Researchers at the HUN-REN Wigner Research Center have proposed a new method for distilling quantum entanglement

Quantum communication, quantum computing, and quantum sensing all regard entanglement as an essential resource. Quantum entanglement is valuable: it is not easy to create, yet it is easily damaged and degrades. Another fundamental property is that once it has not been present between distant systems, it can no longer be created. The general procedure is to create entanglement at a given location, then send the subsystems—such as photons—to remote users, taking care to ensure that the entanglement is not damaged en route. However, some degradation is essentially unavoidable. In such cases, all we can do is concentrate or distill the entanglement. In these procedures, we use multiple distributed entangled systems, such as photon pairs: the remote laboratories perform certain operations and measurements on the half-pairs they possess, then, after comparing the measurement results, discard some of the pairs. The entanglement of the retained pairs can thus become greater than before, provided that the entanglement of the discarded pairs is smaller than or equal to zero, thereby ensuring that the law of the average entanglement decrease is not violated.

Orsolya Kálmán, Aurél Gábris, Tamás Kiss (researchers at the HUN-REN Wigner FK) and Igor Jex (professor at the Czech Technical University in Prague) proposed a new method in their paper published in Physical Review Letters that is both universal and practical. Its universality lies in the fact that, aside from the entanglement of the initial states being only slightly degraded, no other information is required about the state of the incoming system—in this case, a pair of qubits. This represents an advance over previous methods, which generally work only for certain types of initial states. Practicality refers, on the one hand, to the number of input systems used (quantum bit pairs, which could be, for example, photon pairs), and on the other hand, to the number of operations used in the procedure and its feasibility in practice.

The new method produces a nearly perfect so-called Bell pair from eight input quantum bit pairs, which is a pair in a maximally entangled state. The method is probabilistic in nature but unambiguous: if we receive a green signal based on the control measurements, then we are certain to find the desired state at the output. The procedure based on this new idea also brings something new to non-universal distillation. If we know that the input pairs are slightly degraded instances of the desired Bell state, then four input pairs are sufficient for distillation. Due to its unique combination of universal, practical, and scalable properties, the new approach presented here is ideally suited for future quantum communication, distributed quantum computing, or multi-core quantum computers, where devices require high-quality, shared entanglement for efficient operation.