Quantum-limited detection

The beating of a faint source with a high power coherent source is a well known process to detect its phase and amplitude. The same detection equipment allows the evaluation of the power of the source with theoretical limits similar to a noiseless photon counter. Such detection apparatus are limited by the bandwidth of the electronic component as this bandwidth is rapidly increasing, this may be a competitive solution for quantum limited detection in the far infra red. The phase of a thermal source is an useless information ... 

Optical heterodyne detection is based on interferometric mixing of the weak signal beam to be detected with a frequency offset local oscillator beam on the surface of the detector. It can be shown that, in principal, ideal (quantum limited) detection can be achieved however, it requires a complex optoelectronic system with rigorous mechanical tolerances. 

Finally, although heterodyne receiver is basically an amplitude-phase detector, its detectivity as a power receiver is similar to a quantum limited detector ... 

Gas absorptions in the ultraviolet are due to electronic transitions in the target species and are relatively strong. They lead to well-defined line spectra that allow gases to be detected with considerable sensitivity. When combined with the use of detectors with near-quantum limited performance this leads to detection sensitivities of a few nanomoles per mole over typical path lengths. 

The sensitivity of systems which detect changes in phase or wavelength rrltimately depends on the linewidth of the sottrce, i.e., the narrower source linewidth the better. Similarly, the sensitivity of sensing systems which detect changes in intensity may depend on the source intensity noise. Fiber lasers can have close to quantum-limited linewidth and intensity noise, making them excellent sottrces for marty sensing applications. 

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. 

A more far-reaching phenomenon is the possibility of generating radiation in "squeezed" states [4.19]. Such radiation exhibits reduced noise below the quantum limit and could have important applications for optical communication and precision interferometric measurements of small displacements, e.g. in gravity-wave detection experiments. A considerable degree of "squeezing" has recently been experimentally demonstrated [4.20, 21]. Various aspects of modern quantum optics have been discussed in [4. 22-25]. 

Gaudino, R., and Poggiolini, P. (2003) Quantum limit of direct-detection receivers using duobinary transmission. IEEE Photonics TechnoL Lett, 15 (1), 102-104. 

The total noise budget must be compared to the standard quantum limit. Using the spread Mco Gx in the uncertainty principle, it is possible to deduce the minimum detectable strain ... 

In optical domain, preamplifier is no more an utopia and is in actual use in fiber communication. However quantum physics prohibits the noiseless cloning of photons an amplifier must have a spectral density of noise greater than 1 photon/spatial mode (a "spatial mode" corresponds to a geometrical extent of A /4). Most likely, an optical heterodyne detector will be limited by the photon noise of the local oscillator and optical preamplifier will not increase the detectivity of the system. 

Heterodyne is a very efficient tool for detecting the phase of a "coherent" signal i.e. a signal which has a stable phase relation to the local oscillator. The detector is only limited by the quantum fluctuation of vacuum. This property is common use in coherent lidar. Satellite to satellite optical communications using laser as a local oscillator are under development (Fig. 3). 

There are methods available to quantify the total mass of americium in environmental samples. Mass spectrometric methods provide total mass measurements of americium isotopes (Dacheux and Aupiais 1997, 1998 Halverson 1984 Harvey et al. 1993) however, these detection methods have not gained the same popularity as is found for the radiochemical detection methods. This may relate to the higher purchase price of a MS system, the increased knowledge required to operate the equipment, and the selection by EPA of a-spectrometry for use in its standard analytical methods. Fluorimetric methods, which are commonly used to determine the total mass of uranium and curium in environmental samples, have limited utility to quantify americium, due to the low quantum yield of fluorescence for americium (Thouvenout et al. 1993). 

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