On Friday, an international team of researchers led by David Christle at the University of Chicago published research that demonstrates the feasibility of repurposing materials and protocols commonly used in existing telecommunications networks for quantum communications. The work marks a major step toward the practical application of quantum phenomena.
As detailed in the physics journal Physical Review X, Christle and his colleagues used atomic imperfections in silicon carbide wafers—a material commonly found in everyday electronic devices—to entangle particles. Entanglement is a quantum phenomenon which links two distinct particles so strongly that acting on one of these particles automatically influences the other. In fact, these particles are so tightly linked that the resulting effect is basically like having a single particle exist in multiple locations at once.
The defects in the silicon carbide wafers used by Christle and his colleagues consisted of a missing atom in the material, which caused atoms adjacent to this defect to rearrange their electrons. The researchers then leveraged a quantum property of these electrons called spin to store information as quantum bits. Quantum bits are just like bits in a normal computer, except for rather than being either 1 or 0, these quantum bits can exist as both at the same time.
"A key advance in this work is that we have found a mechanism that is built into the defect that can allow us to convert that spin state into light," Christle explained to me via email. "This means we have found a way to entangle the state of the spin with the state of the photon. Essentially, this means that instead of quantum information being stationary, it is now mobile, in a way."
To understand how this might be applied in a simple quantum communication system, imagine two silicon carbide wafers separated by some given length of optical fiber. Researchers would manipulate the defects in these wafers with a laser to generate an electron spin state and produce a photon that is entangled with the electron.
Each of the photons created at the silicon wafers would then travel through the optical fiber toward one another. When the photons interact with one another at some central meeting point in the optical fiber, this will result in the entanglement of the two defects at each end of the optical fiber. At this point, a quantum communication channel will have been established between the two locations of the silicon wafers.
Photon entanglement is at the core of quantum communications. It is considered to be one of the most secure methods of exchanging information between two locations because of its extreme fragility. When the state of entangled photons is measured, this necessarily alters the quantum system. This means that any eavesdropper attempting to intercept the information being sent between two parties in a quantum system will disturb the system and alert both parties to the breach.
While the fragility of quantum key distribution is considered a positive from an infosec standpoint, the fragility of the system also poses a number of problems in terms of creating practical quantum communication networks. This is because interference from the photons interacting with optical fiber or the air will increasingly degrade a quantum system as the distance of the transmission increases. At a certain point, the quantum system will "decohere" entirely and become useless as a vehicle for information.
So far, the record for using atomic defects to generate entanglement is just under a mile. In this case, the entanglement was generated using imperfections in diamonds called nitrogen vacancy centers. Like defects in silicon carbide, nitrogen vacancy centers in diamonds are able to maintain electron spin states for long periods of time and also function as an interface for converting spin states into light.
Still, using silicon carbide defects to generate photon entanglement has a number of advantages over diamond defects. In the first place, silicon carbide wafers are widely used in commercial electronics that operate at high voltages or temperatures. Unlike diamonds, these wafers are easily and cheaply obtained, making them better for use in quantum networks at scale.
But the main advantage is that the silicon carbide defect works at light wavelengths that are compatible with optical fiber. Diamonds, on the other hand, function at wavelengths in the visible spectrum, which makes them less cut out for fiber transmissions.
"That means for every kilometer inside the fiber, you lose photons ten times more often if you use a diamond versus silicon carbide," Christle said. "Lower losses mean that the attempt to generate entanglement between distant defects connected by fiber has a higher chance of succeeding."
As Christle pointed out, minimizing photon loss is crucial to creating a practical quantum communication network. In previous experiments, the success rates for generating entanglement using atomic defects in diamonds has been incredibly low. In the case of the record-breaking diamond entanglement, the researchers succeeded in entangling particles about once every hour, or approximately every 156 million attempts.
Christle and his colleagues also think they can generate far more photons from each spin state created in the silicon wafers compared to spin states generated in diamond defects. According to their measurements, they should be able to create anywhere from 5,000 to 30,000 photons before the spin state decoheres.
"That's a very wide uncertainty on our prediction, but it is still quite promising since even our lower estimate is a big improvement over the roughly 1,000 photons the nitrogen vacancy center in a diamond emits," Christle said.
Only one of these thousands of photons would actually be used to transmit information over an optical fiber. The rest allow the researchers to get a better measurement of the quantum state, a crucial element when securely sending information over a quantum network depends on knowledge of the quantum states involved. If you can only produce 100 photons per each quantum state generated in the wafer, your measurement of these photons will be far less accurate than if you can measure the quantum state over 10,000 photons.
The final benefit of using silicon carbide wafers to generate entangled photons is the length of time the quantum spin state can be maintained in the defect, which is essentially measuring how long the defect can store information. According to Christle, silicon carbide spin states last for about 1 millisecond before they decohere. This might not sound like much, but in the quantum realm this is a long, long time.
"This is an essential element for a quantum repeater," Christle said. "You need to be able to store quantum information and also convert it to light. In the real world, losses in optical fiber will always be present, but if quantum repeaters are placed periodically along a long-distance fiber link, then the distance over which entanglement can be successful can be enhanced further. Our system has the essential elements for this key future technology."
For now, Christle and his colleagues' quantum communication technology remains experimental. The next step is to actually measure the number of photons that can be produced from a spin state generated in the silicon carbide defect and to test their system over distance.
While we still have awhile to go before you're sending email over a quantum network, our entangled future seems a lot closer now.