Groups of photons prepared in certain quantum states can noninvasively probe ultrasensitive objects such as individual atoms or living cells, overcoming the standard quantum limit for the first time. The results from the Institute of Photonic Sciences (ICFO) have defeated the limit imposed by quantum mechanics that requires all photons to be polarized in one direction and also in the opposite direction; that is, in two different states at once. This is similar to a thought experiment described by Erwin Schrödinger in 1935 in which he imagined a hypothetical cat in a “superposition of states” – simultaneously dead and alive. In the experiment, the researchers prepared pairs of photons in a “Schrödinger cat” state and sent them through a cloud of rubidium atoms, measuring their polarization on the way out. In this way, they deduced the number of atoms in the cloud and the magnetic field of their surroundings. At the same time, they were able to assess the damage, i.e., the number of photons absorbed by the cloud. “Atoms of rubidium are a good model because, on one hand, they share the same characteristics as the cells in relation to the information obtained and damage caused and, on the other hand, we have very precise knowledge of their characteristics,” said Morgan Mitchell of ICFO and a professor at the Institució Catalana de Recerca i Estudis Avançats. The Schrödinger cat results showed that photons organized in certain quantum states can provide more information, giving a clearer picture of ultrasensitive objects while causing less damage. The team overcame the standard quantum limit, which quantifies the maximum amount of information obtainable with any traditional probing. “Overcoming this limit provides rigorous proof of the effectiveness of quantum physics for measuring delicate objects,” Mitchell said. Details of the research were published in Nature Photonics (doi: 10.1038/nphoton.2012.300).