45 research outputs found
Mutual Unbiasedness in Coarse-grained Continuous Variables
The notion of mutual unbiasedness for coarse-grained measurements of quantum
continuous variable systems is considered. It is shown that while the procedure
of "standard" coarse graining breaks the mutual unbiasedness between conjugate
variables, this desired feature can be theoretically established and
experimentally observed in periodic coarse graining. We illustrate our results
in an optics experiment implementing Fraunhofer diffraction through a periodic
diffraction grating, finding excellent agreement with the derived theory. Our
results are an important step in developing a formal connection between
discrete and continuous variable quantum mechanics.Comment: 5 pages, 3 figures + Supplemental Material (1 page) v2: Introduction
expanded, minor typos correcte
Optimizing the use of detector arrays for measuring intensity correlations of photon pairs
Intensity correlation measurements form the basis of many experiments based on spontaneous parametric down-conversion. In the most common situation, two single-photon avalanche diodes and coincidence electronics are used in the detection of the photon pairs, and the coincidence count distributions are measured by making use of some scanning procedure. Here we analyze the measurement of intensity correlations using multielement detector arrays. By considering the detector parameters such as the detection and noise probabilities, we found that the mean number of detected photons that maximizes the visibility of the two-photon correlations is approximately equal to the mean number of noise events in the detector array. We provide expressions predicting the strength of the measured intensity correlations as a function of the detector parameters and on the mean number of detected photons. We experimentally test our predictions by measuring far-field intensity correlations of spontaneous parametric down-conversion with an electron multiplying charge-coupled device camera, finding excellent agreement with the theoretical analysis
EPR-based ghost imaging using a single-photon-sensitive camera
Correlated photon imaging, popularly known as ghost imaging, is a technique whereby an image is formed from light that has never interacted with the object. In ghost imaging experiments, two correlated light fields are produced. One of these fields illuminates the object, and the other field is measured by a spatially resolving detector. In the quantum regime, these correlated light fields are produced by entangled photons created by spontaneous parametric down-conversion. To date, all correlated photon ghost imaging experiments have scanned a single-pixel detector through the field of view to obtain spatial information. However, scanning leads to poor sampling efficiency, which scales inversely with the number of pixels, N, in the image. In this work, we overcome this limitation by using a time-gated camera to record the single-photon events across the full scene. We obtain high-contrast images, 90%, in either the image plane or the far field of the photon pair source, taking advantage of the EinsteinâPodolskyâRosen-like correlations in position and momentum of the photon pairs. Our images contain a large number of modes, >500, creating opportunities in low-light-level imaging and in quantum information processing
Testing for entanglement with periodic coarse-graining
Continuous variables systems find valuable applications in quantum
information processing. To deal with an infinite-dimensional Hilbert space, one
in general has to handle large numbers of discretized measurements in tasks
such as entanglement detection. Here we employ the continuous transverse
spatial variables of photon pairs to experimentally demonstrate novel
entanglement criteria based on a periodic structure of coarse-grained
measurements. The periodization of the measurements allows for an efficient
evaluation of entanglement using spatial masks acting as mode analyzers over
the entire transverse field distribution of the photons and without the need to
reconstruct the probability densities of the conjugate continuous variables.
Our experimental results demonstrate the utility of the derived criteria with a
success rate in entanglement detection of relative to studied
cases.Comment: V1: revtex4, 10 pages, 4 figures + supp. material (4 pages, 1 figure)
V2: Substantial revisions implemented both in theory and experimental data
analysi
Uncertainty Relations for coarse-grained measurements: an overview
Uncertainty relations involving complementary observables are one of the
cornerstones of quantum mechanics. Aside from their fundamental significance,
they play an important role in practical applications, such as detection of
quantum correlations and security requirements in quantum cryptography. In
continuous variable systems, the spectra of the relevant observables form a
continnuum and this necessitates the coarse graining of measurements. However,
these coarse-grained observables do not necessarily obey the same uncertainty
relations as the original ones, a fact that can lead to false results when
considering applications. That is, one cannot naively replace the original
observables in the uncertainty relation for the coarse-grained observables and
expect consistent results. As such, a number of uncertainty relations that are
specifically designed for coarse-grained observables have been developed. In
recognition of the 90 anniversary of the seminal Heisenberg uncertainty
relation, celebrated last year, and all the subsequent work since then, here we
give a review of the state of the art of coarse-grained uncertainty relations
in continuous variable quantum systems, as well as their applications to
fundamental quantum physics and quantum information tasks. Our review is meant
to be balanced in its content, since both theoretical considerations and
experimental perspectives are put on an equal footing.Comment: Review article, 35 page
Optical computing of quantum revivals
Interference is the mechanism through which waves can be structured into the
most fascinating patterns. While for sensing, imaging, trapping, or in
fundamental investigations, structured waves play nowadays an important role
and are becoming subject of many interesting studies. Using a coherent optical
field as a probe, we show how to structure light into distributions presenting
collapse and revival structures in its wavefront. These distributions are
obtained from the Fourier spectrum of an arrangement of aperiodic diffracting
structures. Interestingly, the resulting interference may present quasiperiodic
structures of diffraction peaks on a number of distance scales, even though the
diffracting structure is not periodic. We establish an analogy with revival
phenomena in the evolution of quantum mechanical systems and illustrate this
computation numerically and experimentally, obtaining excellent agreement with
the proposed theory.Comment: 10 pages, 4 figure
The CD85j+ NK Cell Subset Potently Controls HIV-1 Replication in Autologous Dendritic Cells
Natural killer (NK) cells and dendritic cells (DC) are thought to play critical roles in the first phases of HIV infection. In this study, we examined changes in the NK cell repertoire and functions occurring in response to early interaction with HIV-infected DC, using an autologous in vitro NK/DC coculture system. We show that NK cell interaction with HIV-1-infected autologous monocyte-derived DC (MDDC) modulates NK receptor expression. In particular, expression of the CD85j receptor on NK cells was strongly down-regulated upon coculture with HIV-1-infected MDDC. We demonstrate that CD85j+ NK cells exert potent control of HIV-1 replication in single-round and productively HIV-1-infected MDDC, whereas CD85jâ NK cells induce a modest and transient decrease of HIV-1 replication. HIV-1 suppression in MDCC by CD85j+ NK cells required cell-to-cell contact and did not appear mediated by cytotoxicity or by soluble factors. HIV-1 inhibition was abolished when NK-MDDC interaction through the CD85j receptor was blocked with a recombinant CD85j molecule, whereas inhibition was only slightly counteracted by blocking HLA class I molecules, which are known CD85j ligands. After masking HLA class I molecules with specific antibodies, a fraction of HIV-1 infected MDDC was still strongly stained by a recombinant CD85j protein. These results suggest that CD85j+ NK cell inhibition of HIV-1 replication in MDDC is mainly mediated by CD85j interaction with an unknown ligand (distinct from HLA class I molecules) preferentially expressed on HIV-1-infected MDDC