The establishment of a worldwide quantum Internet would provide individuals, businesses, organizations, and governments access to intrinsically secure communications. However, absorption of photons in transit between internet nodes can dramatically reduce the efficiency of operating a quantum Internet. Now a research group at Australia's (CQCCT) has invented a way to recover some of the lost quantum information by teleporting the original information to another photon.

Quantum communication depends on maintaining the fidelity of the quantum information encoded on, e.g, a photon. If the fidelity is reduced, the efficiency of communication can be dramatically lowered. For example, in the likely case that a photon carrying a quantum bit (qubit) might be absorbed in transit, the resulting "noisy photon" is an entangled version of the photon with the quantum state of the vacuum, which contains no photons whatsoever. (Note that all the transmitter photons become noisy photons simply by travelling through conditions in which they might be absorbed.) Some of the information carried by the original photon is lost in the noisy photon, which can dramatically lower the efficiency of quantum communication applications.

The Center for Quantum Computation and Communication Technology (CQCCT) team has now invented a workaround for such information loss, which they call heralded state amplification. A herald is a classical signal that alerts one to an upcoming event. Imagine that, as a polite neighbor, you don't like to keep visitors waiting at the door. One approach is to stand at the door in case a visitor arrives, but this takes a lot of effort and prevents you from working on other tasks. It is more efficient to ignore the door until you hear the doorbell. The doorbell is a herald of arriving visitors.

The basic idea of heralded state amplification is that even though quantum information has been lost in a noisy photon, it is possible to improve the situation. Noisy photons pass through an analyzer that teleports the information to a second photon, and generates a herald when the second photon is less noisy than the original noisy photon.

The analyzer combines a noisy photon with a pure photon, producing three quantum states. Two of these are measured by photon detectors, while the third is the output of the analyzer. These three states are constructed so that when zero or two photons are detected, the output of the analyzer is noisier than is the input, while when exactly one photon is detected, the output is less noisy – the information has been amplified on the heralded output states.

To ensure that only amplified outputs are used for continued processing, a classical herald is generated by sending the detector outputs through an exclusive-or gate, so that a herald is generated when only one photon is detected by the detectors. The heralded state amplifier typically produces an information gain of five or less, but this is sufficient to rescue a large number of marginal communications applications. CQCCT's new heralded state amplifier thus represents an important addition to the quantum communications toolbox.

Source: Nature Physics