If quantum mechanics had a mascot, Schrödinger’s cat would certainly be it. Now, the famous thought experiment has been upgraded to explain an even weirder system that was achieved for the first time: a spatially separated entangled system of photons in a two-mode superposition.
Although it might sound like sciency jibberish, the new system is quite the breakthrough. It shows that researchers can manipulate complex quantum states, and this discovery, published in Science, has applications in computation and long-distance communication.
But how does the cat come into it? Schrödinger’s cat explains the curious phenomenon of quantum superposition. In the thought experiment, the imaginary cat is locked in a box with a vial of poison that is activated by a binary quantum mechanical process, like a quantum switch on or off. But until it is observed, this process is in a state of superposition, meaning it exists as a combination of all of its states – it is both on and off.
The state of the cat depends on quantum mechanics, so the cat is not alive, it’s not dead, it’s both alive and dead. Therefore, the "quantum cat" is a state in a two-mode superposition.
To construct the new state, the researchers from Yale University used another quirk of quantum mechanics: entanglement. The entangled particle cannot be described independently, and even if they’re separated, they’ll act as a single system. When the property of one particle is measured, the system instantaneously collapses, but no information is transferred so it doesn’t violate relativity.
The team constructed this entangled quantum cat state in a very specific wave. They used two separated cavities (think high-tech microwave ovens) that emit light particles only at a specific wavelength. The cavities were connected by a supercurrent – an electric current with no dissipation – which allowed the photons in the two cavities to become entangled. The cat is now alive and dead and in both boxes at the same time.
The photons in one cavity were then forced into a superposition state, and the researchers observed the photons in the other cavity. This entangled cat state can be constructed using up to 80 photons, but researchers think larger systems can be made.
The construction of such a large system is another great achievement. Macroscopic quantum coherence states exhibit quantum properties at an everyday scale that can then be harnessed in technology. Laser and superconductors are examples of highly coherent systems.
The team believes that this type of state is the first step in the construction of the logical operation needed for quantum programs, thus bringing us a step closer to quantum computers.