Radiation and noise in quantum electronics

Avraham Gover, Avner Amir, Luis R. Elias

The problem of fundamental laser line broadening due to random spontaneous emission of radiation and amplification of thermal radiation noise is analyzed in terms of a classical fluctuating field phasor model. We derive a general expression for the intrinsic linewidth, given in terms of the spectral power of the radiation noise source, which can be classical or quantum mechanical in nature. In the case of a two-level atomic laser, we recover by the use of Einstein relations, the traditional linewidth formula of the Schalow-Townes form. In the case of the free-electron laser (FEL), using the explicit expression for the spontaneous emission, we present calculation of the laser linewidth by purely classical methods. The result agrees with the one obtained in the framework of a quantum-mechanical model. By using ``extended Einstein relations'' which are applicable to classical radiators, we show that a Schalow-Townes-type formula can also be obtained for the FEL. The theory predicts extremely narrow intrinsic linewidth ${(10}^{\mathrm{\ensuremath{-}}7}$ Hz) for cw FEL's with parameters similar to those of the FEL experiment of Elias et al.

E. S. R. Gopal

Gershon Kurizki

It is suggested that the quantum uncertainty of cavity radiation is measurable via Aharonov-Bohm effects for electrons in a ring which is threaded by the magnetic cavity flux. When the electrons follow this oscillatory flux nearly adiabatically, and dissipation is negligible, the noise in the ring current should provide a quantum-nondemolition measure of the flux uncertainty. Unlike standard photodetection, such measurements should reveal the scaling of the flux uncertainty with the diamagnetic or dielectric index of the cavity medium, depending on the dimensions of the electron ring.

J. P. Gordon

Hans Erik Ekström, Boris S. Karasik

An experimental study is presented of the output noise in hot-electron superconducting NbN bolometer mixers. Fluctuations of the electron temperature have been shown to be the main source of electrical noise. The rolloff frequency of the spectral noise density corresponds well to the inverse electron temperature relaxation time (≊230 ps). Equivalent noise data have been obtained for the device at a physical temperature within superconducting transition region, and at a somewhat lower physical temperature with the device pumped by 350 GHz radiation to reach the same operating point. The single sideband (SSB) mixer noise temperature due to intrinsic noise mechanisms for the NbN device is estimated to 360 K independent of radiation frequency. A larger critical current density may promote a further reduction of the intrinsic conversion loss of the mixer device.

M. W. P. Strandberg