Papers by Chiara Macchiavello
Physical Review Letters, 2010
A recently introduced family of multipartite entangled states, the 4-qubit phased Dicke states, h... more A recently introduced family of multipartite entangled states, the 4-qubit phased Dicke states, has been created by 2-photon hyperentanglement. Our experimental method allows high state fidelity and generation rate. By introducing quantum noise in the multipartite system in a controlled way, we have tested the robustness of these states. To this purpose the entanglement of the resulting multipartite entangled mixed
Physical Review A, 2002
We introduce a general method for the experimental detection of entanglement by performing only f... more We introduce a general method for the experimental detection of entanglement by performing only few local measurements, assuming some prior knowledge of the density matrix. The idea is based on the minimal decomposition of witness operators into a pseudo-mixture of local operators. We discuss an experimentally relevant case of two qubits, and show an example how bound entanglement can be detected with few local measurements. 03.67.Dd, 03.67.Hk, A central aim in the physics of quantum information is to create and detect entanglement -the resource that allows to realize various quantum protocols. Recently, much progress has been achieved experimentally in creating entangled states . In every real experiment noise and imperfections are present so that the generated states, although intended to be entangled, may in fact be separable. Therefore, it is important to find efficient experimental methods to test whether a given imperfect state ρ is indeed entangled.
Sealing information means making it publicly available, but with the possibility of knowing if it... more Sealing information means making it publicly available, but with the possibility of knowing if it has been read. Commenting on [1], we will show that perfect quantum sealing is not possible for perfectly retrievable information, due to the possibility of performing a perfect measurement without disturbance, even on unknown states. The measurement is a collective one, and this makes the
Siam Journal on Computing, 1996
We propose a method for the stabilisation of quantum computations (including quantum state storag... more We propose a method for the stabilisation of quantum computations (including quantum state storage). The method is based on the operation of projection into $\cal SYM$, the symmetric subspace of the full state space of $R$ redundant copies of the computer. We describe an efficient algorithm and quantum network effecting $\cal SYM$--projection and discuss the stabilising effect of the proposed
We show how procedures which can correct phase and amplitude errors can be directly applied to co... more We show how procedures which can correct phase and amplitude errors can be directly applied to correct errors due to quantum entanglement. We specify general criteria for quantum error correction, introduce quantum versions of the Hamming and the Gilbert-Varshamov bounds and comment on the practical implementation of quantum codes.
Physical review letters, Jan 3, 2015
We provide an interpretation of entanglement based on classical correlations between measurement ... more We provide an interpretation of entanglement based on classical correlations between measurement outcomes of complementary properties: States that have correlations beyond a certain threshold are entangled. The reverse is not true, however. We also show that, surprisingly, all separable nonclassical states exhibit smaller correlations for complementary observables than some strictly classical states. We use mutual information as a measure of classical correlations, but we conjecture that the first result holds also for other measures (e.g., the Pearson correlation coefficient or the sum of conditional probabilities).
Quantum and Semiclassical Optics: Journal of the European Optical Society Part B, 1997
We study both systematic and statistical errors in radiation density matrix measurements. First w... more We study both systematic and statistical errors in radiation density matrix measurements. First we estimate the minimum number of scanning phases needed to reduce systematic errors below a fixed threshold. Then, we calculate the statistical errors, intrinsic in the procedure that gives the density matrix. We present a detailed study of such errors versus the detectors quantum efficiency η and the matrix indexes in the number representation, for different radiation states. For unit quantum efficiency, and for both coherent and squeezed states, the statistical errors of the diagonal matrix elements saturate for large n. On the contrary, off-diagonal errors increase with the distance from the diagonal. For non unit quantum efficiency the statistical errors along the diagonal do not saturate, and increase dramatically versus both 1 − η and the matrix indexes.
Physical Review A, 2014
We introduce a class of mixed multi-qubit states, that corresponds to a randomized version of gra... more We introduce a class of mixed multi-qubit states, that corresponds to a randomized version of graph states. It is shown that unitary equivalences are lost by randomization using a rank argument. We study the entanglement features of these states by investigating both bipartite and genuine multipartite entanglement. Bipartite entanglement is studied via the concepts of connectedness and persistency, defined for graph states and strictly related to measurement based quantum computation. The presence of multipartite entanglement is characterized by witness operators which are subsequently adapted to study non-local properties through the violation of suitable Bell inequalities. Finally, we present results on the entanglement detection of particular randomized graph states, and we propose a method to further improve the detection of genuine multipartite entanglement.
Physical Review Letters, 1999
We introduce a new decomposition of the multiqubit states of the form ρ ⊗N and employ it to const... more We introduce a new decomposition of the multiqubit states of the form ρ ⊗N and employ it to construct the optimal single qubit purification procedure. The same decomposition allows us to study optimal quantum cloning and state estimation of mixed states.
Restoration of quantum states after measurements
Applied Physics B-lasers and Optics - APPL PHYS B-LASERS OPT, 1997
. We discuss possibilities of protecting quantum states against disturbances introduced by quan... more . We discuss possibilities of protecting quantum states against disturbances introduced by quantum measurements. We specify conditions under which it is possible to restore an unknown state of a combined quantum system after measurements which were performed on some (but not on all) of its components.
Physical Review A - PHYS REV A, 2006
We describe a general framework to study covariant symmetric broadcasting maps for mixed qubit st... more We describe a general framework to study covariant symmetric broadcasting maps for mixed qubit states. We explicitly derive the optimal N-->M superbroadcasting maps, achieving optimal purification of the single-site output copy, in both the universal and phase-covariant cases. We also study the bipartite entanglement properties of the superbroadcast states.
Stabilization of Quantum Computations by Symmetrization
SIAM Journal on Computing, 1997
We propose a method for the stabilization of quantum computations (including quan- tum state stor... more We propose a method for the stabilization of quantum computations (including quan- tum state storage). The method is based on the operation of projection intoSYM, the symmetric subspace of the full state space of R redundant copies of the computer. We describe an ecient algorithm and quantum network eectingSYM{projection and discuss the stabilizing eect of the proposed method in the
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 1998
Quantum computers use the quantum interference of different computational paths to enhance correc... more Quantum computers use the quantum interference of different computational paths to enhance correct outcomes and suppress erroneous outcomes of computations. A common pattern underpinning quantum algorithms can be identified when quantum computation is viewed as multi-particle interference. We use this approach to review (and improve) some of the existing quantum algorithms and to show how they are related to different instances of quantum phase estimation. We provide an explicit algorithm for generating any prescribed interference pattern with an arbitrary precision.
Physical Review Letters, 2007
We address the problem of estimating the phase given N copies of the phase-rotation gate u . We c... more We address the problem of estimating the phase given N copies of the phase-rotation gate u . We consider, for the first time, the optimization of the general case where the circuit consists of an arbitrary input state, followed by any arrangement of the N phase rotations interspersed with arbitrary quantum operations, and ending with a general measurement. Using the polynomial method, we show that, in all cases where the measure of quality of the estimate for depends only on the difference ÿ , the optimal scheme has a very simple fixed form. This implies that an optimal general phase estimation procedure can be found by just optimizing the amplitudes of the initial state.
Physical Review Letters, 1998
We derive a tight upper bound for the fidelity of a universal N → M qubit cloner, valid for any M... more We derive a tight upper bound for the fidelity of a universal N → M qubit cloner, valid for any M ≥ N , where the output of the cloner is required to be supported on the symmetric subspace. Our proof is based on the concatenation of two cloners and the connection between quantum cloning and quantum state estimation. We generalise the operation of a quantum cloner to mixed and/or entangled input qubits described by a density matrix supported on the symmetric subspace of the constituent qubits. We also extend the validity of optimal state estimation methods to inputs of this kind. 03.65.Bz, 03.67.-a
Quantum Error Correction for Communication
Physical Review Letters, 1996
We show how procedures which can correct phase and amplitude errors are in themselves sufficient ... more We show how procedures which can correct phase and amplitude errors are in themselves sufficient to correct errors due to quantum entanglement, generalizing earlier results. We specify general criteria for quantum error correction, introduce quantum versions of the Hamming and the ...
Physical Review Letters, 2002
We study optimal eavesdropping in quantum cryptography with three-dimensional systems, and show t... more We study optimal eavesdropping in quantum cryptography with three-dimensional systems, and show that this scheme is more secure against symmetric attacks than protocols using two-dimensional states. We generalize the according eavesdropping transformation to arbitrary dimensions, and discuss the connection with optimal quantum cloning. 03.67.Dd, 03.67.Hk, 03.67.-a
Physical Review Letters, 2005
We derive the optimal universal broadcasting for mixed states of qubits. We show that the nobroad... more We derive the optimal universal broadcasting for mixed states of qubits. We show that the nobroadcasting theorem cannot be generalized to more than a single input copy. Moreover, for four or more input copies it is even possible to purify the input states while broadcasting. We name such purifying broadcasting superbroadcasting. PACS numbers: 03.65.-w, 03.67.-a
Physical Review Letters, 2003
We report on the first experimental realization of the entanglement witness for polarization enta... more We report on the first experimental realization of the entanglement witness for polarization entangled photons. It represents a recently discovered significant quantum information protocol which is based on few local measurements.
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Papers by Chiara Macchiavello