Issue Date: September 11, 2017 | Web Date: September 7, 2017
Seeking materials to send unbreakable codes
In June, Chinese researchers announced a remarkable feat. With the help of an engineered crystal aboard a satellite orbiting Earth, they had beamed pairs of quantum-entangled photons from the satellite to receiving stations on two Tibetan mountaintops—located 1,200 km apart—and successfully measured the photons’ quantum properties (Science 2017, DOI: 10.1126/science.aan3211). This 1,200-km separation was more than 10 times the previous record, and it marked a major advance in the quest to use the quantum properties of photons to encode information. Quantum communications, as the field is known, holds the promise of sending information under unbreakable encryption.
This form of communication relies on the quantum entanglement of photons, in which a collection of photons generated together each have complementary spins or polarizations. These properties remain correlated even if the photons are separated. The idea that entangled photons could be used to encode data has been around since the 1960s, and scientists demonstrated the first use of quantum communication to send invincibly encrypted information in 1984.
This encryption method is called quantum key distribution (QKD). Similar to other methods, a person looking to decrypt a message needs a key. In QKD, the data that make up the key are encoded on some quantum property of a photon. Because measuring a quantum property changes it, if someone were to intercept the encoded photons, the intended recipient would see changes in the photons and know there was an eavesdropper. The integrity of the message is protected by the laws of physics; there’s no higher level of security.
But for QKD to be practical, scientists need to find ways to generate large enough quantities of entangled photons to produce a reliable signal that can travel long distances without losing information. Finding materials to do this has been a stumbling block for quantum communication. The Chinese satellite, called Micius, relied on a commercially available engineered crystal. Researchers are also developing materials to use in quantum communication systems on Earth.
Micius generated entangled photons by firing an ultraviolet laser through a potassium titanyl phosphate crystal (KTP). The crystal has alternating, regularly spaced layers with crystalline domains that are oriented in different directions. Most photons pass through unaffected, but a fraction get split into pairs of lower energy photons—one with horizontal and one with vertical polarization. The technique is called parametric down-conversion. The spacing of the layers in the crystal determines the wavelength of the resulting entangled photons.
Jian-Wei Pan, a physicist at the University of Science & Technology of China who led the Micius experiment, says this type of crystal was right for the satellite design because it can efficiently generate high-intensity entangled photons with a low-power laser suitable to use in space. Despite that, the team detected only about one entangled pair out of 6 million generated because of photons lost during their passage through the atmosphere. That’s not a practical rate for quantum communications.
Still, that detection is an impressive feat of engineering, says Alexander Ling, a physicist at the National University of Singapore’s Centre for Quantum Technologies. He’s also working on space-based QKD, placing his experiments on high-altitude balloons and small satellites called CubeSats. Like Pan, his team uses commercially available crystals such as β-barium borate to down convert a photon into an entangled pair. Barium borate is very resistant to space radiation, he says, whereas KTP requires careful temperature stabilization, rendering the emissions system more complex. Other teams have studied crystals with similar structures, including lithium niobate and lead tetraborate.
Even though many QKD-capable materials are available, Ling says, what are needed are materials that generate large numbers of photon pairs with each laser pulse. The more pairs that are created, the higher the communication rate, even with the inevitable losses of photons that result during transmission and detection.
Even better, increasing the number of photons entangled together would also increase the potential of quantum communications, Ling says. Having four photons entangled together instead of just two would quadruple the amount of information that could be encoded in the system. Researchers have generated up to 12 entangled photons, but the frequency of such generations is far too rare for practical applications. “At the moment, to get anything more than two photons at a time is very complicated,” Ling says.
One photon at a time
The satellite tests performed so far have been proof-of-concept experiments and have used existing materials, but researchers also are developing new, more efficient sources of photons. Many are focusing on single-photon emitters, which produce a stream of identical photons that can be entangled by splitting the beam with an interferometer (Nat. Photonics 2016, DOI: 10.1038/nphoton.2016.186). In this context, “single photon” means that each laser pulse creates one photon, which can be individually manipulated, whereas ordinary lasers produce hundreds or thousands of photons per pulse.
One contender for a single-photon emitter is a diamond containing a “color center” defect, so called because it produces a glint of color when light passes through. In the diamond’s crystalline structure, one carbon atom is replaced with a nitrogen atom, and a neighboring carbon atom is missing, producing the color center. The crystal becomes negatively charged at the site of the defect and changes the wavelength of light entering the crystal.
The trouble with diamond, though, is that it’s hard to get that light out, says Marko Loncar, an electrical engineer at Harvard University’s Center for Nanoscale Systems. Only about 5% of the generated photons are sufficiently uniform to be entangled. Of those, 5% escape the diamond; the rest get reflected back thanks to the crystal’s high refractive index. Loncar is trying to engineer diamonds’ structure on a nanometer scale using techniques from the microelectronics industry, such as lithography and reactive etching, to create an optical cavity that gives his team more control over the light. “That actually has been a big challenge because diamond’s a pretty tough material to work with,” he says. Though if he can make it work, the approach could be easily scaled up to produce many photons from a small device, placing perhaps 100 synthetic diamonds on a 1-cm2 chip.
Other types of diamond defects could work. “There are hundreds and hundreds of defects in diamonds that one can go through and hopefully engineer something that has good properties,” Loncar says.
Another approach is to put color centers into other types of materials—perhaps silicon carbide or a semiconductor such as gallium nitride. “Diamond at the moment is probably the best and most promising solid-state platform,” Loncar says, though he admits he’s biased because that’s the material he works on.
Another route to single photons involves quantum dots, small clusters of semiconductor material that emit different wavelengths of light depending on their size and composition. Pan, for example, created a single-photon emitter using a quantum dot made of indium arsenide and gallium arsenide embedded in a thin cavity of gallium arsenide. It generates approximately 25 million identically polarized photons per second, one at a time. The only trouble is, it operates at cryogenic temperatures—even colder than those in space—making it impractical for a space-borne system, Pan says.
Unlike arsenide versions, nitride-based quantum dots operate at room temperature. They have not yet, however, been made to emit a pure stream of single photons, according to Igor Aharonovich, a physicist at the University of Technology Sydney. Similarly, quantum dots made from perovskites can emit single photons and work at room temperature, but they perform inconsistently and have a low rate of photon emission.
Bringing things down to Earth
One of the reasons satellites are a good starting point for QKD is that photons traveling through the vacuum of space are unlikely to hit an atom that will absorb them or alter their quantum properties. Traveling through air or through optical fiber, though, is another story. In these environments, the number of photons that make it to their destination drops significantly. Still, researchers would like to transmit photons on Earth, too. To send photons through optical fibers, scientists need to use the wavelengths of light—1,330 and 1,550 nm—that other optical communications now use to transmit data and phone calls, so the challenge is to tune emitters to operate at those wavelengths.
The solution may be to add defects to carbon nanotubes (Nat. Photonics 2017, DOI: 10.1038/nphoton.2017.119). “They can act as single-photon emitters at telecom wavelengths at room temperature, and no other material can do that,” says Stephen K. Doorn, a physical chemist at Los Alamos National Laboratory’s Center for Integrated Nanotechnologies.
Carbon nanotubes of the right size emit light at between 1,000 and 1,200 nm. Adding a benzene-based group to the wall of the nanotube creates a covalently bonded defect that acts as an emission site and pushes the wavelength higher into the infrared. “We can tune the emission to be deeper into this telecommunication wavelength,” says Han Htoon, a physicist at Los Alamos who works with Doorn.
All these technologies need further development to the point where they’ll make quantum communications practical. Doorn says there is a long way to go in understanding and controlling the chemistry of the nanotubes. Loncar hopes he and other researchers can demonstrate systems that use diamond defects within about five years. And Ling says a quantum future is coming soon. “We think from a technology perspective it’s only a few years out,” he says.
Neil Savage is a freelance writer. A version of this story appeared in ACS Central Science: cenm.ag/entanglement.
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