When measuring photons, it's a case of 'wanted, dead' — catching them alive is not an option. But we can observe how a superposition of many photon waves progressively collapses as it interacts with a beam of atoms.rnEarlier this year, a team from the Ecole Nor-male Superieure in Paris recorded jumps of light heralding the birth and death of a photon trapped in a cavity. As they describe in this issue (Guerlin et al., page 889), the same researchers have now performed a similar, more complex trick — recording exactly how a coherent state of many photons collapses as it is measured.
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