If quantum
mechanics had a mascot, Schrödinger’s cat would surely be it. Now, the renowned
thought experiment has been upgraded to explain an even stranger system that
was accomplished for the first time: a spatially separated entangled system of
photons in a two-mode superposition.
Although
it might sound like sciency nonsense, the new system is quite the breakthrough.
It indicates that scientists can influence complex quantum states, and this
discovery, published in Science, has uses in computation and long-distance
communication.
But how
does the cat come into it? Schrödinger’s cat describes the curious phenomenon
of quantum superposition. In the thought experiment, the imaginary cat is
locked in a box with a flask of poison that is triggered by a binary quantum
mechanical process, like a quantum switch on or off. But until it is detected,
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, and it’s both alive and dead. Thus, the "quantum cat" is a
state in a two-mode superposition.
To create
the new state, the scientists from Yale University used another coincidence of
quantum mechanics: entanglement. The entangled particle cannot be defined
independently, and even if they’re separated, they’ll act as a single system.
When the property of one particle is caculated, the system immediately collapses,
but no information is transferred so it doesn’t violate relativity.
Researchers
constructed this entangled quantum cat state in a very precise wave. They used
two separated cavities (think high-tech microwave ovens) that emit light
particles only at a definite wavelength. The cavities were connected by a
supercurrent – an electric current with no dissipation – which permitted 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 scientists detected
the photons in the other cavity. This entangled cat state can be constructed
using up to 80 photons, but scientists think larger systems can be made.
The construction
of such a large system is another great accomplishment. Macroscopic quantum
coherence states show quantum properties at an everyday scale that can then be connected
in technology. Laser and superconductors are examples of highly coherent
systems.
Researchers
believes that this type of state is the first step in the construction of the
logical operation required for quantum programs, thus bringing us a step closer
to quantum computers.
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