An optical analogue to Schrödinger’s cat

Entanglement between a rubidium-87 atom and a laser pulse leads to the creation of macroscopic superposition states.

A rubidium-87 atom (blue sphere) is entangled with a light pulse that is in two distinct superpositions of phase states. A measurement of spin collapses the atom state and leads to the odd (top) or even (bottom) superposition state of the light.

Erwin Schrödinger’s cat gedanken experiment of 1935 presented the possibility of entanglement between microscopic and macroscopic physical systems and the resulting quantum behavior of macroscopic objects. In the years since, researchers have realized analogous systems with entanglement between microscopic and macroscopic structures—for example, the entanglement between a trapped ion and the vibrational state in the trap described in David Wineland’s Nobel Prize–winning work. However, the resulting macroscopic quantum superposition states typically had limited utility because their creation had a low probability or they had short transmission distances. Now Gerhard Rempe of the Max Planck Institute of Quantum Optics in Germany and his colleagues have deterministically produced a macroscopic light pulse with a controlled superposition state.

Rempe and his colleagues used a single rubidium-87 atom trapped in an optical cavity as the microscopic object and a laser pulse as the macroscopic cat. After putting the atom into a quantum superposition of up- and down-spin states, the researchers reflected the light pulse off the optical cavity. Although the light would not gain a phase shift for an atom in the up-spin state, it would gain one for the down-spin state. Because the atom had both up and down spin, the light experienced both phase shifts and became entangled with it. After rotating the atom’s spin by 90 degrees, the researchers measured the spin state, projecting it into either the up or down state. For a spin-up measurement, the light was in an odd superposition of the phase states; for a spin-down measurement, it was in an even superposition, as shown in the figure. Rempe and his colleagues confirmed the creation of a macroscopic superposition state by measuring the probability distribution in phase space with a homodyne detector.

The experiment performed by Rempe and his colleagues offers more than the realization of a famous thought experiment. Compared with previous work, the results provide some important advantages, such as the creation of atom-light entanglement in every trial. Furthermore, because it is optical, the macroscopic object can propagate without losing its superposition state. Those features make the new technique particularly promising for quantum network applications. Schrödinger’s cat, simultaneously alive and dead, may become a new mode of information transmission. (B. Hacker et al., Nat. Photonics 13, 110, 2019.)


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