plane-polarized beams incident on a quantum-noise-limited detector under ideal conditions [7.4-7], i.e.. 

For quantum-noise limited detectors such as photoemitters and reverse-biased photodiodes operating in the infrared and optical [7.4—7,10,14,15], assuming that the incident radiation and the coherent LO are polarized in the same plane, the input SNR to the nonlinear device is [see (7.1) and (7.42b)]... 

If the modulation frequency X2 is chosen sufficiently high Q > 1000 MHz), the technical noise may drop below the quantum-noise limit set by the statistical fluctuations of detected photons. In this case, the detection limit is mainly due to the quantum limit [6.4]. Since lock-in detectors cannot handle such high frequencies, the signal input has to be downconverted in a mixer, where the difference frequency between a local oscillator and the signal is generated. 

Due to the conversion process an absorbed photon give rise to less than one electron generated in the CCD. This phenomenon, also called a "quantum sink" shows that the detector is degrading the S/N ratio of the image. The quality of an image being mainly limited by the quantum noise of the absorbed gamma this effect is very important. 

It is possible to approach shot-noise-limited performance in many optical experiments. When light levels are low, photomultipliers serve as noise-free quantum amplifiers with a gain of 10 . For absorption measurements, detectors with the highest quantum efficiency and uniformity of response, such as end-on semitransparent photocathode styles, are better than the high gain, opaque photocathode, low dark count types that are used for luminescence measurements. If one needs to measure absorption with a precision of AA 10, then 10 photons need to be accumulated at each data point. At these light levels, the dark count usually may not contribute greatly to the S/N. However, in absorption... 

In the shot noise limit, we observe the familiar increase in SNR with It is worth noting that the SNR cannot exceed that given by Eq. (4.6), even for a perfect spectrometer. The best possible Raman measurement, with 4 7r collection, 100 per cent spectrometer transmission, 100 per cent quantum efficiency, and zero background is still limited in SNR by Eq. (4.6). As long as the photons arrive randomly (and any other case is hard to envision), the maximum SNR will be given by Eq. (4.5) and (4.6). Figure 4.4 is an example of a Raman spectrum with an SNR determined mainly by sample shot noise. 

Detector Type Pixel area/pm2 Number of pixels (area/mm2 j Max. quantum (10% limits) Full well capacity/e Read-out noise (e rms)... 

One of the parameters characterising the EELS detection system is its Detective Quantum Efficiency, DQE, defined as the ratio of the number of counts to the mean square fluctuation in them. A detection system is said to have unit DQE if it is shot noise limited, i.e. the mean square signal variation in a chaimel is equal to the number of counts within it. However, channel-to-channel gain variations in photodiode arrays, dark current, and detector noise... 

Example 9.18 The shot-noise limit of an optical detector with the quantum efficiency r] < 1 irradiated by N photons per second leads to a minimum relative fluctuation A5/5 of the detector signal 5, which is for a detection bandwidth A/ given by... 

H.A. Bachor, PT. Fisk, Quantum noise—a limit in photodetection. Appl. Phys. B 49, 291... 

The output waveform of a stable, single-frequency laser far above the threshold of oscillation may be approximated by an almost perfect sine wave with nearly constant amplitude and frequency. For a laser operating in an ideal environment, the spectral purity is measured by a linewidth which is determined by frequency fluctuations caused by random walk of the oscillation phase under the Influence of spontaneous emission (quantum) noise. In their fundamental 1958 paper, Schawlow and Townes predicted that the quantum phase noise limited line profile will be a Lorentzian with a full width between the half power points (FVIHM) that may be approximated by ... 

Electronic detectors offer the ultimate in frequency response, as high as tens of gigahertz, and especially in the visible, approach photon-counting or quantum-limited performance. As such, they offer magnitudes of improvement in sensitivity over thermal devices. In the limit of photon-counting performance, the signal measurement fluctuation or noise is produced by the random production of photo electrons. In many cases, electrical noise in the postdetection amplifier, rather than photon noise, limits the sensitivity. 

Equation (13) is applicable to signal-noise-limited detector performance where the noise is the quantum-limited fluctuation of the signal itself. Using Eqs. (1), (11), and (13), the signal-to-noise ratio becomes... 

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