PORTLAND, Ore.—National Institute of Standards and Technology (NIST, Boulder, Colo.) has announced a breakthrough allowing secure quantum encryption keys to be teleported over 63 miles without a repeater.
Uncrackable encryption keys use what’s called quantum key distribution (QKD), which transmits the decryption key to the receiver over special communications lines using quantum teleportation to tell whether the key has been observed during transmission. Unfortunately, that requires the transmission of up to a 128,000 single photons—one at a time—through a single optical fiber, requiring ultra-sensitive sensors at the receiving end that detects single photons. So far, only about 15 miles was the longest distance a quantum key could be teleported.
“The biggest breakthrough of our announcement is the ultra-sensitive photon detectors we have made here at NIST,” Matin Stevens, a scientist at NIST told EE Times. “We also want to thank Hiroki Takesue, a NIST guest researcher from NTT Corp. in Japan, who brought along many special pieces of his own equipment to help make this new distance record possible.”
Hard encryption is easy on a user’s device itself—just set your solid-state- or hard-drive to encrypt the contents of your drive with a long random passcode. However, today transmitting encrypted messages over public networks depends on the computational impracticality of deriving a properly generated private decryption key from its public key and a passcode. Depending on public key encryption is “good enough” for most users, because if the difficulty of cracking the passcode. But for banks, stock exchanges and government “top secrets,” its possible for foreign governments to still crack public key encrypted messages with supercomputers that try every passcode (the hard way, but widely available) or with quantum computers that can guess many passcode simultaneously (the easy way, but not widely available, yet).

Today, since quantum computers are not widely available, private decryption keys transmitted to receivers using quantum-key distribution is the ultimate in security, because the very act of observing the decryption key can be detected, thereby disqualifying that key from use. On the other hand, reception of a key that has not been observed guarantees that it can be used without fear to decrypt the next data transmission. Typically, such users change their private decryption key every day—just to be super-sure—and for “top secret” data transmission the decryption key might be changed for every new transmission. (And when quantum computers become widely available, what’s called post-quantum cryptography will be ready to keep encryption uncrackable.)
id Quantize SA has been securely transmitting single photons encoded with polarization since 2002. But NIST’s new world record in distance is the first long-range use of quantum teleportation, which has an every wider field of applications than cryptography.
“One of the possible applications of teleportation is to use it in quantum cryptography, but there are other possible applications too, such as remote sensing or connecting two distant quantum computers,” Stevens told us.

The NIST-based technique encodes by entangling one photon with a second photon so that the two remain identical as long as no one has observed either one during quantum teleportation. Any attempt to measure the encoded state changes it, and more importantly can be detected as having been observed. For encryption applications, NIST combines quantum entanglement with quantum superposition (see figure below) to insure that it quantum teleportation algorithm is absolutely bulletproof.
Quantum Teleportation
Unlike physical teleportation of matter through empty space—à la Star Trek—quantum teleportation transfers quantum states encoded, here, on photons then remotely reconstructs the original information—usually a decryption key. Local teleportation within a quantum computer is also useful in cracking even the longest private decryption keys reconstructed from a passcode and a pubic key (the method used for the “hard encryption” law enforcement is currently complaining to Apple and Google that they cannot crack).
Quantum teleportation was first conceived 20 years ago and has been demonstrated in a number of mediums, including open air, but never at such long distances as to make it feasible for quantum key distribution. Even longer distances could be enabled by configuring quantum repeaters that resend photons to extend a network’s reach eventually enabling a “quantum internet,” according to NIST. Previously most scientists believed that supercooled atoms would be required to build quantum repeaters, but NIST’s work puts pure photonic quantum repeaters within reach.

The reason that the technique is so difficult to realize is that it requires individual entangled photons to be transmitted (see diagram), thus requiring unbelievably sensitive detectors. NIST, with the help of NTT, uses supercooled superconducting molybdenum silicide (MoSi) detectors and still only got a one percent success rate through 63 miles of fiber. The algorithm it used has only a 25 percent maximum success rate of which NIST only attained 80 percent. Luckily it takes only one-eight of a second to transmit 256,000 individual photons. As a result, NIST can transmit eight uncrackable 256-bit private keys, per second.
Even with all the mitigating qualifiations, however, the 63 mile demonstration recently made by NIST would still be beyond belief had it not been carefully peer reviewed by Optica.
Get all the details in today’s edition of Optica, an online-only open-access journal of The Optical Society (OSA) dedicated to rapid dissemination of only the highest-impact peer-reviewed research.
— R. Colin Johnson, Advanced Technology Editor, EE Times 
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This is great work, the kind of limit pushing that NIST can be very good at. Thanks for the excellent article on it.