Quantum Cloning for Absolute Radiometry

In the quantum regime information can be copied with only a finite fidelity. This fidelity gradually increases to 1 as the system becomes classical. In this Letter we show how this fact can be used to directly measure the amount of radiated power. We demonstrate how these principles can be used to build a practical primary standard.

Figure 1

(a) Model of a nontotally inverted medium as succession of infinitesimal gain elements $G_n$ and loss elements ${\eta }_n$. (b) Each loss element ${\eta }_n$ within the fiber is equivalent to a smaller loss element $Q_n$ before the amplifier.

Figure 2

Experimental arrangement: a broadband source with controllable polarization and power is amplified by an erbium doped fiber amplifier. The degree of polarization (DOP) of the amplified light is then measured with a polarimeter. Spectral bandwidth is determined by a tunable filter. A value for the input power can be calculated from the DOP and compared with a calibrated power meter (monitor).

Figure 3

Optical frequency-domain reflectometer measurement showing homogenous gain per unit length within the ${\mathrm{Er}}^{3+}$ doped fiber. The solid line is an exponential fit of the data.

Figure 4

Autocorrelation function of the source after the filter. $\gamma (\tau )$ is the fringe visibility measured with a low-coherence interferometer; ${\tau }_c$ will simply be the numerical integral of the square of this data. The solid line is a Gaussian fit.

Figure 5

Fidelity versus number of input photons per mode, fitted with Eq. (5). Representative errors are shown as boxes on some points. The inset shows the output versus input number of photons per mode, where the line is a fit on the first data points (${\mu }_{\mathrm{in}}^{*}<1$), showing reduced gain as ${\mu }_{\mathrm{in}}$ grows.