Quantum mechanics has led to the biggest technological breakthroughs of last century. However, many paradoxes still remain in the foundations of quantum theory. In this research we study one effect that leads to such paradoxes; quantum correlations. Quantum nonlocality refers to the set of quantum correlations that are not explainable using local models. It is quantum nonlocality that has troubled physicists since the inception of quantum theory nearly 100 years ago. However, these nonlocal quantum correlations offer a range of advantages in quantum information tasks. For example, quantum key distribution and quantum computing are two tasks that are powered by quantum correlations. For this reason a wide range of quantum correlations are studied experimentally in this research. These include: entanglement, EPR-steering, Bell nonlocality and bilocality. We study these correlations using the framework of quantum information, using single photons as our chosen quantum information carriers. The first quantum correlation tested in this research is EPR-steering. EPR-steering is the application of the EPR-paradox to a quantum information task. We implement three different experiments on EPR-steering. By studying the properties of the quantum information task we find that: EPR-steering is more robust to depolarisation noise than Bell-nonlocality; EPR-steering is absolutely loss tolerant; and tests of EPR-steering are easier to carry out, than tests of Bell nonlocality, but harder to implement compared to tests of non-separability. As a result of these three findings we experimentally show that EPR-steering can be demonstrated on Belllocal states, we close the detection loophole in a photonic test of quantum nonlocality, and we discover and implement maximally parsimonious tests of non-separability, EPR-steering and Bell nonlocality.
Quantum mechanics has led to the biggest technological breakthroughs of last century. However, many paradoxes still remain in the foundations of quantum theory. In this research we study one effect that leads to such paradoxes; quantum correlations. Quantum nonlocality refers to the set of quantum correlations that are not explainable using local models. It is quantum nonlocality that has troubled physicists since the inception of quantum theory nearly 100 years ago. However, these nonlocal quantum correlations offer a range of advantages in quantum information tasks. For example, quantum key distribution and quantum computing are two tasks that are powered by quantum correlations. For this reason a wide range of quantum correlations are studied experimentally in this research. These include: entanglement, EPR-steering, Bell nonlocality and bilocality. We study these correlations using the framework of quantum information, using single photons as our chosen quantum information carriers. The first quantum correlation tested in this research is EPR-steering. EPR-steering is the application of the EPR-paradox to a quantum information task. We implement three different experiments on EPR-steering. By studying the properties of the quantum information task we find that: EPR-steering is more robust to depolarisation noise than Bell-nonlocality; EPR-steering is absolutely loss tolerant; and tests of EPR-steering are easier to carry out, than tests of Bell nonlocality, but harder to implement compared to tests of non-separability. As a result of these three findings we experimentally show that EPR-steering can be demonstrated on Belllocal states, we close the detection loophole in a photonic test of quantum nonlocality, and we discover and implement maximally parsimonious tests of non-separability, EPR-steering and Bell nonlocality.
Quantum information science explores the foundational aspects of quantum mechanics and its ombination with information science for new information technologies. The underlying key resource is non-classical correlations. These counter-intuitive correlations between quantum systems can be used for encoding, transmitting and measuring information in quantum information tasks. Although quantum properties can be used in a variety of systems, here we explore photons. These information carriers are fast, easy to generate and manipulate and only interact weakly with the environment. These properties make them to excellent candidates to be employed in experiments. Developing quantum technology, such as single photon sources and single photon detectors, allows us to investigate the foundational aspects of quantum mechanics in quantum information tasks. These tasks use non-classical correlations, which form a hierarchy, from Bell nonlocality to Einstein-Podolsky-Rosen (EPR) steering to quantum nonseparability. Two systems are nonlocally correlated, when measuring one system affects the measurement results on the other system, hence the name `steering. In test for non-classical correlations, we share a quantum state between two observers which are trusted or untrusted. While observers are both untrusted or trusted in entanglement witness tests and Bell inequality violations, respectively, EPR-steering is distinct from these two classes by its fundamental asymmetry: one party is trusted while the other is untrusted.
Quantum entanglement, quantum steering and Bell nonlocality, as significant quantum resources in the field of quantum information science, can achieve variously valuable quantum information tasks. Among of them, quantum entanglement and Bell nonlocality are the weakest and strongest nonlocal correlations, respectively. One can capture the quantum steering and Bell nonlocality via violating steering inequality and Bell inequality, respectively. In general, the detections of quantum steering and Bell nonlocality are strictly harder than entanglement detection. Here, based on steering inequality test and quantum state tomography, we attain various nonlocal correlations and experimentally demonstrate that the estimations of quantum steering and Bell nonlocality can be realized according to the quantum entanglement of the prepared two-photon test states. The estimated efficiency of quantum steering is stronger than the one of Bell nonlocality in this scenario, i.e., more steerable two-photon test states can be verified through quantum entanglement. In addition, quantum steering and Bell nonlocality are bounded by the corresponding upper and lower bounds, and these bounds cannot be punctured by all prepared two-photon states in experiment. These results are conducive to understand the relations among these nonlocal correlations.
Abstract Entanglement and Einstein–Podolsky–Rosen (EPR) steering are nonlocal quantum correlations, which are relevant resources for quantum information protocols. EPR steering, or quantum steering, refers to the correlation where a party might ‘steer’, or modify, the state of another, which is spatially separated. Entanglement is a symmetric resource while steering is asymmetrical, since it depends on the direction of the effect. Due to these different characteristics and the therefore different possible applications, there has been both theoretical and experimental research on forms to certify the distinct quantum nonlocal correlations. In recent years, alongside the investigation on quantum correlations between two systems, there has been a great interest in investigating multipartite/multimode entanglement as well as steering, since they include a high dimension and it may be possible to store more information than in a single qubit. In this review, we will summarize the different criteria and measures that have been developed for the characterization of these two kinds of correlations. We first focus on bipartite entanglement and steering. We then review the progress that has been made in the investigation of multipartite quantum correlations. We revise the theoretical work in quantum nonlocal correlation witnesses and measures, which respectively allow one to certify that the system is entangled or presents EPR steering, and give a quantification of the content of these correlations in the system. Then, we briefly review the experiments that have been designed and that demonstrate multipartite quantum correlations. We also include applications in quantum information protocols, in particular in quantum teleportation and quantum cryptography.
Shared quantum correlations find application in a wide range of modern quantum information protocols. This thesis presents several optical experiments towards the improved certification of shared quantum correlations, using photonic qubits. The first experiment demonstrates the Einstein–Podolsky–Rosen (EPR) steering effect with no detection loophole. Polarisation–entangled photon pairs generated in a Sagnac interferometer were used to test new loss–tolerant EPR steering inequalities. To test the robustness of the loss–tolerant EPR steering nequalities, a transmission loss of −4.3dB was introduced using a 1km optical fibre loop, finding inequality violations for n = 10 and n = 16 measurement settings with no detection loophole (equating to a total loss of 87%). The findings demonstrate how the new loss–tolerant protocols can be used to certify the EPR steering effect for arbitrarily high losses, and may find application in one–sided device–independent quantum key distribution protocols. The second experiment introduces EPR steering in the context of an EPR steering game. The rules of the game allow for certification of shared quantum correlations via EPR steering inequalites without requiring trust in devices or parties external to a quantum referee. In particular, the experiment successfully demonstrates EPR steering in a measurement–device independent framework; a framework which has typically been accessible only through Bell inequality violations. The findings suggest the possibility for quantum–refereed one–sided device–independent quantum key distribution protocols.
Quantum nonlocality has recently been classified into three distinct types: quantum entanglement, Einstein-Podolsky-Rosen (EPR) steering, and Bell's nonlocality. Experimentally Bell's nonlocality is usually tested by quantum violation of the Clause-Horne-Shimony-Holt (CHSH) inequality in the two-qubit system. Bell's nonlocality is the strongest type of nonlocality, also due this reason Bell-test experiments have encountered both the locality loophole and the detection loophole for a very long time. As a weaker nonlocality, EPR steering naturally escapes from the locality loophole and is correspondingly easier to be demonstrated without the detection loophole. In this work, we trigger an extraordinary approach to investigate Bell's nonlocality, which is strongly based on the EPR steering. We present a theorem, showing that for any two-qubit state τ, if its mapped state ρ is EPR steerable, then the state τ must be Bell nonlocal. The result not only pinpoints a deep connection between EPR steering and Bell's nonlocality, but also sheds a new light to realize a loophole-free Bell-test experiment (without the CHSH inequality) through the violation of steering inequality.
The Einstein-Podolsky-Rosen (EPR) paradox is one of the earliest paradoxes, arising from the lack of local realistic description of mechanics. The EPR paradox leads to the important concept of quantum which manifests itself by three different phenomena: entanglement, steering, and Bell's nonlocality. Two well-known paradoxes related to Bell's nonlocality, i.e. the Greenberger-Horne-Zeilinger paradox and the Hardy paradox, have been widely studied and also experimentally confirmed. However, an experimental test of the EPR paradox is still lacking. The EPR paradox is essentially a steering paradox, and recently it has been mathematically formulated as a contradiction equality k=1, thus making it amenable to an experimental verification. Moreover, based on the EPR paradox we also derive a generalized linear steering inequality. In this work, we perform the first experimental test of the EPR paradox in a two-qubit scenario, and also verify the corresponding generalized steering inequality, which has an advantage over the usual one by detecting more steerability of states. Experimental results confirm the theoretical predictions within the experimental errors, and provide a deeper understanding of foundation.