Scientists break distance record for quantum teleportation
A new record distance has been set for the quantum teleportation of information over optical fibers. Researchers working at the NationalInstitute of Standards and Technology (NIST) claim to have transmitted thequantum information carried in light particles over 100 km (62 miles), fourtimes farther than previously achieved.
Breakthroughs in quantum physics continue to accelerate, havingalready shown the practical potential of quantum cryptography and increasingly making progress toward powerful, everyday, quantumcomputers. This new record set by NIST adds to this momentum by providingthe ability to transmit quantum state information much farther than previouslythought practicable.
Accordingto the researchers, this capability was only possible through the advancementof NIST’s own bespoke single-photon detectors made in its laboratory inColorado. Utilizing superconducting nanowires created from molybdenum silicide,these detectors are so sensitive that they can record the arrival of more than80 percent of the photons transmitted, even after traveling more than 100 km –unboosted – down an optical fiber.
"Onlyabout 1 percent of photons make it all the way through 100 km of fiber,"said NIST’s Marty Stevens. "We never could have done this experimentwithout these new detectors, which can measure this incredibly weaksignal."
In thisparticular experiment, the scientists used quantum states to indicate preciselywhen in an arrangement of time slots a single photon arrived. In this way, theinformation stored as a qubit (a quantum bit) was encoded as a sort of "timestamp". To do this, a single photon was emitted which then had twopathways to travel down. One was a short length and the other considerablylonger. The path the photon traveled was completely random, however, and each pathdetermined the time – "early" or "late" – at which thephoton appeared.
Because the state of the photons is entirely random, they could be in either ofthe two time states. If, as a result of the travel times, the photon is"in phase" where the superposition of states is the same, then thepeaks of the waves will align and the signal will carry through. If, on theother hand, they are out of phase, then the peaks will align with the troughsof the wave, and they will effectively cancel each other out.
The first step in the teleportation process is to generate a photon in a superposition of all possible states. It is thenfired into a crystal splitter where the photon is cleaved into two entangled photons (which NIST dub "helper" and "output" photons). Asecond photon (randomly encoded with a "late" or "early"timestamp) is generated simultaneously and fired into a beam splitter exactly asthe "helper" photon arrives. Two detectors at this point determine if thehelper photon and the input photon are in or out of phase.
Becausethe helper photon is entangled with the output photon, when the output photonreaches the end of its long journey down the optical fiber, it should arrive atthe detector end mirroring the state of its entangled twin. In this way, thesuperposition of the entangled pair – in phase/out of phase can be determinedto verify that the states have remained intact and the data of that state hasbeen effectively transmitted.
Prior tothis research, a great deal of quantum information was lost in the fibertransmission medium, meaning that data transfer speeds and achievable distanceswere both low. Previous attempts to solve this problem sought the answer insuch things as exceptionally complex quantum memory systems that could briefly store, and thenre-transmit, quantum data without upsetting the finely-strung quantum spinstates.
The newteleportation method may, say the researchers, obviate the need for suchstorage systems by providing the long-sought ability to add a type ofrepeater/amplifier system into an optical network that would emulate thosealready used in current light communications systems. They also believe thatthis may one day help build a type of "quantum Internet" where vast numbers ofthese devices periodically resend data in order to almost infinitely extendsuch a network.
The results of this research were recently published in the journal Optica.