Quantum-Limited Noise Measurement

Our noise measurement system achieves accuracy to the quantum limit—exceeding any radio astronomy or standards laboratory. Conventional noise measurement methods utilize thermal noise reference standards, typically at 295 K room temperature or 77 K liquid nitrogen temperature. These noise reference levels and associated uncertainties are much too great for measurement of amplifiers having noise temperature less than 5 K.

Over the years, many schemes have been tried for improved accuracy in amplifier noise measurement and noise parameter extraction. These typically involve redundant measurements at different source impedances and temperatures. However, no great measure of success is possible with these attempts because fundamental weaknesses of the method including high reference temperature are not addressed.

Absolute Quantum Reference

The noise reference of our system is the quantum unit or one photon. Shown below is the relation of noise temperature and equivalent photon noise at 1.4 GHz. One photon is the zero-point noise and an absolute reference level. The input noise for an uncooled 3 K amplifier is 45 photons and noise temperature measurement uncertainty is 0.1 K or about 1 photon. A cooled device operates near the quantum noise limit where an absolute quantum reference is essential for measurement.

Equivalent system input noise comprises quantum and thermal noise components. The usual kTsys approximation overestimates thermal noise density by one-half photon.

Ultralow-noise amplifier device development was the motivation for this measurement system. Speed and accuracy far surpass those of any "automatic" instrument or manual hot–cold method, and no cryogenics or cooled components are used. The techniques employed are also applicable to the field of radiometry. nn