Europe and China are gaining the upper hand in the race to bounce perfectly secure messages off satellites in low Earth orbit.
One of the great benefits of quantum communication is the ability to send messages from one point in space to another with perfect security. Not so great is the fact that so-called quantum cryptography is limited to distances of around 100 kilometers.
That’s because over longer distances, photons tend to be absorbed by the glass in fiber-optic cables and by the atmosphere when beamed from one location to another. That causes errors that are too great for perfect privacy.
But there is a potential way around this–to send photons to an orbiting spacecraft, which then retransmits the message securely when it is over another part of the planet. That’s possible because the photons traveling straight up only have to negotiate a few tens of kilometers of the atmosphere before reaching space.
So it’s not surprising that governments all over the world are keen on exploiting space-based quantum cryptography. Indeed, last year we reported on a Chinese team that had successfully reflected individual photons off an orbiting satellite, to simulate a satellite sending photons to the ground.
The Chinese team said the demonstration was a crucial step toward space-based quantum cryptography. However, the ability to send single photons from orbit and receive them on the ground is not enough.
A key factor is the error rate in this process. If the error rate is above 11 percent, quantum cryptography does not work.
So an important unanswered question is whether the error rate is small enough.
Today, we get an answer thanks to the work of Giuseppe Vallone at the University of Padova in Italy and a few pals. These guys have bounced polarized photons off a number of different satellites and measured the error rate in the photons that return to Earth.
And they have good news. They clearly show that the error rate can be made smaller than the critical threshold.
link.
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