The light an atom emits is not just a flash. It carries an imprint of the atom's state at the moment of emission. Physicists took a superconducting qubit, an artificial atom the size of a microchip, and made it glow into an open waveguide, like a bulb in a long corridor. Then they caught every photon and watched how its phase changed.
Quantum superposition is when an atom exists in two states at once, like a coin spinning in the air, neither heads nor tails. It turns out that the phase statistics of the emitted photons preserve this duality. Even with significant noise, the phase distribution remains distinguishable: it is broader than that of a regular, 'collapsed' state.
Over time, the quantum coin slows and falls: the qubit loses coherence. The scientists saw this through two signs. The signal envelope, the overall brightness of the flash, decays, showing how quickly the qubit gives up energy. Meanwhile, the phase distribution widens, pointing to pure decoherence — the process where quantum information leaks into the environment without changing energy.
Phase statistics turned out to be a direct quantitative probe of coherence. This method allows monitoring a qubit's health simply by looking at the light it emits. This matters for quantum computers, where each qubit must stay in superposition as long as possible.
Light turned out to be a mirror of the quantum world: it returns to us the atom's history at the moment of emission. And the closer we look at the phase, the clearer we see the fragile quantum coin fall onto the table.
A superconducting qubit operates at temperatures below 0.1 kelvin — colder than outer space. At such temperatures, it behaves like a single atom, even though it consists of billions of metal atoms.
The idea of reading a quantum state from emitted light echoes the 'lightsabers' of Star Wars: there, the weapon glows depending on the crystal's setting; here, the radiation reveals the qubit's internal state.