Quantum computers scarcely exist yet, but scientists have already begun working on shrinking them down by creating smaller components. A group of researchers have devised a way to use an LED to generate the entangled photon pairs needed for quantum teleportation, computing, and encryption. While the LED is not quite as reliable as lasers, its smaller footprint should help make quantum applications a bit more practical.
To generate entangled pairs of photons, researchers often use a process called parametric down-conversion. A laser sends its light at a crystal that can split each individual photon in two photons that are entangled so their properties are linked to one another. This method may be popular, but still has some problems.
The laser and crystal setup has a large likelihood of emitting zero or multiple entangled pairs at once, either giving us nothing to work with, or way too much for the detector that reads them. The researchers also point out that the lasers used for these purposes are often rather bulky, or else very expensive.
Overcoming these issues might be as simple as using stimulated quantum dots instead of laser-splitting crystals. Researchers have known for a while that it is possible to charge up a quantum dot and make it deliver a single photon at a time, but a more recent approach has used quantum dots to create pairs of entangled photons.
To get an entangled pair, researchers have to get the quantum dots to occupy what is called a "biexciton state." An exciton is a quasiparticle consisting of an electron hole and an electron, and a biexciton is a set of two electrons and two holes. When an exciton decays, the electron occupies the hole and puts its excess energy into a photon. When a quantum dot has two excitons that both decay, it generates exactly two entangled photons.
Researchers embedded a layer of quantum dots inside the active region of an LED, and designed a special cavity around them to minimize destruction of the generated photons' entanglement. They found the type of current supplied to the LED influenced its ability to generate photon pairs, so they tried using both direct and alternating currents.
The direct current was less complex, and allowed scientists to see how often the LED was able to generate coherent photons. Direct current produced photon pairs that were correctly entangled 71 percent of the time—better than chance, but still a cut below the typical 89 percent accuracy scientists are able to get using parametric down-conversion with lasers.
Another problem with direct current was that the constant flow of electricity tended to immediately regenerate electrons and holes. Once excited out, the electrons would drop back down into the holes and emit photons, often adding up to thousands of excess entangled pairs.
With alternating current, driving the electrons back and forth wasn't as much of a problem. Using AC improved the fidelity of the entangled photons to 78 percent. Narrowing the time allowed for the detection of a single photon pair and also reduced the problems associated with entanglement-breaking activities like scattering and re-excitation even further, making the highest achievable fidelity 83 percent.
This fidelity measure is still significantly lower than a laser-crystal setup. However, as with the laser and crystal, it is possible to overcome the disconnect created by less-than-perfect transmissions by simply using more photon pairs, which ensures the receiver is hearing the sender correctly.
Even though LEDs need to create more photon pairs to get their message across, they solve their own problem in a way, since researchers get much more control over the number of pairs they send. The authors of the paper speculate that the fidelity could still be improved by reducing background light emission and making the device's photon generation more agile, so it can respond to more frequent electricity pulses.
Either way, LEDs are an improvement footprint-wise over lasers, and could ensure quantum computers won't end up being room-sized devices.
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