NTT researchers entangle photons 300 kilometers apart using fiber optics
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Japanese telecom giant NTT is concerned with more than routing phone calls — it is exploring the technology to enable a new kind of super-fast communication based on quantum mechanics. NTT’s recent experiments with quantum entanglement have reached a true milestone in the understanding of this phenomenon. A team led by Takahiro Inagaki at the NTT Basic Research Laboratories in Kanagawa has managed to keep two particles entangled over a distance of 300 kilometers through fiber optic cable.
Entanglement has been of huge interest to physicists over the years for its ability to seemingly transfer information simultaneously from one location to the other without actually crossing the space in between. Two particles that are entangled share the same quantum state, describing properties like spin, momentum, and polarization. Thus, observing one of the particles tells you about the other.
Any system that can lead to a faster, more secure method of sending information would naturally be of use to a telecom company, hence NTT’s interest in researching entanglement. The record-setting experiment used a standard process for creating an entangled pair of photons called parametric down conversion. This consists of passing a single photon through a crystal of lithium niobate, which splits it into two entangled particles.
In NTT’s experiment, the two entangled photons were routed down separate 150 kilometer lengths of optical fiber to detectors at the end. The readings from each detector were compared in order to confirm that the particles were indeed still entangled at the end of their journey. It’s not just the distance that is of note here, but the medium through which the photons passed.
Previous experiments with entanglement have been able to pass data between distant points, but they usually relied on lasers. That’s considerably less convenient for our existing telecommunications infrastructure, but demonstrating this is possible in optical cable is a big deal.
The trouble with sending entangled photons down optical fiber is that many of them are absorbed or altered by the trip, and that effect only increases the farther they travel. Photon detectors are also prone to false positives, which leads to high background in the signal, known as dark noise. That means the entangled photons that do make it through are harder to detect. The NTT researchers were able to partially avoid this by employing a new type of superconducting photon detector that can take sensitive readings with much lower dark noise.
While this is an impressive feat, there are still fundamental issues with transmitting data through fiber optics with entangled particles. The researchers estimate that signal loss would mean a functional data rate of 1 bit every 10 million seconds. At that rate, it would take about 2.7 million years to transfer a single 1MB JPEG. Improved detectors could one day reduce dark noise and improve this speed by orders of magnitude, but the NTT experiment still shows the potential value of the technology.
Research paper: arXiv:1310.5473: “Entanglement distribution over 300 km of fiber”