As a direct consequence of the no-cloning theorem, the deterministic amplification as in classical communication is impossible for quantum states. This calls for more advanced techniques in a future global quantum network, e.g. for cloud quantum computing. A unique solution is the teleportation of an entangled state, i.e. entanglement swapping, representing the central resource to relay entanglement between distant nodes. Together with entanglement purification and a quantum memory it constitutes a so-called quantum repeater. Since the aforementioned building blocks have been individually demonstrated in laboratory setups only, the applicability of the required technology in real-world scenarios remained to be proven. Here we present a free-space entanglement-swapping experiment between the Canary Islands of La Palma and Tenerife, verifying the presence of quantum entanglement between two previously independent photons separated by 143 km. We obtained an expectation value for the entanglement-witness operator, more than 6 standard deviations beyond the classical limit. By consecutive generation of the two required photon pairs and space-like separation of the relevant measurement events, we also showed the feasibility of the swapping protocol in a long-distance scenario, where the independence of the nodes is highly demanded. Since our results already allow for efficient implementation of entanglement purification, we anticipate our assay to lay the ground for a fully-fledged quantum repeater over a realistic high-loss and even turbulent quantum channel.
Teleportation of an entangled state, also known as entanglement swapping, plays a vital role in the vision of a global quantum internet, providing unconditionally secure communication, blind cloud computing, and an exponential speedup in distributed quantum computation. In contrast to the teleportation of a single quantum state from one qubit to another, entanglement swapping generates entanglement between two independent qubits that have never interacted in the past. Therefore this protocol represents a key resource for numerous quantum-information applications that has been implemented in many different systems to date. We experimentally demonstrated entanglement swapping over 143 km between the Canary Islands of La Palma and Tenerife, proving the feasibility of this protocol to be implemented in a future global scenario.
Significance – Twisted photon entanglement through turbulent air across Vienna
The spatial structure of photons provides access to a very large state space. It enables the encoding of more information per photon, useful for (quantum) communication with large alphabets and fundamental studies of high-dimensional entanglement. However, the question of the distribution of such photons has not been settled yet, as they are significantly influenced by atmospheric turbulence in free-space transmissions. In the present paper we show that it is possible to distribute quantum entanglement of spatially structured photons over a free-space intracity link. We demonstrate the access to four orthogonal quantum channels in which entanglement can be distributed over large distances. Furthermore, already available technology could provide access to even larger quantum state spaces.
Abstract – Twisted photon entanglement through turbulent air across Vienna
Photons with a twisted phase front can carry a discrete, in principle, unbounded amount of orbital angular momentum (OAM). The large state space allows for complex types of entanglement, interesting both for quantum communication and for fundamental tests of quantum theory. However, the distribution of such entangled states over large distances was thought to be infeasible due to influence of atmospheric turbulence, indicating a serious limitation on their usefulness. Here we show that it is possible to distribute quantum entanglement encoded in OAM over a turbulent intracity link of 3 km. We confirm quantum entanglement of the first two higher-order levels (with OAM=± 1ℏ and ± 2ℏ). They correspond to four additional quantum channels orthogonal to all that have been used in long-distance quantum experiments so far. Therefore, a promising application would be quantum communication with a large alphabet. We also demonstrate that our link allows access to up to 11 quantum channels of OAM. The restrictive factors toward higher numbers are technical limitations that can be circumvented with readily available technologies.