The Long Path of the Quantum Repeater

The Long Path of the Quantum Repeater
Long Path of Quantum Repeater - Figure : A quantum repeater equipped with trapped ion quantum memories is used to distribute entangled telecom wavelength photons between two network nodes separated by 50 km. APS/M. Hajdusek/C. Cain

Quantum Entanglement information can be transmitted via a quantum repeater based on 50 km long photons, trapped ions. In the last 50 years, communication networks have completely changed our society and we now have a hard time imagining life without them. Scientists are excited about the possibility of connecting quantum devices to networks as a result of recent advances in quantum technologies. Long-distance quantum communication heralds the possibility of capabilities not available in traditional networks. Quantum networks exchange signals at the single photon level to take full advantage of entanglement and other quantum effects. Consequently, the main cause of failure in these systems is fiber attenuation.

However, photon loss can be avoided by using a group of intermediary network nodes known as quantum repeaters that directly entangle the two scattered mesh nodes. The entanglement of two mesh nodes separated by 32 m was recently achieved using a quantum repeater based on nitrogen void centers in diamond. Using trapped ions as quantum repeaters, Victor Krutyanskiy of the University of Innsbruck in Austria and colleagues was able to combine two 25 km long entangled links into a single 50 km long link. This distance is the kind of distance required by functional quantum networks in the real world.

The importance of the success of Krutyanskiy and his colleagues can be understood, given the three ideal properties that functional quantum repeaters should possess. The first of these is to have access to quantum memory [5]. The method of generating remote entanglement is unclear due to photon loss and other hardware inadequacies. If an end-to-end connection could only be made if all short-distance connections were successful simultaneously, the overall success rate would be exponentially small. Quantum memories store short-range entanglement, allowing failed connections to repeat entanglement attempts.

The "addition" of entanglement depends on the third desired property. A fixed quantum memory and a "flying" photon traveling along the fiber become entangled thanks to the repeater. It repeats the process using a new memory to create a second flying photon. Two separate entangled links are created by sending two photons to two different, remote network nodes. The repeater then uses a process known as entanglement swapping to combine these links. To maintain the invaluable overall success rate of end-to-end entanglement, the defragmentation process must be deterministic rather than probabilistic.

These three features were combined into a single system by Krutyanskiy and his team. They also deployed entanglement between two network nodes A and B, which are 50 km apart, a suitable distance for practical uses of quantum networks. The team was able to achieve this feat by capturing two calcium 40Ca+ ions and using them as two quantum memories. The two ions are first initialized to their ground state and then repeatedly illuminated with laser pulses as part of the repeater protocol. The ions are receiving enough energy from the laser to ascend to a higher energy state. As a result of the subsequent disintegration of the ions, each ion emits a photon, which keeps the ion-photon pair entangled.

The photons are collected in a wavelength converter, a device that converts the original wavelength of the emitted photons into a suitable telecom wavelength for their subsequent travel. The two photons are then directed to nodes A and B using 25 km long spools of optical fiber. The ion-photon entanglement is then converted by the repeater into a photon-photon entanglement spanning 50 km by performing a deterministic entanglement swap on the two ions it holds.

By repeatedly repeating the entanglement distribution and measuring the photons at nodes A and B, state tomography can determine the final photon-photon state and create a statistical measure of how faithful the shared photon-photon state is.

A perfect ideal situation is represented by unit fidelity. Nodes A and B were able to achieve entanglement with a success rate of 9,2 Hz and a probability of success of 9,2 per trial, resulting in a fidelity of 104. This fidelity is much higher than the 0,72 required for photon entanglement. The researchers also conducted an experiment in which photon-photon entanglement was distributed over a distance of 0,5 km without using a repeater. The benefit of using repeater assisted techniques is clearly demonstrated by the low success rate of 50 Hz. At the working distances of the experiment, this advantage may seem insignificant. However, at distances greater than 6,7 km, the success rate drops dramatically when there are no repeaters.

In their analysis, the Innsbruck team considered how much better the experimental settings would have to be for multiple coupled repeaters to span an end-to-end distance of 800 km. Surprisingly, few changes need to be made to many of the features. The most significant improvement is required in the nondeterministic photon entanglement modifier required to interconnect several repeaters. Researchers make strong cases as to why the improvements are feasible in the near future.

Exciting experimental examples of quantum communication have occurred recently. In light of the long-distance capabilities demonstrated in these studies, it is clear that quantum networks are rapidly advancing from theoretical concepts to practical applications. It is very important to remember two important lessons learned from the internet, which is a traditional network. First of all, having good equipment is not enough to enable communication on a global scale. However, a robust software architecture is needed. Second, good software takes a long time to mature. To keep hardware and software running in tandem, physicists and technologists collaborate to create custom link layer protocols and complete architectures for the quantum internet of the future.





Günceleme: 23/05/2023 12:58

Similar Ads