Quantum cryptography is hard — we knew that. But researchers might have solved one of the most challenging problems of quantum communications, by showing that diamonds can be used as ultra-bright single photon emitters. This could bring us a big step closer to the development of quantum computers and secure communication lines that could operate at room temperature for the first time.
Until now, quantum dots have been the closest we’ve come to real-world quantum cryptographic systems, and bit leakage isn’t the only vulnerability in quantum cryptography. Other architectures using attenuated lasers have been susceptible to bit leakage by way of their tendency to emit multiple extra photons at a time, any of which could be intercepted by an eavesdropper without the sender or recipient ever knowing. But emitting more than one photon at a time is fundamentally impossible with their crystal architecture, according to researchers Dmitry Fedyanin from the Laboratory of Nanooptics and Plasmonics at MIPT, and Mario Agio from the University of Siegen.
Their report is built on the idea that properly doped diamonds can be grown in such a way as to create a tiny, deliberate point flaw called a color center. Diamonds aren’t all clear; the colored ones often include minute traces of other elements. In yellow diamonds, the color centers are single dopant atoms (nitrogen) swapped in where a carbon atom should have been, which also makes a break in the lattice because of molecular geometry. These researchers got their results using diamonds doped with nitrogen or silicon to create color centers, and therefore point flaws, in the crystal lattice.
Applying a low voltage to a diamond with one of these critical color centers makes each single-atom point flaw act like a sort of electroluminescent ramp that throws off energy in the form of photons, singly and sequentially. Certain color centers can even serially emit two photons at two different wavelengths, from two different charge states, in a single act of electroluminescence. Getting a single photon out of a macroscale piece of diamond just hinges on manufacturing tolerances, which are getting pretty good.
To give a sense of the network performance capabilities of such a diamond, let’s start with this: a qubit can be encoded in the polarization of a single photon. Photons also have the properties (and additional bit depth) of wavelength, amplitude, and frequency: the brightness and color of the light emitted can create extra bandwidth in the network. At the same time, diamonds are attractive because they work just fine at room temperature. But the researchers found that their throughput increased as they heated their diamonds — moving from 100,000 photons per second at SATP to more than 100 million photons per second at 200°C. “Our single-photon source is one of few, if not the only optoelectronic device that should be heated in order to improve its performance,” Fedyanin said.
This could be a fundamental breakthrough in quantum communications, though building devices that practically take advantage of these capabilities is still years away. Previous single-photon generation methods required extremely low temperatures to operate, meaning expensive and bulky cooling equipment was a practical necessity. This discovery won’t magically create the kind of quantum infrastructure to make practical use of quantum cryptography. But it could prove a vital step in creating that infrastructure — and offer a practical method for absolutely securing communications in ways no CPU or group of CPUs on Earth could crack.