A new way to intertwine the fates of light shards has overcome some serious obstacles in the way of photon-based quantum computing.
Researchers from the Max Planck Institute for Quantum Optics in Germany have successfully entangled 14 photons in a state considered ideal for qubits, more than double previous attempts – while also improving their efficiency.
Unlike the binary code ‘bits’ behind more traditional forms of computing technology, qubits exist in a state of probability called superposition, and behave like an inverted currency as it rolls through the air.
Algorithms that rely on the way groups of quantum coins fall can make some fairly complex computations short, but only if their collective path is not inadvertently blown away by the environment.
This discontinuity in particle superposition is referred to as decoherence, and is a significant obstacle for engineers designing useful Quantum computers.
In theory, almost anything can exist in a quantum superposition of states, from electrons to atoms to whole (or larger) molecules. But to limit disconnection, smaller, simpler things take the cake.
Photons make perfect qubits. Unfortunately, practical quantum computers need a lot of qubits. In the thousands. Even millions. The most the best. Not only do they all need to spin in superposition at once, their fates must be shared. Or, to use the term physics, synaptic.
This is where the challenge comes in.
There are relatively easy ways to entangle pairs of photons. Force the atom to emit a wave of light and then split it using a special screen, and you’ll get two photons with a common history.
While they remain in flight with their own yet unmeasured characteristics, they behave somewhat like that spinning coin. Eventually, one will go up the head and the other tails.
The entanglement of more than two photons becomes more difficult.
Experiments with objects called quantum dots have succeeded in entangling strings of three to four photons. Not only is it unlikely to produce the hundreds and thousands needed for As far as the computerHowever, the entanglement condition using this approach is not as reliable as the engineers might like.
More recent studies using atoms with large electron orbitals, called Rydberg atoms, have produced up to six entangled photons, all in an efficiently entangled form. Although the method can make computing components super fast, it is not an easily scalable option either.
This newer solution could, in theory, produce any number of entangled photons, all in the ideal case.
“The trick with this experiment was that we used a single atom to emit photons and weave them in a very specific way,” says Physics PhD student and lead author Philip Thomas.
An atom of rubidium was tickled at emitting light waves, which were directed into a cavity-shaped cavity to reflect back and forth in a very precise manner.
By fine-tuning the way rubidium glows, each photon could be entangled with the entire state of the atom—meaning that each photon bouncing back and forth in the cavity was entangled with a large number of its siblings as well.
“Because the series of photons emerged from a single atom, they can be produced in a deterministic way,” says Thomas.
In this case, the team was able to entangle 12 photons in a less efficient linear array, and 14 in the precious Greenberger-Horn-Zellinger (GHZ) case.
“To our knowledge, 14 entangled optical particles is the largest number of entangled photons generated in the lab to date,” Thomas says.
Not only were they able to entangle many photons, the efficiency of this method improved in previous processes, with roughly one in two photons providing precisely entangled qubits.
Future setups will need to introduce a second atom to provide the qubits needed for many quantum computing operations. The presence of entangled photons at a click could provide the foundations for a technology that transcends computing, and occupies a central role in quantum encoded communications.
This research was published in temper nature.
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