Quantum teleportation combines classical and quantum information channels to transmit the complex state of quantum particles. The communication method conveys complete quantum information without the need to pass directly through the space between communicating parties. Significant challenges interfere with quantum teleportation in practice. Much of the effort employed by the researchers focused on using new approaches to overcome practical barriers. This is the first experiment of its type to use "feed-forward in real time" for quantum teleportation. In the experiment, the sender feeds classical information alongside quantum information to assist the receiver in reconstructing the original message.
A sender wants to share with a receiver the quantum state of a photon. The sending party entangles two photons, keeping one and sharing one with the receiver, to create a quantum channel. The sender performs a quantum-mechanical measurement against two photons: the retained photon and a message photon in the desired state. As a result of the measurement, the entangled photon belonging to the receiver contains information about the state of the message photon. The sender then classically communicates the results of the quantum-mechanical measurement, enabling the receiver to reconstruct the desired original quantum state. The receiver now has a photon with the same quantum state as the original message photon from the sender.
In quantum teleportation, communicating parties transmit information reliably over sizable distances by combining classical and quantum information channels. Entangled particles, in this case photons, respond relatedly regardless of distance. With additional information communicated over a classical channel, the recipient can reconstruct a photon with identical state to a photon transmitted by the sender. Quantum teleportation enables communication innovations, including indefinite information transfer and physical security guarantees, by linking together interlocutors through entangled particles.
The researchers conducted the experiment in the Canary Islands. In the experiment, researchers sent photons between optical stations of the Isaac Newton Group on La Palma and the European Space Agency on Tenerife. A distance of 143 kilometers separates the two locations - the longest ever quantum teleportation.
Quantum teleportation between the Canary Islands La Palma and Tenerife over both quantum and classical 143 km free-space channels. (For detailed explanation see the paper in arXiv)
Over recent years, the authors of this paper and other researchers have applied quantum teleportation over increasing distances. The old record, set earlier this month, reached 97 kilometers across a lake in China. The previous record, also in China, came in at only 16 kilometers.
Earlier research transmitted quantum information about individual photons or entangled photon pairs. To perform quantum teleportation requires three or more photons, and introduces major technical challenges. Extending the distance of communication increases the technical hurdles.
To date, experimenters have performed quantum teleportation over short distances in labs or over fiber networks. The process works over fiber optic cables as well as through free space. Free space offers advantages including greater range, because of more efficient transmission. Optical fiber limits the distance that can separate communicating nodes through signal loss. The new research introduces several methods to extend practical applications through free space.
In the newest experiment, scientists used a titanium-sapphire laser to generate pairs of photons. Pulses of blue light beamed at crystals, which emit correlated photons. Crystal emissions include the entangled photons for the quantum channel, as well as a photon containing the information for communication. The authors performed the quantum-mechanical measurement by overlapping the message photon and one of the entangled photons using a beam splitter. For photon detection, the authors used silicon avalanche photodiodes.
The researchers sent an entangled photon over 143 kilometers of free space from the source to the destination. One telescope at each side of the experiment enabled the transmission. The experimenters started by testing only the simple case where the quantum-mechanical measurement reveals that the receiver can use the entangled photon as received. The researchers used a polarization analyzer to confirm the success of quantum teleportation. Next, the researchers added "real-time feed-forward" capability, to inform the receiver of any necessary adjusments to reconstruct the message photon regardless of the quantum-mechanical measurement results. The classical information about the measurement results also traveled over 143 kilometers by laser via telescope.
Communication via free-space optics continues to evolve. While free-space optical communication poses technical challenges, the process offers the possibility of sending quantum information over long distances. Satellites promise to make this type of transfer globally ubiquitous.
The researchers developed tools, including a new photon source, to add reliability. The experiment synchronized events at the two stations by GPS data. In addition to starting the experimental time by GPS at both stations, new techniques include the use of entanglement to link clocks, reducing the time resolution to one nanosecond. Combining several technical innovations, the experiment resulted in verified quantum communication of practical importance. The capabilities open the way to future applications of quantum information such as space networks.
The researchers battled harsh environmental conditions, including "rapid temperature change, sand storms, rain, fog, strong wind and even snow". A laser tracking system helped the telescopes maintain connectivity throughout the turbulent conditions. The authors note that compared to this experiment, atmospheric conditions pose less of a challenge for satellite communications, where optical signals have clearer paths.
The experiment resulted in high state fidelity transmission rates, exceeding classical limits to demonstrate quantum communication. Quantum process tomography, a method of identifying quantum states, verified the experimental results. For the sent photon and the received photon, the authors analyze each element. Ideally, quantum teleportation sends the photon state losslessly. The experiment surpassed classical limits in both the simple and feed-forward phases, clearly demonstrating quantum teleportation.
As a foreseeable objection to the experimental setup, the sender performed the quantum-mechanical measurement while the receiving photon remained in transit. However, as the authors note, quantum teleportation calls for the receiver to apply the classically transferred information about the measurement in order to reconstruct the message quantum state. Therefore, no special meaning attaches to the quantum-mechanical measurement alone, so when the receiver reconstructs the state of the message photon, the quantum teleportation succeeds.
Achieving practical quantum teleportation over large distances of free space opens the way to future applications. The authors speculate that by storing many entangled particles in quantum memory, two parties could establish a more reliable means of communication through quantum teleportation compared to transferring the same information directly. With quantum teleportation and a "stockpile" of entangled states, the sender can broadcast the quantum-mechanical measurement results classically to complete the transmission, regardless of any quantum link with the receiver.
The new research demonstrates the viability of quantum teleportation through free space over long distances, with applications including satellite transmissions. As an example, the experimental communication between Tenerife and La Palma encompasses greater atmospheric travel compared to space communications. The authors foresee experiments involving quantum teleportation from Earth to satellites, and between satellites, leading to "future quantum networks in space".
By Frank Smith. Frank Smith writes about science and technology, with a focus on light.