Quantum physics is an often mind-boggling branch of science filled with strange behavior and bizarre implications. For many people, the mere mention of the term is enough to send us hurtling in the opposite direction, like an electron bouncing off the center of an atom.
But evidence is mounting that the future of technology lies in quantum mechanics, which focuses on how the smallest things in our universe work. And a new breakthrough by scientists in China has just brought the world one very big step closer to this quantum revolution. Hundreds of miles closer, in fact. So it's as good a time as any to understand why quantum physics is making such waves.
The Background
Quantum physics is all about waves. And particles. Together. Sort of.
Mostly, we think of light as something that occurs in waves and matter as distinct particles. But theorist Max Planck's attempt in 1900 to explain observations about colors emitted from hot objects started scientists down a path that transformed our understanding of how life works at the very smallest scale.
The first step was realizing that light behaves like a stream of individual particles, called photons. Albert Einstein came to this conclusion following Planck's work. Each photon contains a discrete amount of energy.
Subsequent research by Niels Bohr and others disrupted what physicists understood about electrons, the negatively charged particles that swirl around the heavy centers of the atoms that make up the elements (gold, silver, potassium, calcium, etc.…) that in turn make up matter. That disruption was accentuated by Louis de Broglie, who realized that if light can behave like a particle, then maybe electrons, which physicists had always thought of as particles, could behave like waves. Numerous experiments proved that to be the case. Photons behave like waves and particles. Electrons behave like waves and particles. The type of measurement you do determines how a photon or an electron behave.
One of the most intriguing effects of quantum physics is something called entanglement. With quantum entanglement, two particles derived from the same source behave the same way, even when they are far apart. The state of either particle cannot be determined until it is measured, and the act of measuring is what determines its state. And the measurement of one particle affects the measurement of the other particle. This thinking is embodied by Erwin Schrödinger's thought problem about his famous cat.
If you split photon A into a photon pair—B and C—measuring B will tell you, with absolute certainty, the measure of C. Paul Kwiat, physicist at the University of Illinois, gives the analogy of flipping a coin. If one flipped coin results in heads, heads, tails, heads, tails, tails, head, then the "entangled" coin, placed hundreds of miles away, would follow the same sequence. "That's not a behavior you see with coins," says Kwiat. "That's where quantum entanglement is pretty weird." Two things hundreds of miles away behaving as one: That's quantum entanglement. And it's real. Albert Einstein called it "spuckhafte ferwirklung," or "spooky action at a distance."
For more on the history of quantum physics and the entanglement phenomenon, author Chad Orzel, who teaches physics at Union College in Schenectady, New York, has some excellent videos.
Beyond the weirdness factor, quantum entanglement has broad implications for computing and information sharing. Entanglement distribution—for example, the splitting of a single photon into two linked photons—could be used to create a secure internet connection. The technology, called quantum cryptography, would allow the users to detect any eavesdropper on the channel. The reason you can detect the eavesdropper is that such an intruder would necessarily alter the entangled photons by his or her presence.
The principle allows for "a secure communication channel that is unhackable," says Jonathan Dowling, a physicist at Louisiana State University "When the Chinese roll out this type of communications nationwide, which is their plan," says Dowling, "then no matter how many NSA computers you string together, you are never going to be able to tap into their system."
The Breakthrough
A new study in Science, by Juan Yin and colleagues at the University of Science and Technology in China and several other institutions there, has brought this future technology within much closer reach. The researchers split a photon on a satellite and sent the two resulting photons in two different directions, aimed at ground stations in China. The ground stations were more than 700 miles apart from one another. The distance from the satellite, which was constantly in motion, to each ground station varied from 300 to 1,200 miles.
The researchers managed to send photon pairs to different ground stations repeatedly and confirmed that the photons were entangled. Using a laser pointer-like source, they made about 6 million photon pairs per second. About one pair per second reached the ground stations. Kwiat says it's like throwing a dime into a toll booth bucket while driving at high speed, only you're throwing a much tinier object from much farther away and at a much faster speed. Measurements confirmed that the photon pairs had the same polarization, proving that they were entangled.
Although previous studies have managed to achieve similar results, never has it been done over such a great distance and from a satellite. (The prior record demonstrated entanglement across two of the Canary Islands, about 89 miles apart.) "It's a beautiful experiment," says Kwiat. "They demonstrated the persistence of entanglement over a longer distance than any experiment before by roughly a factor of 10."
Dowling says that this achievement proves that the quantum-based technologies many physicists envision are attainable. "The long-term goal is to build a quantum internet where future computers around the globe are linked together in an uncrackable network of extraordinary computational power," says Dowling. "The satellite will go down in history as the first link in the quantum internet."
The Chinese physicists are not the only team on the quest for this technology. Quantum cryptography systems are commercially available and researchers in several countries, including the U.S., Canada, Italy and Singapore are also forging the way ahead, says Kwiat, who is among them. Google is also working on quantum information science.
Still, the new study is a huge breakthrough because it proves entanglement can be achieved from a satellite and across this large distance. "We have done something that was absolutely impossible without the satellite," says senior author Jian-Wei Pan. The next step, he says, is to perform more experiments with light from space, across yet longer distances and at faster speeds, with a goal of controlling quantum states and understanding how gravity affects quantum behavior.
