Entropic tests of multipartite nonlocality and state-independent contextuality

We introduce a multipartite extension of an information-theoretic distance first introduced in [Nature 341, 119 (1989)]. We use this new distance to derive entropic tests of multipartite nonlocality for three and for an arbitrary even number of qubits as well as a test of state-independent contextuality. In addition, we re-derive the tripartite Mermin inequality and a state-independent non-contextuality inequality by Cabello [Phys. Rev. Lett. 101, 210401 (2008)]. This suggests that the information-theoretic distance approach to multipartite nonlocality and state-independent contextuality can provide a more general treatment of nonclassical correlations than the orthodox approach based on correlation functions.Comment: 5 pages, 3 figures, extended version of arXiv:1409.7290 with more general result

Topics on the information theoretic limits of quantum information processing and its implementation

Recent advances in quantum technologies enabled us to make large quantum states and pushed towards examining quantum theory at the macroscopic level. However observation of quantum e ects at a macroscopic level still remains a demanding task. In this thesis we try to address one of the challenges and propose and explore some new solutions. One of the obstacles for observation of macroscopic quantum e ects is the sensitivity to the measurement resolution. For many di erent cases, it has been observed that the precision requirement for measuring quantum e ects increases with the system size. We formalize this as a conjecture that for observation of macroscopic quantum e ects, either the outcome precision or the control precision of the measurements has to increase with system size. This indicates that the complexity of macroscopic quantum measurement increases with the system size and sheds some lights on the quantum-to-classical transition at the macroscopic level. We also introduce a technique to go around the sensitivity problem for observation of micro-macro entanglement. We propose that using a unitary deampli cation process, one can bring the system back to the microscopic level where the measurements are less demanding and quantum e ects are easier to verify. As the unitary processes do not change the entanglement, this serves as a veri cation tool for micro-macro entanglement. We also explored the connection between quantum e ects and thermodynamics of macroscopic quantum systems for two speci c cases. For one, we investigated the e ect of entanglement in composite bosons and Bose-Einstein condensation. We showed that as the state of the composite boson approaches a maximally entangled state, the condensation rate also approaches one. The other case we considered was heat-bath algorithmic cooling. We found the cooling limit of this class of thermodynamic transformations and showed that it decreases exponentially with the number of qubits. We also developed an entropic version of Mermin's inequality. Here the idea is to develop a tool to reveal the entanglement in many-body quantum systems based on the entropy of the measurement outcomes. We introduce a new inequality that holds for locally realistic models, yet can be violated with quantum measurements. One of the nice features of this inequality is that it can be violated maximally with quantum measurements. This resembles the GHZ paradox but for entropies of the measurement outcomes

Coarse Graining Makes It Hard to See Micro-Macro Entanglement

Observing quantum effects such as superpositions and entanglement in macroscopic systems requires not only a system that is well protected against environmental decoherence, but also sufficient measurement precision. Motivated by recent experiments, we study the effects of coarse-graining in photon number measurements on the observability of micro-macro entanglement that is created by greatly amplifying one photon from an entangled pair. We compare the results obtained for a unitary quantum cloner, which generates micro-macro entanglement, and for a measure-and-prepare cloner, which produces a separable micro-macro state. We show that the distance between the probability distributions of results for the two cloners approaches zero for a fixed moderate amount of coarse-graining. Proving the presence of micro-macro entanglement therefore becomes progressively harder as the system size increases.Comment: 5 pages, 3 figure

Quantum information approach to Bose-Einstein condensation of composite bosons

We consider composite bosons (cobosons) comprised of two elementary particles, fermions or bosons, in an entangled state. First, we show that the effective number of cobosons implies the level of correlation between the two constituent particles. For the maximum level of correlation, the effective number of cobosons is the same as the total number of cobosons, which can exhibit the original Bose-Einstein condensation (BEC). In this context, we study a model of BEC for indistinguishable cobosons with a controllable parameter, i.e., entanglement between the two constituent particles. We find that bi-fermions behave in a predictable way, i.e., the effective number of the ground state coboson is an increasing function of entanglement between a pair of constituent fermions. Interestingly, bi-bosons exhibit the opposite behaviour - the effective number of the ground state coboson is a decreasing function of entanglement between a pair of constituent bosons.Comment: 10 pages, 5 figures, accepted for publication in New J. Phy