Session X30: Focus Session: Quantum Information for Quantum Foundations - Foundational Experiments and Experimental Proposals

Experimental violation of the Leggett-Garg inequality under decoherence

Despite the great success of quantum mechanics, questions regarding its application still exist and the boundary between quantum and classical mechanics remains unclear. Based on the philosophical assumptions of macrorealism and noninvasive measurability, Leggett and Garg devised a series of inequalities (LG inequalities) involving a single system with a set of measurements at different times. Introduced as the Bell inequalities in time, the violation of LG inequalities excludes the hidden-variable description based on the above two assumptions. Here, we experimentally investigate the single photon LG inequalities in a dephasing environment simulated by birefringent media. By implementing an optical Controlled-Not gate on a single photon, the LG inequalities are shown to be maximally violated in a coherent evolution process. This disproves its classical realistic description with the two assumptions of the LG inequalities. With the increase of birefringent media, the violation of LG inequalities becomes weaker and is shown to be not violated anymore at some time. The ability to violate the LG inequalities can be used to set the boundary of the classical realistic description.   

A condition for macroscopic realism beyond the Leggett-Garg inequalities

In 1985, Leggett and Garg have put forward the concept of macroscopic realism (macrorealism), stating that the properties of macroscopic objects exist independent of and are not influenced by measurement. In analogy to Bell's theorem, they derived a necessary condition in terms of inequalities, which are now known as the Leggett-Garg inequalities. In this talk, a mathematical condition is introduced which is not only necessary but also sufficient for macrorealism. More importantly, the structure of this condition intuitively encompasses the physical meaning of macrorealism and allows for its experimental test in situations where the paradigm of Leggett-Garg inequalities might not be applicable.   

A quantum delayed-choice {\em gedanken} experiment

{\em Gedanken} experiments are important conceptual tools in the quest to reconcile our classical intuition with quantum mechanics and nowadays are routinely performed in the laboratory. An important open question is the quantum behaviour of the controlling devices in such experiments. We propose a framework to analyse quantum-controlled experiments and illustrate the implications by discussing a quantum version of Wheeler's delayed-choice experiment. The introduction of a quantum-controlled device (i.e., quantum beamsplitter) has several consequences. First, it implies that we can measure complementary phenomena with a single experimental setup, thus pointing to a redefinition of complementarity principle. Second, a quantum control allows us to prove there are no consistent hidden-variable theories in which ``particle'' and ``wave'' are realistic properties. Finally, it shows that a photon can have a morphing behaviour between ``particle" and ``wave''; this further supports the conclusion that ``particle" and ``wave'' are not realistic properties but merely reflect how we ``look'' at the photon. The framework developed here can be extended to other experiments, particularly to Bell-inequality tests.   

The Pauli Exclusion Principle for electrons -- a high sensitivity test in Gran Sasso underground laboratory

One of the fundamental rules of nature and a pillar in the foundation of quantum theory and thus of modern physics is represented by the Pauli Exclusion Principle. We know that this principle is extremely well fulfilled due to many observations like the order of the elements and the stability of matter. Numerous experiments were performed to search for tiny violation of this rule in various systems. The experiment VIP at the Gran Sasso underground laboratory is searching for possible small violations of the Pauli Exclusion Principle for electrons leading to an ``anomalous'' X-ray transition in copper atoms. VIP is aiming at a test oft he Pauli Exclusion Principle for electrons with unprecedented accuracy, down to the level of 10-29 - 10-30, thus improving the previous limit by 3-4 orders of magnitude. The experimental method, the setup, results obtained so far and future plans to further increase the precision by 2 orders of magnitude will be presented.   

Quantum metamaterials as a tool for investigating the quantum-classical transition

Quantum metamaterials are optical media comprised of individually controllable unit elements (e.g., qubits), which maintain quantum coherence for periods sufficient for an electromagnetic wave to pass through the system. They represent macroscopic, spatially extended quantum scatterers, which can be put in a superposition of states with different properties (e.g., different refractive indexes) and can thus provide new ways of testing different scenarios of quantum-classical transition. We consider an inverse of the classic double-slit experiment, where a classical electromagnetic wave is scattered by a quantum metamaterial in a superposition of states, and discuss the possibilities of its experimental realization.   

Progress towards a loophole-free test of nonlocality

We report on our progress towards a loophole-free test of nonlocality using spontaneous parametric down-conversion (SPDC). While the timing loophole can be easily closed in such a system by moving the detectors far apart [1], closing the detector loophole is significantly more difficult. In the standard Bell entangled states with the maximal violation of the CHSH inequality [2], an overall efficiency of 83\% is required. This limit can be lowered to 67\% by using non-maximally entangled states (although sensitivity to noise is greatly increased) [3]. We are carefully engineering our source to achieve maximal heralding efficiency, by optimizing both the spatial and spectral filtering, while keeping noise low using high-extinction-ratio polarizing beamsplitters. Combined with high-efficiency detectors, either optimized visible-light photon counters [4] or transition-edge sensors [5], closure of the detection loophole is within reach. \\[4pt] [1] G. Weihs et al., Phys. Rev. Lett. 81, 5039 (1998).\newline [2] J. F. Clauser et al., Phys. Rev. Lett. 23, 880 (1969).\newline [3] P.H. Eberhard, Phys. Rev. A 47, R747 (1993).\newline [4] S. Takeuchi et al., Appl. Phys. Lett. 74, 1063 (1999).\newline [5] A. E. Lita, A. J. Miller, and S. Nam, Opt. Exp. 16, 3032 (2008).   

Loophole-free Quantum Steering

Experiments testing quantum mechanics have provided increasing evidence against local realistic theories. However, a conclusive test that simultaneously closes all major loopholes (the locality, freedom-of-choice, and detection loopholes) remains an open challenge. An important class of local realistic theories can be tested with the concept of ``steering.'' Schr\"{o}dinger introduced this term for entanglement seemingly allowing to remotely steer the state of a distant system [1]. Einstein called this ``spooky action at a distance.'' Steering was recently formalized by deriving steering inequalities allowing experimental tests. Here, we present the first loophole-free steering experiment [2]. We use entangled photons shared between two distant laboratories and close all loopholes by a large separation, ultra-fast switching and quantum random number generation, and high, overall detection efficiency. Beside its foundational importance loop-hole-free steering is relevant for is relevant for device-independent certification of quantum entanglement. \\[4pt] [1] E. Schr\"{o}dinger, Proc. Camb. , Phil. Soc. 31, 553 (1935) \\[0pt] [2] B. Wittmann, S. Ramelow, F. Steinlechner, N. K. Langford, N. Brunner, H. Wiseman, R. Ursin, A. Zeilinger, arXiv:1111.0760, (2011)   

Conclusive quantum steering with superconducting transition edge sensors

Quantum steering allows two parties to verify shared entanglement even if one measurement device is untrusted. A conclusive demonstration of steering through the violation of a steering inequality is of considerable fundamental interest and opens up applications in quantum communication. To date all experimental tests with single photon states have relied on post-selection, allowing untrusted devices to cheat by hiding unfavorable events in losses. Here we close this ``detection loophole'' by combining a highly efficient source of entangled photon pairs with superconducting transition edge sensors. We achieve an unprecedented $\sim $62{\%} conditional detection efficiency of entangled photons and violate a steering inequality with the minimal number of measurement settings by 48 standard deviations. Our results provide a clear path to practical applications of steering and to a photonic loophole-free Bell test.   

Experimental Violation of Heisenberg's Precision Limit by Weak Measurements

Along with the uncertainty principle, Heisenberg postulated another set of relations, which set a lower limit on the disturbance caused by a measurement [1]. These relations were shown by Ozawa to be inaccurate [2], shedding doubt on widely accepted bounds on the information left in a system after a measurement, and offering new insights into the foundations of quantum physics and quantum information. A theoretical scheme for testing Ozawa's precision-disturbance relations was proposed [3]. In this proposal the hurdle of destructive measurements is addressed by the weak value approach [4]. This scheme is based on a 3-qubit quantum circuit that requires two CNOT gates of variable strength with a common control qubit. Here, we present an experimental realization of Heisenberg's precision limit violation based on weak value measurements. We implement a technique inspired by the one-way quantum computing using entanglement as the substrate for quantum gates. In this way, we demonstrate a violation of Heisenberg's relation for measurement disturbance, confirming the revised bound due to Ozawa. \\[4pt] [1] Z. Phys. 43 172(1927); [2] Ann. Phys. NY 311 350(2004); [3] New J. Phys. 12 093011(2010); [4] Phys. Rev. Lett. 60 1351(1988)   

The rise of long-distance entanglement within a linear chain of ions

One stumbling block which limits our observation of quantum effects in the macroscopic world is decoherence. For this reason the study of decoherence and dissipation in open quantum systems has attracted a lot of attention. It has been shown that the generation of long distance entanglement is possible between oscillators via a harmonic crystal (Wolf et al, EPL, 95(2011) 60008). The aim of this current work is to propose an experimentally feasible setup to test the possibility of the creation of long distance entanglement. For this purpose we consider an ion chain in a linear Paul trap with two embedded impurities, whose transverse modes resemble the two degrees of freedom that we aim to entangle via the rest of the chain. With the aid of appropriately designed laser fields, the dynamics described in (Wolf et al, EPL, 95(2011) 60008) is reproduced. The resulting entanglement between the transverse modes of the impurities is analysed by means of the logarithmic negativity.   

Popper's Thought Experiment Reinvestigated

Karl Popper posed an interesting thought experiment in 1934. With it, he meant to question the completeness of quantum mechanics. He claimed that the notion of quantum entanglement leads to absurd scenarios that cannot be true in real life and that an implementation of his thought experiment would not give the results that QM predicts. Unfortunately for Popper, it has taken until recently to perform experiments that test his claims. The results of the experiments do not refute QM as Popper predicted, but neither do they confirm what Popper claimed QM predicted. Kim and Shih implemented Popper's thought experiment in the lab. The results of the experiment are not clear and have instigated many interpretations of the results. The results show some correlation between entangled photons, but not in the way that Popper thought, nor in the way a simple application of QM might predict. A ghost-imaging experiment by Strekalov, et al. sheds light on the physics behind Popper's thought experiment, but does not try to directly test it. I will build the physics of Popper's thought experiment from the ground up and show how the results of both of these experiments agree with each other and the theory of QM, but disprove Popper.   

Positive Noise Cross Correlation in a Copper Pair Splitter.

Entanglement is in heart of the Einstein-Podolsky-Rosen (EPR) paradox, in which non-locality is a fundamental property. Up to date spin entanglement of electrons had not been demonstrated. Here, we provide direct evidence of such entanglement by measuring: non-local positive current correlation and positive cross correlation among current fluctuations, both of separated electrons born by a Cooper-pair-beam-splitter. The realization of the splitter is provided by injecting current from an Al superconductor contact into two, single channel, pure InAs nanowires - each intercepted by a Coulomb blockaded quantum dot (QD). The QDs impedes strongly the flow of Cooper pairs allowing easy single electron transport. The passage of electron in one wire enables the simultaneous passage of the other in the neighboring wire. The splitting efficiency of the Cooper pairs (relative to Cooper pairs actual current) was found to be $\sim $ 40{\%}. The positive cross-correlations in the currents and their fluctuations (shot noise) are fully consistent with entangled electrons produced by the beam splitter.   

Scalable fiber integrated source for higher-dimensional path-entangled photons

Higher dimensional Hilbert spaces are expected to show intriguing higher order effects. Examples are higher dimensional perfect correlations or the unsolved problem of finding a complete set of MUBs for dimension 6. Higher dimensional systems also have advantages for QKD protocols. Our approach is to build an easy to use platform to access these dimensions. We realized an in-fiber, high brightness and high fidelity source for path-entangled quNits in the telecom band. It is purely integrated in fiber and only standard off-shelf components are used. This results in high stability and scalability in terms of complexity with increasing dimension. In order to manipulate and transform the produced entangled states we implemented multiports in integrated optical technology, enabling us to perform any unitary transformation depending on its internal settings[1]. Results up to dimension 4 (ququarts) will be presented in the talk. [1] M. Reck, A. Zeilinger, H. J. Bernstein and P. Bertani, PRL 73, 1 (1994)   

New Near-Deterministic Teleportation Protocol with Linear Optics

We present a new near-deterministic method of separating all four photon Bell states by means of concatenated Mach-Zehnder interferometers. Realistic proposals for implementations of teleportation, superdense coding, and cryptographic ping-pong protocols will be presented. Discrimination of the Bell states is made possible by two two linear chains of concatenated Mach-Zehnder interferometers each fed with photons emerging from two opposite sides of a beam splitter. This amounts to detecting two Bell states $|\Phi^\pm\rangle$ while keeping the third one $|\Psi^+\rangle$ conditionally at bay---thus going around Vaidman-L{\"u}tkenhaus 50\%-limit. Realistic implementation with an efficiency of 90\%\ is feasible with today's technology. Channel capacity of 1.98 can be ideally achieved for superdense coding with 5 concatenated Mach-Zehnder interferometers but already with only two easily implementable ones we obtain 1.74 capacity. The setup is based on a revised and corrected method given in M.\ Pavi{\v c}i{\'c}, {\it Phys.\ Rev\ Lett.\/} {\bf 107}, 080403 (2011).