The academic research into entanglement nicely illustrates the interplay between fundamental science and applications, and the need to foster both aspects to advance either one. For instance, the possibility to distribute entangled photons over tens or even hundreds of kilometers is fascinating because it confirms the quantum predictions over large distances, while quantum theory is often presented to apply to the very small (see Figure 1). On the other hand, entanglement enables quantum key distribution (QKD) [1]. This most advanced application of quantum information processing allows one to distribute cryptographic keys in a provably secure manner. For this, one merely has to measure the two halves of an entangled pair of photons. Surprisingly, and being of both fundamental and practical interest, the use of entanglement removes even the necessity for trusting most equipment used for the measurements [5]. Furthermore, entanglement serves as a resource for quantum teleportation (see Figure 2) [1]. In turn, this provides a tool for extending quantum key distribution to arbitrarily large distances and building large-scale networks that connect future quantum computers and atomic clocks [6]. In the following, we describe the counter-intuitive properties of entangled particles as well as a few recent experiments that address fundamental and applied aspects of quantum teleportation. While a lot of work is being done using different quantum systems, including trapped ions, color centers in diamond, quantum dots, and superconducting circuits, we will restrict ourselves to experiments involving photons due to their suitability for building future quantum networks.
No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
Networks have advanced the study of nonlocality beyond Bell's theorem. Here, we introduce the concept of full network nonlocality, which describes correlations that necessitate all links in a network to distribute nonlocal resources. Showcasing that this notion is stronger than standard network nonlocality, we prove that the most well-known network Bell test does not witness full network nonlocality. In contrast, we demonstrate that its generalisation to star networks is capable of detecting full network nonlocality in quantum theory. More generally, we point out that established methods for analysing local and theory-independent correlations in networks can be combined in order to systematically deduce sufficient conditions for full network nonlocality in any network and input/output scenario. We demonstrate the usefulness of these methods by constructing polynomial witnesses of full network nonlocality for the bilocal scenario. Then, we show that these inequalities can be violated via quantum Elegant Joint Measurements.
In the last few years the world in which Quantum Cryptography evolves has deeply changed. On the one side the revelations of Snowden, though he said nothing really new, made the world more aware of the importance to protect sensitive data from all kinds of adversaries, including sometimes "friends." On the other side, breakthroughs in quantum computation, in particular in superconducting qubits and surface-codes, made it possible that in 15 to 25 years there might be a quantum machine able to break today's codes. This implies that in order to protect today's data over a few decades, one has to act now and use some quantum-safe cryptography.
Recently a new Bell inequality has been introduced (CGLMP,KKCZO) that is strongly resistant to noise for maximally entangled states of two $d$-dimensional quantum systems. We prove that a larger violation, or equivalently a stronger resistance to noise, is found for a non-maximally entangled state. It is shown that the resistance to noise is not a good measure of non-locality and we introduce some other possible measures. The non-maximally entangled state turns out to be more robust also for these alternative measures. From these results it follows that two Von Neumann measurements per party may be not optimal for detecting non-locality. For $d=3,4$, we point out some connections between this inequality and distillability. Indeed, we demonstrate that any state violating it, with the optimal Von Neumann settings, is distillable.
Quantum communication is built on a set of disruptive concepts and technologies. It is driven by fascinating physics and by promising applications. It requires a new mix of competencies, from telecom engineering to theoretical physics, from theoretical computer science to mechanical and electronic engineering. First applications have already found their way to niche markets and university labs are working on futuristic quantum networks, but most of the surprises are still ahead of us. Quantum communication, and more generally quantum information science and technologies, are here to stay and will have a profound impact on the XXI century.
The nature of quantum correlations in networks featuring independent sources of entanglement remains poorly understood. Here, focusing on the simplest network of entanglement swapping, we start a systematic characterization of the set of quantum states leading to violation of the so-called "bilocality" inequality. First, we show that all possible pairs of entangled pure states can violate the inequality. Next, we derive a general criterion for violation for arbitrary pairs of mixed two-qubit states. Notably, this reveals a strong connection between the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality and the bilocality inequality, namely, that any entangled state violating CHSH also violates the bilocality inequality. We conclude with a list of open questions.Received 1 February 2017DOI:https://doi.org/10.1103/PhysRevA.96.020304©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEntanglement measuresNonlocalityQuantum communicationGeneral PhysicsQuantum Information
For any nonfactorable state of two quantum systems, it is possible to find pairs of observables whose correlations violate Bell's inequality. In the case of two particles of spin j prepared in a single state, the violation of Bell's inequality remains maximal for arbitrary large j. It is thus seen that large quantum numbers are no guarantee of classical behaviour.
Quantum cryptography over 23 km of installed telecommunications fibre using a novel interferometer with Faraday mirrors is presented. The interferometer needs no alignment nor polarisation control and features 99.8% fringe visibility. A secret key of 20 kbit length with an error rate of 1.35% for 0.1 photon per pulse was produced.
