76 research outputs found
Nature of light correlations in ghost imaging
We investigate the nature of correlations in Gaussian light sources used for
ghost imaging. We adopt methods from quantum information theory to distinguish
genuinely quantum from classical correlations. Combining a microscopic analysis
of speckle-speckle correlations with an effective coarse-grained description of
the beams, we show that quantum correlations exist even in `classical'-like
thermal light sources, and appear relevant for the implementation of ghost
imaging in the regime of low illumination. We further demonstrate that the
total correlations in the thermal source beams effectively determine the
quality of the imaging, as quantified by the signal-to-noise ratio.Comment: 12 pages, 5 figures. To appear in Scientific Reports (NPG
Imaging high-dimensional spatial entanglement with a camera
The light produced by parametric down-conversion shows strong spatial
entanglement that leads to violations of EPR criteria for separability.
Historically, such studies have been performed by scanning a single-element,
single-photon detector across a detection plane. Here we show that modern
electron-multiplying charge-coupled device cameras can measure correlations in
both position and momentum across a multi-pixel field of view. This capability
allows us to observe entanglement of around 2,500 spatial states and
demonstrate Einstein-Podolsky-Rosen type correlations by more than two orders
of magnitude. More generally, our work shows that cameras can lead to important
new capabilities in quantum optics and quantum information science.Comment: 5 pages, 4 figure
Quantum-inspired interferometry with chirped laser pulses
We introduce and implement an interferometric technique based on chirped
femtosecond laser pulses and nonlinear optics. The interference manifests as a
high-visibility (> 85%) phase-insensitive dip in the intensity of an optical
beam when the two interferometer arms are equal to within the coherence length
of the light. This signature is unique in classical interferometry, but is a
direct analogue to Hong-Ou-Mandel quantum interference. Our technique exhibits
all the metrological advantages of the quantum interferometer, but with signals
at least 10^7 times greater. In particular we demonstrate enhanced resolution,
robustness against loss, and automatic dispersion cancellation. Our
interferometer offers significant advantages over previous technologies, both
quantum and classical, in precision time delay measurements and biomedical
imaging.Comment: 6 pages, 4 figure
Spectral compression of single photons
Photons are critical to quantum technologies since they can be used for
virtually all quantum information tasks: in quantum metrology, as the
information carrier in photonic quantum computation, as a mediator in hybrid
systems, and to establish long distance networks. The physical characteristics
of photons in these applications differ drastically; spectral bandwidths span
12 orders of magnitude from 50 THz for quantum-optical coherence tomography to
50 Hz for certain quantum memories. Combining these technologies requires
coherent interfaces that reversibly map centre frequencies and bandwidths of
photons to avoid excessive loss. Here we demonstrate bandwidth compression of
single photons by a factor 40 and tunability over a range 70 times that
bandwidth via sum-frequency generation with chirped laser pulses. This
constitutes a time-to-frequency interface for light capable of converting
time-bin to colour entanglement and enables ultrafast timing measurements. It
is a step toward arbitrary waveform generation for single and entangled
photons.Comment: 6 pages (4 figures) + 6 pages (3 figures
Signal-Locality and Subquantum Information in Deterministic Hidden-Variables Theories
It is proven that any deterministic hidden-variables theory, that reproduces quantum theory for a 'quantum equilibrium' distribution of hidden variables, must predict the existence of instantaneous signals at the statistical level for hypothetical 'nonequilibrium ensembles'. This 'signal-locality theorem' generalises yet another feature of the pilot-wave theory of de Broglie and Bohm, for which it is already known that signal-locality is true only in equilibrium. Assuming certain symmetries, lower bounds are derived on the 'degree of nonlocality' of the singlet state, defined as the (equilibrium) fraction of outcomes at one wing of an EPR-experiment that change in response to a shift in the distant angular setting. It is shown by explicit calculation that these bounds are satisfied by pilot-wave theory. The degree of nonlocality is interpreted as the average number of bits of 'subquantum information' transmitted superluminally, for an equilibrium ensemble. It is proposed that this quantity might provide a novel measure of the entanglement of a quantum state, and that the field of quantum information would benefit from a more explicit hidden-variables approach. It is argued that the signal-locality theorem supports the hypothesis, made elsewhere, that in the remote past the universe relaxed to a state of statistical equilibrium at the hidden-variable level, a state in which nonlocality happens to be masked by quantum noise
All-depth dispersion cancellation in spectral domain optical coherence tomography using numerical intensity correlations
In ultra-high resolution (UHR-) optical coherence tomography (OCT) group velocity dispersion (GVD) must be corrected for in order to approach the theoretical resolution limit. One approach promises not only compensation, but complete annihilation of even order dispersion effects, and that at all sample depths. This approach has hitherto been demonstrated with an experimentally demanding ‘balanced detection’ configuration based on using two detectors. We demonstrate intensity correlation (IC) OCT using a conventional spectral domain (SD) UHR-OCT system with a single detector. IC-SD-OCT configurations exhibit cross term ghost images and a reduced axial range, half of that of conventional SD-OCT. We demonstrate that both shortcomings can be removed by applying a generic artefact reduction algorithm and using analytic interferograms. We show the superiority of IC-SD-OCT compared to conventional SD-OCT by showing how IC-SD-OCT is able to image spatial structures behind a strongly dispersive silicon wafer. Finally, we question the resolution enhancement of 2–? that IC-SD-OCT is often believed to have compared to SD-OCT. We show that this is simply the effect of squaring the reflectivity profile as a natural result of processing the product of two intensity spectra instead of a single spectrum
Optical coherency matrix tomography
The coherence of an optical beam having multiple degrees of freedom (DoFs) is described by a coherency matrix G spanning these DoFs. This optical coherency matrix has not been measured in its entirety to date—even in the simplest case of two binary DoFs where G is a 4 × 4 matrix. We establish a methodical yet versatile approach—optical coherency matrix tomography—for reconstructing G that exploits the analogy between this problem in classical optics and that of tomographically reconstructing the density matrix associated with multipartite quantum states in quantum information science. Here G is reconstructed from a minimal set of linearly independent measurements, each a cascade of projective measurements for each DoF. We report the first experimental measurements of the 4 × 4 coherency matrix G associated with an electromagnetic beam in which polarization and a spatial DoF are relevant, ranging from the traditional two-point Young’s double slit to spatial parity and orbital angular momentum modes
Bell's measure in classical optical coherence
The statistical description of optical fields in classical coherence theory is the foundation for many applications in metrology, microscopy, lithography and astronomy. Partial coherence is commonly attributed to underlying fluctuations originating at the source or arising upon passage through a random medium. A less acknowledged source of uncertainty (partial coherence) stems from the act of ignoring a degree of freedom of a beam when observing another degree of freedom coupled to (or classically entangled with) it. We demonstrate here that Bell’s measure, which is commonly used in tests of quantum non-locality, may be used as a quantitative tool in classical optical coherence to delineate native incoherence associated with statistical fluctuations from correlation- (or, entanglement-) based incoherence. Our results demonstrate the applicability of the concepts recently developed in quantum information science to classical optical coherence theory and optical signal processing
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