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One of the emerging uses for single photons is to pack them with quantum information and send them to another location. This technique, known as quantum communication, exploits the laws of physics to make sure the information cannot be read by any eavesdropper.
One challenge is to find ways to send this quantum information around the world. That’s difficult because the information is fragile—any interaction between the photons and their environment destroys it. Photons cannot travel more than a hundred kilometers or so through the atmosphere or through optical fibers without the quantum information they carry being destroyed.
So Chinese physicists have come up with a workaround: beam the photons to an orbiting satellite, which relays them to another location on Earth’s surface. In this way, the uncomfortable passage through the atmosphere can be minimized. If photons are transmitted from ground stations at high altitude, their journey is mostly through the vacuum of empty space.
But there is a problem. Quantum communication requires detectors that can spot and measure single photons. In recent years, physicists have designed and built increasingly sensitive devices that can do this.
However, this sensitivity makes them vulnerable to any kind of background noise, which can overwhelm the signal from the photons themselves. And space is filled with unwanted noise in the form of high-energy particles, extreme temperatures, and extraneous light from sources such as the sun.
Building single-photon detectors that can operate in this environment is a significant challenge. So it’s no surprise that physicists have been scratching their heads over this issue for some time.
Today, Meng Yang and colleagues at the University of Science and Technology of China in Hefei say they have solved the problem. They have even tested their machine over the last two years on an orbiting satellite and say it works well.
The team’s detector exploits a phenomenon known as avalanche breakdown, which occurs in semiconductor chips under special circumstances. A semiconductor such as silicon conducts electric current in the form of free electrons and holes that can move through the material lattice under the influence of an electric field.
Under normal circumstances, these charge carriers are bound to the lattice and so cannot move. In these circumstances, the material acts as an insulator.
But if an electron is set free, perhaps by thermal fluctuations or a kick from an incident photon, it can travel through the structure, creating a current. In these circumstances, the material becomes a conductor
Of course, a single electron freed in this way creates a tiny current that is hard to detect. So the trick with avalanche breakdown is to set up a voltage that rapidly accelerates a free electron to high enough speeds to knock other conducting electrons free. This creates a chain reaction—an avalanche—that results in a much larger and more easily detectable current.
In recent years, physicists have made these devices so sensitive that a single photon of a specific wavelength can trigger this kind of avalanche. The result is a single-photon detector capable of spotting most of the photons that hit it.
However, this sensitivity comes at a price. It’s easy to see how a high-energy particle can tear through a silicon photodiode, kicking out electrons and triggering an avalanche. And in space, this kind of effect creates so much background noise—called a dark count rate—that it swamps the signal from the photons physicists hope to measure.
So the task for Yang and co was to find ways to protect and enhance the performance of commercial off-the-shelf single-photon detectors so that they can operate in space.
Their first fix was straightforward—surrounding the detector with shielding that blocks high-energy particles. This is a delicate balancing act because shielding is heavy and thus expensive to put in orbit. The interaction between the shielding and the high-energy particles can also create showers of secondary particles that make the dark rate count even worse.
Yang and co eventually settled for a shield consisting of two layers. The outer layer is 12-millimeter sheet of aluminum, and the inner layer is a 4mm sheet of the much denser and heavier element tantalum. The resulting shield reduces the radiation dose by a factor of 2.5.
This shielding also acts as a thermal insulator, which allows the team to cool the detectors to -15 °C. This also reduces dark counts by minimizing thermal fluctuations in the silicon detector.
Finally, the team developed electronic drivers that switch off the detectors during periods when they are vulnerable to background noise, a technique known as after-pulsing resistance.
The effect of all these approaches was significant. For unprotected single-photon detectors, the expected dark count rate is over 200 counts per second. This is too high for quantum communication in space.
However, the modified detectors have a dark count rate of just 0.54 counts per second. That’s two orders of magnitude better.
In 2016, Yang and co launched their detectors onboard the Chinese Micius satellite, a quantum technology demonstrator that has notched up an impressive series of breakthroughs. For example, the detectors were a key component in teleporting the first object from Earth to orbit—a single photon in 2017. The satellite also enabled the first quantum encrypted video call between continents.
These experiments have set the scene for a new generation of space-based quantum communication. “Our single photon detectors open new windows of opportunities for space research and applications in deep-space optical communications, single-photon laser ranging, as well as for testing the fundamental principles of physics in space,” say Yang and co.
In the meantime, the rest of the quantum physics world has looked on with envy. China has a clear lead in space-based quantum communication, albeit with help from European researchers in key areas.
Europe is working on an orbiting quantum technology demonstrator called the Security and Cryptographic mission, or SAGA. This is part of much larger plan to create a quantum communication network across the continent. However, no launch date has been set.
By contrast, US plans have stalled. In 2012, the military technology research agency DARPA started a program called Quiness to test quantum communication technologies in space. But the program—and the field in general—has suffered from a severe lack of funding.
An important question now is how the rest of the world, particularly the US, plans to catch up.