@inproceedings{42033, author = {{Rastelli, A and Ding, F and Plumhof, J.D and Kumar, S and Trotta, R and Deneke, C and Malachias, A and Atkinson, P and Zallo, E and Zander, T and Herklotz, A and Singh, R and Krápek, V and Schrötel, J.R and Kiravittaya, S and Benyoucef,, M and Hafenbrak, R and Jöns, Klaus D. and Thurmer, D.J and Grimm, D and Bester, G and Dörr, K and Michler, P and Schmidt, O.G}}, number = {{4}}, pages = {{687--696}}, title = {{{Controlling quantum dot emission by integration of semiconductor nanomembranes onto piezoelectric actuators}}}, volume = {{249}}, year = {{2012}}, } @inproceedings{42028, author = {{Hermannstädter, C and Witzany, M and Heldmaier, M and Hafenbrak, R and Jöns, Klaus D. and Beirne, G.J and Michler, P}}, title = {{{Polarization anisotropic luminescence of tunable single lateral quantum dot molecules}}}, volume = {{111}}, year = {{2012}}, } @inproceedings{42029, author = {{Jöns, Klaus D. and Hafenbrak, R and Atkinson, P and Rastelli, A and Schmidt, O.G and Michler, P}}, number = {{4}}, pages = {{697--701}}, title = {{{Quantum state tomography measurements on strain-tuned In(x)Ga(1−x)As/GaAs quantum dots}}}, volume = {{249}}, year = {{2012}}, } @inproceedings{8169, abstract = {{The polynomial hierarchy plays a central role in classical complexity theory. Here, we define a quantum generalization of the polynomial hierarchy, and initiate its study. We show that not only are there natural complete problems for the second level of this quantum hierarchy, but that these problems are in fact hard to approximate. Our work thus yields the first known hardness of approximation results for a quantum complexity class. Using these techniques, we also obtain hardness of approximation for the class QCMA. Our approach is based on the use of dispersers, and is inspired by the classical results of Umans regarding hardness of approximation for the second level of the classical polynomial hierarchy (Umans 1999). We close by showing that a variant of the local Hamiltonian problem with hybrid classical-quantum ground states is complete for the second level of our quantum hierarchy.}}, author = {{Gharibian, Sevag and Kempe, Julia}}, booktitle = {{International Colloquium on Automata, Languages, and Programming (ICALP 2012)}}, editor = {{Czumaj, Artur and Mehlhorn, Kurt and Pitts, Andrew and Wattenhofer, Roger}}, isbn = {{978-3-642-31594-7}}, location = {{Warwick, UK}}, pages = {{387--398}}, publisher = {{Springer Berlin Heidelberg}}, title = {{{Hardness of Approximation for Quantum Problems}}}, doi = {{10.1007/978-3-642-31594-7_33}}, year = {{2012}}, } @article{8175, abstract = {{Approximation algorithms for classical constraint satisfaction problems are one of the main research areas in theoretical computer science. Here we define a natural approximation version of the QMA-complete local Hamiltonian problem (where QMA stands for Quantum Merlin Arthur) and initiate its study. We present two main results. The first shows that a nontrivial approximation ratio can be obtained in the class NP using product states. The second result (which builds on the first one) gives a polynomial time (classical) algorithm providing a similar approximation ratio for dense instances of the problem. The latter result is based on an adaptation of the “exhaustive sampling method” by Arora, Karger, and Karpinski [J. Comput. System Sci., 58 (1999), p. 193] to the quantum setting and might be of independent interest.}}, author = {{Gharibian, Sevag and Kempe, Julia}}, issn = {{0097-5397}}, journal = {{SIAM Journal on Computing}}, number = {{4}}, pages = {{1028--1050}}, publisher = {{Society for Industrial & Applied Mathematics (SIAM)}}, title = {{{Approximation Algorithms for QMA-Complete Problems}}}, doi = {{10.1137/110842272}}, volume = {{41}}, year = {{2012}}, } @article{8174, abstract = {{We propose a measure of non-classical correlations in bipartite quantum states based on local unitary operations. We prove the measure is non-zero if and only if the quantum discord is non-zero; this is achieved via a new characterization of zero discord states in terms of the state's correlation matrix. Moreover, our scheme can be extended to ensure the same relationship holds even with a generalized version of quantum discord in which higher-rank projective measurements are allowed. We next derive a closed form expression for our scheme in the cases of Werner states and (2 x N)-dimensional systems. The latter reveals that for (2 x N)-dimensional states, our measure reduces to the geometric discord [Dakic et al., PRL 105, 2010]. A connection to the CHSH inequality is shown. We close with a characterization of all maximally non-classical, yet separable, (2 x N)-dimensional states of rank at most two (with respect to our measure).}}, author = {{Gharibian, Sevag}}, journal = {{Physical Review A}}, pages = {{042106}}, publisher = {{American Physical Society}}, title = {{{Quantifying nonclassicality with local unitary operations}}}, doi = {{10.1103/PhysRevA.86.042106}}, volume = {{86}}, year = {{2012}}, } @inproceedings{42031, author = {{Plumhof, J.D and Krápek, V and Ding, F and Jöns, Klaus D. and Hafenbrak, R and Klenovský, P and Herklotz, A and Dörr, K and Michler , P and Rastelli,, A and Schmidt, O.G}}, title = {{{Strain-induced anticrossing of bright exciton levels in single self-assembled GaAs/Al(x)Ga(1−x)As and In(x)Ga(1−x)As/GaAs quantum dots}}}, volume = {{83}}, year = {{2011}}, } @inproceedings{42032, author = {{Jöns, Klaus D. and Hafenbrak, R and Singh, R and Ding, F and Plumhof, J.D and Rastelli, A and Schmidt, O.G and Bester, G and Michler, P}}, pages = {{217402}}, title = {{{Dependence of the Redshifted and Blueshifted Photoluminescence Spectra of Single In(x)Ga(1−x)As/GaAs Quantum Dots on the Applied Uniaxial Stress}}}, volume = {{107}}, year = {{2011}}, } @article{8178, abstract = {{In [Piani et al., PRL106 (2011) 220403], an activation protocol was introduced which maps the general non-classical (multipartite) correlations between given systems into bipartite entanglement between the systems and local ancillae by means of a potentially highly entangling interaction. Here, we study how this activation protocol can be used to entangle the starting systems themselves via entanglement swapping through a measurement on the ancillae. Furthermore, we bound the relative entropy of quantumness (a naturally arising measure of non-classicality in the scheme of Piani et al. above) for a special class of separable states, the so-called classical–quantum states. In particular, we fully characterize the classical–quantum two-qubit states that are maximally non-classical.}}, author = {{Gharibian, Sevag and PIANI, MARCO and ADESSO, GERARDO and CALSAMIGLIA, JOHN and HORODECKI, PAWEŁ}}, issn = {{0219-7499}}, journal = {{International Journal of Quantum Information}}, number = {{07n08}}, pages = {{1701--1713}}, publisher = {{World Scientific Pub Co Pte Lt}}, title = {{{Characterizing Quantumness via Entanglement Creation}}}, doi = {{10.1142/s0219749911008258}}, volume = {{09}}, year = {{2011}}, } @inproceedings{8176, abstract = {{Approximation algorithms for classical constraint satisfaction problems are one of the main research areas in theoretical computer science. Here we define a natural approximation version of the QMA-complete local Hamiltonian problem and initiate its study. We present two main results. The first shows that a non-trivial approximation ratio can be obtained in the class NP using product states. The second result (which builds on the first one), gives a polynomial time (classical) algorithm providing a similar approximation ratio for dense instances of the problem. The latter result is based on an adaptation of the "exhaustive sampling method" by Arora et al. [J. Comp. Sys. Sci. 58, p.193 (1999)] to the quantum setting, and might be of independent interest.}}, author = {{Gharibian, Sevag and Kempe, Julia}}, booktitle = {{IEEE Annual Conference on Computational Complexity (CCC 2011)}}, isbn = {{9781457701795}}, location = {{San Jose, USA}}, publisher = {{IEEE}}, title = {{{Approximation Algorithms for QMA-Complete Problems}}}, doi = {{10.1109/ccc.2011.15}}, year = {{2011}}, } @article{8177, abstract = {{We devise a protocol in which general nonclassical multipartite correlations produce a physically relevant effect, leading to the creation of bipartite entanglement. In particular, we show that the relative entropy of quantumness, which measures all nonclassical correlations among subsystems of a quantum system, is equivalent to and can be operationally interpreted as the minimum distillable entanglement generated between the system and local ancillae in our protocol. We emphasize the key role of state mixedness in maximizing nonclassicality: Mixed entangled states can be arbitrarily more nonclassical than separable and pure entangled states.}}, author = {{Piani, Marco and Gharibian, Sevag and Adesso, Gerardo and Calsamiglia, John and Horodecki, Paweł and Winter, Andreas}}, issn = {{0031-9007}}, journal = {{Physical Review Letters}}, number = {{22}}, publisher = {{American Physical Society (APS)}}, title = {{{All Nonclassical Correlations Can Be Activated into Distillable Entanglement}}}, doi = {{10.1103/physrevlett.106.220403}}, volume = {{106}}, year = {{2011}}, } @inproceedings{42030, author = {{ Richter, D and Hafenbrak, R and Jöns, Klaus D. and Schulz, W-M and Eichfelder, M and Heldmaier, M and Roßbach, R and Jetter, M and Michler, P}}, title = {{{Low-density MOVPE grown InGaAs QDs exhibiting ultra-narrow single exciton linewidths}}}, volume = {{21}}, year = {{2010}}, } @article{8179, abstract = {{Given the density matrix rho of a bipartite quantum state, the quantum separability problem asks whether rho is entangled or separable. In 2003, Gurvits showed that this problem is NP-hard if rho is located within an inverse exponential (with respect to dimension) distance from the border of the set of separable quantum states. In this paper, we extend this NP-hardness to an inverse polynomial distance from the separable set. The result follows from a simple combination of works by Gurvits, Ioannou, and Liu. We apply our result to show (1) an immediate lower bound on the maximum distance between a bound entangled state and the separable set (assuming P != NP), and (2) NP-hardness for the problem of determining whether a completely positive trace-preserving linear map is entanglement-breaking.}}, author = {{Gharibian, Sevag}}, journal = {{Quantum Information & Computation}}, number = {{3{\&}4}}, pages = {{343--360}}, title = {{{Strong NP-hardness of the quantum separability problem}}}, volume = {{10}}, year = {{2010}}, } @article{8180, abstract = {{Given a bipartite quantum state rho with subsystems A and B of arbitrary dimensions, we study the entanglement detecting capabilities of locally noneffective, or cyclic, unitary operations [L. B. Fu, Europhys. Lett., vol. 75, pp. 1-7, 2006]. Local cyclic unitaries have the special property that they leave their target subsystem invariant. We investigate the distance between rho and the global state after local application of such unitaries as a possible indicator of entanglement. To this end, we derive and discuss closed formulae for the maximal such distance achievable for three cases of interest: (pseudo)pure quantum states, Werner states, and two-qubit states. What makes this criterion interesting, as we show here, is that it surprisingly displays behavior similar to recent anomalies observed for non-locality measures in higher dimensions, as well as demonstrates an equivalence to the CHSH inequality for certain classes of two-qubit states. Yet, despite these similarities, the criterion is not itself a non-locality measure. We also consider entanglement detection in bound entangled states.}}, author = {{Gharibian, Sevag and Kampermann, Hermann and Bru{\ss}, Dagmar}}, journal = {{Quantum Information & Computation}}, number = {{11}}, pages = {{1013--1029}}, title = {{{On global effects caused by locally noneffective unitary operations}}}, volume = {{9}}, year = {{2009}}, } @article{8181, abstract = {{We investigate signatures of non-classicality in quantum states, in particular, those involved in the DQC1 model of mixed-state quantum computation [Phys. Rev. Lett. 81, 5672 (1998)]. To do so, we consider two known non-classicality criteria. The first quantifies disturbance of a quantum state under locally noneffective unitary operations (LNU), which are local unitaries acting invariantly on a subsystem. The second quantifies measurement induced disturbance (MID) in the eigenbasis of the reduced density matrices. We study the role of both figures of non-classicality in the exponential speedup of the DQC1 model and compare them vis-a-vis the interpretation provided in terms of quantum discord. In particular, we prove that a non-zero quantum discord implies a non-zero shift under LNUs. We also use the MID measure to study the locking of classical correlations [Phys. Rev. Lett. 92, 067902 (2004)] using two mutually unbiased bases (MUB). We find the MID measure to exactly correspond to the number of locked bits of correlation. For three or more MUBs, it predicts the possibility of superior locking effects.}}, author = {{Datta, Animesh and Gharibian, Sevag}}, issn = {{1050-2947}}, journal = {{Physical Review A}}, number = {{4}}, publisher = {{American Physical Society (APS)}}, title = {{{Signatures of nonclassicality in mixed-state quantum computation}}}, doi = {{10.1103/physreva.79.042325}}, volume = {{79}}, year = {{2009}}, } @article{37680, author = {{Silberhorn, Ch. and Korolkova, N. and Leuchs, G.}}, issn = {{0031-9007}}, journal = {{Physical Review Letters}}, keywords = {{General Physics and Astronomy}}, number = {{16}}, publisher = {{American Physical Society (APS)}}, title = {{{Quantum Key Distribution with Bright Entangled Beams}}}, doi = {{10.1103/physrevlett.88.167902}}, volume = {{88}}, year = {{2002}}, }