Quantum entanglement is a phenomenon where particles act in sync, even if they are at separate ends of the universe. Links of this type are so delicate that, if anyone tried to eavesdrop on a message sent using a stream of entangled particles, the disturbance would immediately be obvious. This has led to extraordinarily secure quantum cryptography that can instantly detect any spying.
But if this phenomenon is to be used to keep cellphone conversations private or make sure that no one can sniff out your banking password, scientists have to first generate entangled photons using electronics that can fit onto a microchip. However, until now, entangled photon emitters could only be scaled down to millimeters in size, too large for on-chip applications by several orders of magnitude. In addition, such emitters required much more power than is practical for putting them on, say, a cellular handset.
Now researchers at the University of Pavia in Italy say they have developed a device that can generate a continuous supply of entangled photons and is small enough to fit on a microchip. The scientists detailed their findings on 26 January in the online edition of the journal Optica.
The key component of the device is a "micro-ring resonator," a 20-micrometer-diameter ring etched into a silicon wafer. The ring is 500 nanometers wide and 220 nanometers high. When a laser beam is directed along an optical fiber and into the device, the photons race around the ring and can become entangled. “The key to this result is the ability to confine light and matter in the same microscopic place for as long as possible to force their interaction,” Daniele Bajoni, a physicist at the University of Pavia in Italy who is a member of the research team, told IEEE Spectrum.
The device can generate 10 million entangled pairs of photons per second, and requires less than a milliwatt of power—thousands of times less than was needed by previous entangled photon emitters. The researchers employ lasers with a wavelength of 1,550 nanometers, which is often used in telecommunications. They suggest their device could be readily incorporated into existing silicon chip technologies.
Bajoni and his colleagues now aim to integrate this device onto microchips. “I would like to caution the lay reader that we are not going to see a quantum version of the Internet, in which you can send quantum-encrypted e-mails, anytime soon,” Bajoni said. “It is probable that the first applications will be point-to-point exchanges of information. For instance, one can think of quantum ATM machines where bank clients can exchange quantum cryptography keys to be used for home banking.”
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.