Searcharxiv⌕ Search

arXiv subjects

Marcin Wiesniak

Publications and source records attributed to Marcin Wiesniak.

At least 19 recordsLinked to original sources
arXiv2018

Higher dimensional entanglement without correlations

It has been demonstrated both theoretically and experimentally that genuine multipartite entanglement between qubits can exist even in the absence of multipartite correlations. Here we provide first examples of this effect in higher dimensional systems -- qudits. We construct states in which genuine $N$-partite entanglement between qudits is supported only by correlations involving strictly less than $N$ particles. The construction differs in several aspects from the ones for qubits. The states introduced here are a natural test-bed for candidate quantifiers of genuinely multipartite quantum correlations.

quant-ph↗
arXiv2017

Normalized Stokes operators for polarization correlations of entangled optical fields

Stokes parameters are a standard tool in quantum optics. They involve averaged intensities at exits of polarizers. If the overall measured intensity fluctuates, as e.g. for states with undefined photon numbers, the instances of its increased value contribute more to the parameters. One can introduce normalized quantum Stokes operators. Operationally, for a given single run of the experiment, their values are differences of measured intensities (or photon numbers) at the two exits of a polarizer divided by their sum. Effects of intensity fluctuations are removed. Switching to normalized Stokes operators results in more sensitive entanglement conditions. We also show a general method of deriving an entanglement indicator for optical fields which use polarization correlations, which starts with any two-qubit entanglement witness. This allows to vastly expand the family of such indicators.

quant-ph↗
arXiv2017

Experimental Quantum Solution to the Dining Cryptographers Problem

Quantum resources such as superposition and entanglement have been used to provide unconditional key distribution, secret sharing and communication complexity reduction. In this letter we present a novel quantum information protocol for dining cryptographers problem and anonymous vote casting by a group of voters. We successfully demonstrate the experimental realization of the protocol using single photon transmission. Our implementation employs a flying particle scheme where a photon passes by the voters who perform a sequence of actions (unitary transformations) on the photonic state at their local stations.

quant-ph↗
arXiv2016

Bell Inequalities for Quantum Optical Fields

We show that the "practical" Bell inequalities, which use intensities as the observed variables, commonly used in quantum optics and widely accepted in the community, suffer from an inherent loophole, which severely limits the range of local hidden variable theories of light, which are invalidated by their violation. We present alternative inequalities which do not suffer from any (theoretical) loophole. The new inequalities use redefined correlation functions, which involve averaged products of local rates rather than intensities. Surprisingly, the new inequalities detect entanglement in situations in which the "practical" ones fail. Thus, we have two for the price on one: full consistency with Bell's Theorem, and better device-independent detection of entanglement.

quant-ph↗
arXiv2015

Highly noise resistant multipartite quantum correlations

We analyze robustness of correlations of the $N$-qubit GHZ and Dicke states against white noise admixture. For sufficiently large $N$, the Dicke states (for any number of excitations) lead to more robust violation of local realism than the GHZ states (e.g. for $N=9$ for the W state). We also identify states that are the most resistant to white noise. Surprisingly, it turns out that these states are partially product. As a by-product, we obtain a simple three-setting Bell inequality which is violated by any pure entangled state.

quant-ph↗
arXiv2014

Translating Concepts of State Transfer to Spin-1 Chains

State transfer is a well-known routine for various systems of spins-$\frac{1}2$. Still, it is not well studied for chains of spins of larger magnitudes. In this contribution we argue that while perfect state transfer may seem unnatural in spin-1 systems, it is still feasible for arrays of V-type three-level atoms. Tomography of such 1D array is also shown to be possible by acting on one atom from such an array.

quant-ph↗
arXiv2013

Comment on "Two Fundamental Experimental Tests of Nonclassicality with Qutrits"

In a recent paper [arXiv:1301.2887] Ahrens et al. claim that our "(...) experiment on qutrits does not test Klyachko et al.'s inequality, but an inequality with extra correlations" and that the "(...) experiment cannot be considered a proper test of a noncontextuality inequality, since the same observable is measured with different setups in different contexts". We disagree with these claims. In this note we briefly re-state how our test of the non-contextuality inequality was constructed. We explain how we keep the context of measurements when switching between different terms of the tested inequality, and we argue that we did in fact test the Klyachko, Can, Binicioglu and Shumovsky inequality. In doing so, we also clarify why our experiment is indeed a proper test of noncontextual realism.

quant-ph↗
arXiv2013

Perfect State Transfer without State Initialization and Remote Collaboration

We present a perfect state transfer protocol via a qubit chain with the evolution governed by the $xx$ Hamiltonian. In contrast to the recent protocol announced in [Phys. Rev. Lett. {\bf 101}, 230502 (2008)], our method does not demand any remote-cooperated initialization and sending classical information about measurement outcomes. We achieve the perfect state transfer only with the assumption of access to two spins at each end of the chain, while the initial state of the whole chain is irrelevant.

quant-ph↗
arXiv2013

Experimental multi-location remote state preparation

Transmission of quantum states is a central task in quantum information science. Remote state preparation (RSP) has the same goal as teleportation, i.e. transferring quantum information without sending physically the information carrier, but in RSP the sender knows the state which is to be transmitted. We present experimental demonstrations of RSP for two and three locations. In our experimental scheme Alice (the preparer) and her three partners share four and six photon polarization entangled singlets. This allows us to perform RSP of two or three copies of a single qubit states, a two qubit Bell state, and a three qubit W, or inverse W state. A possibility to prepare a two-qubit non-maximally entangled and GHZ states is also discussed. The ability to remotely prepare an entangled states by local projections at Alice is a distinguishing feature of our scheme.

quant-ph↗
arXiv2012

Communication Complexity Reduction from Globally Uncorrelated States

Bell inequality violating entangled states are the working horse for many potential quantum information processing applications, including secret sharing, cryptographic key distribution and communication complexity reduction in distributed computing. Here we explicitly demonstrate the power of certain multi-qubit states to improve the efficiency of partners in joint computation of some multi-qubit function, despite the fact that there could be no correlations between all distributed particles. It is important to stress that the class of functions that can be computed more efficiently is widened, as compared with the standard Bell inequalities.

quant-ph↗
arXiv2011

Experimental non-classicality of an indivisible quantum system

Quantum theory demands that, in contrast to classical physics, not all properties can be simultaneously well defined. The Heisenberg Uncertainty Principle is a manifestation of this fact. Another important corollary arises that there can be no joint probability distribution describing the outcomes of all possible measurements, allowing a quantum system to be classically understood. We provide the first experimental evidence that even for a single three-state system, a qutrit, no such classical model can exist that correctly describes the results of a simple set of pairwise compatible measurements. Not only is a single qutrit the simplest system in which such a contradiction is possible, but, even more importantly, the contradiction cannot result from entanglement, because such a system is indivisible, and it does not even allow the concept of entanglement between subsystems.

quant-ph↗
arXiv2009

Finding Traps in Non-linear Spin Arrays

Precise knowledge of the Hamiltonian of a system is a key to many of its applications. Tasks such state transfer or quantum computation have been well studied with a linear chain, but hardly with systems, which do not possess a linear structure. While this difference does not disturb the end-to-end dynamics of a single excitation, the evolution is significantly changed in other subspaces. Here we quantify the difference between a linear chain and a pseudo-chain, which have more than one spin at some site (block). We show how to estimate a number of all spins in the system and the intra-block coupling constants. We also suggest how it is possible to eliminate excitations trapped in such blocks, which may disturb the state transfer. Importantly, one uses only at-ends data and needs to be able to put the system to either the maximally magnetized or the maximally mixed state. This can obtained by controlling a global decoherence parameter, such as temperature.

quant-ph↗
arXiv2009

Interference contrast in multi-source few photon optics

Many recent experiments employ several parametric down conversion (PDC) sources to get multiphoton interference. Such interference has applications in quantum information. We study here how effects due to photon statistics, misalignment, and partial distinguishability of the PDC pairs originating from different sources may lower the interference contrast in the multiphoton experiments.

quant-ph↗
arXiv2009

Experimental filtering of two-, four-, and six-photon singlets from single PDC source

Invariant entangled states remain unchanged under simultaneous identical unitary transformations of all their subsystems. We experimentally generate and characterize such invariant two-, four-, and six-photon polarization entangled states. This is done only with a suitable filtering procedure of multiple emissions of entangled photon pairs from a single source, without any interferometric overlaps. We get the desired states utilizing bosonic emission enhancement due to indistinguishability. The setup is very stable, and gives high interference contrasts. Thus, the process is a very likely candidate for various photonic demonstrations of quantum information protocols.

quant-ph↗
arXiv2008

Heisenberg chains cannot mirror a state

Faithful exchange of quantum information can in future become a key part of many computational algorithms. Some Authors suggest to use chains of mutually coupled spins as channels for quantum communication. One can divide these proposals into the groups of assisted protocols, which require some additional action from the users, and natural ones, based on the concept of state mirroring. We show that mirror is fundamentally not the feature chains of spins-1/2 coupled by the Heisenberg interaction, but without local magnetic fields. This fact has certain consequences in terms of the natural state transfer.

quant-ph↗
arXiv2008

Entanglement in doped Resonating Valence Bond states

We investigate the entanglement properties of resonating valence bond states on a two dimensional lattice in the presence of dopants that remove electrons from the lattice creating "holes". The movement of the holes generated by the Hubbard Hamiltonian in the regime of strong Coloumb repulsion in this setting could be responsible for the phenomenon of high temperature superconductivity as hypothesised by Anderson in Science {\bf 235}, 1196, (1987). We argue that there is a particular density of dopants (holes) where the entanglement contained in the lattice attains its maximal value for the nearest-neighbour RVB liquid state.

quant-ph↗
arXiv2008

Heat Capacity as A Witness of Entanglement

We demonstrate that the presence of entanglement in macroscopic bodies (e.g. solids) in thermodynamical equilibrium could be revealed by measuring heat-capacity. The idea is that if the system were in a separable state, then for certain Hamiltonians heat capacity would not tend asymptotically to zero as the temperature approaches absolute zero. Since this would contradict the third law of thermodynamics, one concludes that the system must contain entanglement. The separable bounds are obtained by minimization of the heat capacity over separable states and using its universal low-temperature behavior. Our results open up a possibility to use standard experimental techniques of solid state physics -- namely, heat capacity measurements -- to detect entanglement in macroscopic samples.

quant-ph↗