610 results:
Error protected quantum bits entangled  
For the first time, physicists from physicists from IQOQI-Vienna and Innsbruck have entangled two quantum bits distributed over several quantum objects and successfully transmitted their quantum properties. This marks an important milestone in the development of fault-tolerant quantum computers. The researchers published their report in Nature.  
An international research group including quantum physicists from the Austrian Academy of Sciences and the Scottish Heriot-Watt University has succeeded in producing and verifying high-dimensional entanglements in systems of two photons. This enables faster and more secure communication, as the scientists report in the journal "Quantum".  
Administration Researchers Group Leaders  Group Leaders Researchers Administration  
Anton Zeilinger  
Anton Zeilinger Group Leader Zeilinger Group (+ 43 1) 4277-51201 anton.zeilinger@univie.ac.at Curriculum Vitae Biosketch Scientific Publications  
Administration Researchers Group Leaders  Group Leaders Researchers Administration  
Brukner Group  
Team Quantum Foundations and Quantum Information Theory The goal of our team is to gain insight into quantum foundations and quantum information by exploiting operational and information-theoretic approaches. The team has recently applied them to the field of causality and gravity.  
Information-Theoretic Foundations of Quantum Theory  
Motivated by Wheeler’s idea of “it from bit” – the idea that information is the most fundamental, basic entity – we formulated one of the first proposals for an “information-theoretic” approach to quantum theory in 1999 [1,2]. The basic idea is that every quantum system is associated with a finite amount of information. Alternatively, one can say that the most elementary system carries one bit of information, as represented by the truth value of an elementary proposition. An example of a one-bit proposition is: “The spin is up along z-direction”, which describes a Stern-Gerlach experiment in abstract information-theoretic language. Many fundamental features of quantum systems can be understood as consequences of the constraints that limited information content imposes on the systems [1,2]. Due to the limited information content an increase in the knowledge of one of the observables is at the expense of the corresponding decrease of the knowledge in others, complementary observables. Entanglement arises from the possibility that the information in a composite system may reside more in the correlations between systems than in the individual systems themselves. Finally, the information content of a system remains constant at all times giving rise to unitarity of the dynamics in quantum theory. These considerations led to an information-theoretic reconstruction of quantum theory [3] (See also “Reconstruction of Quantum Theory”).   [1] Č. Brukner and A. Zeilinger, Operationally Invariant Information in Quantum Measurements, Phys. Rev. Lett. 83, 3354 (1999). [2] Č. Brukner and A. Zeilinger, Information and Fundamental Elements of the Structure of Quantum Theory, in “Time, Quantum, Information”, Ed.  L. Castell and O Ischebeck (Springer, 2003). Preprint at quant-ph/0212084. [3] Č. Brukner and A. Zeilinger, Information Invariance and Quantum Probabilities, Found. Phys. 39, 677 (2009). Additional reading: H. C. von Bayer, In the beginning was the bit, Feature of New Scientist (27 March 2004)  
Macroscopic Entanglement Witnesses  
It is commonly believed that for the understanding of the behaviour of large, macroscopic, objects at moderately high temperatures there is no need to invoke any genuine quantum entanglement. This is because decoherence effects arising from many particles interaction with the environment would destroy all quantum correlations. In a series of papers [1-4] we have shown that this belief is fundamentally mistaken and that entanglement is crucial to correctly describe macroscopic properties of solids. Moreover, we demonstrated that macroscopic thermodynamical properties – such as internal energy, heat capacity or magnetic susceptibility – can reveal the existence of entanglement (i.e. are so called “entanglement witnesses) within solids in the thermodynamical limit even at moderately high temperatures. We found the critical values of physical parameters (e.g. the high-temperature limit and the maximal strength of magnetic field) below which entanglement exists in solids. Detection of entanglement in magnetic solids, modelled by the xxx-Heisenberg spin-1/2 (a, top) and spin 1 (b, bottom) chains. The black solid curves are temperature dependences of the zero-field magnetic susceptibilities per particle and the red curves are “entanglement witnesses”. All points below of the intersection points to the left (below the critical temperatures) indicate the existence of entanglement in the solids. [1] Č. Brukner and V. Vedral, Macroscopic Thermodynamical Witnesses of Quantum Entanglement, Preprint at quant-ph/0406040. [2] Č. Brukner, V. Vedral and A. Zeilinger, Crucial Role of Quantum Entanglement in Bulk Properties of Solids, Phys. Rev. A 73, 012110 (2006). [3] M. Wiesniak. V. Vedral and Č. Brukner, Magnetic Susceptibility as Macroscopic Entanglement Witness, New J. Phys. 7, 258 (2005). [4] M. Wieśniak, V. Vedral, and Č. Brukner, Heat capacity as an indicator of entanglement, Phys. Rev. B 78, 064108 (2008).  
Bell’s Inequalities  
No “locally causal” theory agrees with all predictions of quantum mechanics as quantitatively expressed by violation of Bell’s inequalities. Local causality is in agreement with our everyday experience and classical physics. It is compatible with the view that measurement results are predefined prior to and independent of observations, and that they are independent of any action at space-like separations. In 2001 we derived (and independently Werner and Wolf) a single general Bell inequality that summarizes all possible inequalities for the case in which each observer has a choice between two arbitrary dichotomic observables [1]. This inequality is shown to be tight, i.e. it is a sufficient and necessary condition for the correlation function for N particles to be describable by a locally causal model. We also derive a necessary and sufficient condition for an arbitrary N-qubit mixed state to violate this inequality. Interestingly, we constructed a family of multipartite pure entangled states, which satisfy all Bell’s inequalities with two measurement settings per observer [2]. No efficient method for the derivation of Bell’s inequalities with more than two measurement settings per observer is known. A general way is to define the facets of the correlation polytope. Yet, this is computationally hard NP-problem. In 2004 we derived tight Bell’s inequalities for N>2 observers involving more than two alternative measurement settings [3]. Most importantly, we found that the new inequalities are violated by some classes of states, for which all Bell’s inequalities with two measurement settings per observer are satisfied. You might find useful our review on Bell’s inequalities and its relation to quantum communication [4], as well as our paper on fundamental implications of Bell’s theorem for the notion of physical reality [5]. [1] M. Zukowski and Č. Brukner, Bell’s Theorem for General N-qubit States, Phys. Rev. Lett. 88,  210401 (2002). [2] M. Zukowski, Č. Brukner, W. Laskowski and M. Wiesniak, Do All Pure Entangled States Violate Bell’s Inequalities for Correlation Functions?, Phys. Rev. Lett. 88,  210402 (2002). [3] W. Laskowski, T. Paterek, M. Zukowski and Č. Brukner, Tight Multipartite Bell’s Inequalities Involving Many Measurement Settings, Phys. Rev. Lett. 93, 200401 (2004). [4] Č. Brukner and M. Zukowski, Bell’s Inequalities: Foundations and Quantum Communication, in “Handbook of Natural Computing”, Editors: G. Rozenberg, T.H.W. Baeck, J.N. Kok (Springer, 2011). Preprint at arXiv:0909.2611. [5] M. Zukowski, Č. Brukner, Quantum non-locality - it ain’t necessarily so ..., Special issue on 50 years of Bell's theorem, J. Phys. A: Math. Theor. 47, 424009 (2014). Preprint at arXiv:1501.04618.  
Quantum Communication Complexity  
Although entanglement cannot be used for direct communication (as otherwise it would be superluminal), it surprisingly can produce effects as if some information had been transferred: entanglement can save on classical communication when remote parties need to accomplish a joint task. A typical problem is computation of a function whose inputs are distributed among the remote parties. The minimal amount of information that has to be exchanged to accomplish the task defines its complexity. We showed that in certain communication complexity protocols entangled states are useful only to the extent that they exhibit nonlocal correlations. More precisely, we demonstrated that for every Bell’s inequality there exists a communication complexity problem, for which the protocol assisted by states which violate the inequality is more efficient than any classical protocol [1]. On this basis we developed protocols that exploit entanglement between qubits, qutrits and higher dimensional states [1-2]. In Ref. [3] you can read about a simple but insightful example that illustrates how entanglement can help separated individuals to find each other even in the lack of any communication whatsoever. Recently, we showed that “the quantum superposition of the direction of communication” is a useful resource for communication complexity. We found a task for which such a superposition allows for an exponential saving in communication, compared to one-way quantum (or classical) communication [4] (See also “Quantum Theory on Indefinite Causal Structures”). Two partners are on the two poles of the Earth (left). From each pole there are three paths (red 1, yellow 2 and blue 3) and for each path there are two directions (+ and -) (right, view from the North pole). Which path and direction should the partners take to find each other at the equatorial line in the lack of any communication? (For an entanglement-assistant solution see Ref. [3]) [1] Č. Brukner, M. Zukowski, J.-W. Pan and A. Zeilinger, Bell's inequality and Quantum Communication Complexity, Phys. Rev. Lett. 92, 127901 (2004). [2] Č. Brukner, M. Zukowski and A. Zeilinger, Quantum Communication Complexity Protocol with Two Entangled Qutrits, Phys. Rev. Lett. 89,  197901 (2002). [3] Č. Brukner, N. Paunkovic, T. Rudolph and V. Vedral, Entanglement-assisted Orientation in Space, Int. J. of Quant. Inf. 4, 365 (2006). Preprint at quant-ph/0509123. [4] P. A. Guérin, A. Feix, M. Araújo and Č. Brukner, Exponential Communication Complexity Advantage from Quantum Superposition of the Direction of Communication, Phys. Rev. Lett. 117, 100502 (2016).