Quantum cloning: Radiometry on a small scale

Measuring the luminous power of light is a basic task in physics laboratories and telecom applications. Until recently, however, measuring light energy in an absolute manner has required the complex equipment and techniques available only in metrology laboratories.

Theoretical breakthroughs have enabled physicists at the University of Geneva to build a device that can measure the amount of light in an optical fiber without the need for calibration. The setup can determine absolute measurement of luminous power over a broad range, from a few photons to tens of nanowatts. The device’s accuracy was verified by comparing it with an instrument calibrated by the Swiss metrology laboratory.

Although its accuracy of approximately 1 percent is not quite as good as that of the systems in a metrology laboratory, which can reach 0.01 percent under ideal conditions, it offers absolute calibration, is simpler to implement and can be arranged on a small desktop, making it a practical addition to a typical physics lab.

The study, by Bruno Sanguinetti, with co-authors Enrico Pomarico, Pavel Sekatski, Hugo Zbinden and Nicolas Gisin, was published August 2010 in Physical Review Letters.

The cloning process consisted of injecting an unknown number of photons into a 2-m erbium-doped fiber, where excited erbium atoms were stimulated to emit photons.

Researchers at the University of Geneva have built a device that measures the energy of light by cloning photons. The setup consists of three parts: The first section (top) generates light; the second, the cloning apparatus, uses stimulated emission to amplify the incoming light; and the third (below) is a polarimeter, which takes a direct measure of the spectral radiance of the input, determining the fidelity of the cloning process. Courtesy of Bruno Sanguinetti.

“These other photons are the clones, imperfect copies of the input photons,” Sanguinetti said.

Although the polarization of most of the clones matched that of the input light, the polarization of some clones was perpendicular. By comparing the vertically and horizontally polarized light – a relative measurement requiring no calibration – the investigators could determine the cloning fidelity of the device and calculate the absolute amount of incident light.

Because stimulated emission is fundamentally tied to spontaneous emission, some noise is added, reducing the cloning fidelity. In designing their experiment, the investigators devised a formula showing that the more quantum bits, or qubits – in this case, photons – one starts with, the better the reproduction; the larger the system, the greater the accuracy of the cloning process.

The researchers are optimistic that their findings could have real-life applications.

“I have a great deal of hope that this will be a step in going even further toward using fundamental quantum principles in technology development,” Sanguinetti said. “Our device can already be used to calibrate light sources and detectors.

“From a theoretical point of view, the most exciting aspect of this experiment is that it ties a measurement of light intensity directly to a fundamental law of quantum physics: the ‘no cloning theorem.’ ”