Abstract
Self-testing is a method of quantum state and measurement estimation that does not rely on assumptions about the inner working of the devices used. Its experimental realization has been limited to sources producing single quantum states so far. In this work, we experimentally implement two significant building blocks of a quantum network involving two independent sources: namely, a parallel configuration, in which two parties share two copies of a state, and a tripartite configuration, where a central node shares two independent states with peripheral nodes. Then, by extending previous self-testing techniques, we provide device-independent lower bounds on the fidelity between the generated states and an ideal target made by the tensor product of two maximally entangled two-qubit states. Given its scalability and versatility, this technique can find application in the certification of larger networks of different topologies for quantum communication and cryptography tasks and randomness generation protocols.
- Received 21 October 2020
- Revised 25 March 2021
- Accepted 3 May 2021
DOI:https://doi.org/10.1103/PRXQuantum.2.020346
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
In the last few years, new technologies based on quantum mechanics have been designed, exploiting the intrinsic random nature of quantum mechanics. In this context, it is crucial to certify the correct operation of a device, possibly relying on minimal assumptions on their inner functioning (i.e., device independently). A key device-independent protocol is so-called self-testing, where the reliability of a quantum state source is verified. Several self-testing protocols exist, but they mostly deal with ideal cases. Therefore, the experimental implementation of such techniques has been limited so far.
We adopt a self-testing technique that naturally applies to nonideal scenarios. In detail, it requires the parties involved to perform local measurements and it sets a lower bound on the fidelity of the generated state with the desired target. Such a bound is retrieved through a minimization over a superset of the quantum correlations and is compatible with the observed statistics, requiring no further knowledge of the apparatus. From the experimental side, we implement two quantum network building blocks. The first consists of two parties sharing two states in parallel and the second consists of a central node sharing a state with two peripheral ones.
Since it requires only separable measurements, this technique can easily find practical application, especially in view of the creation of large quantum networks. Furthermore, for some target states, such as in our case, the number of required measurements does not depend on the size of the network. This feature could also be exploited for randomness generation.
