Baris I. Erkmen

100 Tbit/s free-space data link enabled by three-dimensional multiplexing of orbital angular momentum, polarization, and wavelength

We investigate the orthogonality of orbital angular momentum (OAM) with other multiplexing domains and present a free-space data link that uniquely combines OAM-, polarization-, and wavelength-division multiplexing. Specifically, we demonstrate the multiplexing/demultiplexing of 1008 data channels carried on 12 OAM beams, 2 polarizations, and 42 wavelengths. Each channel is encoded with 100 Gbit/s quadrature phase-shift keying data, providing an aggregate capacity of 100.8 Tbit/s (12×2×42×100 Gbit/s).

Ghost imaging: from quantum to classical to computational

Ghost-imaging experiments correlate the outputs from two photodetectors: a high-spatial-resolution (scanning pinhole or CCD array) detector that measures a field that has not interacted with the object to be imaged, and a bucket (single-pixel) detector that collects a field that has interacted with the object. We give a comprehensive review of ghost imaging—within a unified Gaussian-state framework—presenting detailed analyses of its resolution, field of view, image contrast, and signal-to-noise ratio behavior. We consider three classes of illumination: thermal-state (classical), biphoton-state (quantum), and classical-state phase-sensitive light. The first two have been employed in a variety of ghost-imaging demonstrations. The third is the classical Gaussian state that produces ghost images that most closely mimic those obtained from biphoton illumination. The insights we develop lead naturally to a new, single-beam approach to ghost imaging, called computational ghost imaging, in which only the bucket detector is required. We provide quantitative results while simultaneously emphasizing the underlying physics of ghost imaging. The key to developing the latter understanding lies in the coherence behavior of a pair of Gaussian-state light beams with either phase-insensitive or phase-sensitive cross correlation.

Unified theory of ghost imaging with Gaussian-state light

The theory of ghost imaging is developed in a Gaussian-state framework that both encompasses prior work---on thermal-state and biphoton-state imagers---and provides a complete understanding of the boundary between classical and quantum behavior in such systems. The core of this analysis is the expression derived for the photocurrent-correlation image obtained using a general Gaussian-state source. This image is expressed in terms of the phase-insensitive and phase-sensitive cross correlations between the two detected fields, plus a background. Because any pair of cross correlations is obtainable with classical Gaussian states, the image does not carry a quantum signature per se. However, if the image characteristics of classical and nonclassical Gaussian-state sources with identical autocorrelation functions are compared, the nonclassical source provides resolution improvement in its near field and field-of-view improvement in its far field.

Computational ghost imaging for remote sensing.

Computational ghost imaging is a structured-illumination active imager coupled with a single-pixel detector that has potential applications in remote sensing. Here we report on an architecture that acquires the two-dimensional spatial Fourier transform of the target object (which can be inverted to obtain a conventional image). We determine its image signature, resolution, and signal-to-noise ratio in the presence of practical constraints such as atmospheric turbulence, background radiation, and photodetector noise. We consider a bistatic imaging geometry and quantify the resolution impact of nonuniform Kolmogorov-spectrum turbulence along the propagation paths. We show that, in some cases, short-exposure intensity averaging can mitigate atmospheric-turbulence-induced resolution loss. Our analysis reveals some key performance differences between computational ghost imaging and conventional active imaging, and identifies scenarios in which theory predicts that the former will perform better than the latter.

Classical capacity of bosonic broadcast communication and a minimum output entropy conjecture

Previous work on the classical information capacities of bosonic channels has established the capacity of the single-user pure-loss channel, bounded the capacity of the single-user thermal-noise channel, and bounded the capacity region of the multiple-access channel. The latter is a multiple-user scenario in which several transmitters seek to simultaneously and independently communicate to a single receiver. We study the capacity region of the bosonic broadcast channel, in which a single transmitter seeks to simultaneously and independently communicate to two different receivers. It is known that the tightest available lower bound on the capacity of the single-user thermal-noise channel is that channels capacity if, as conjectured, the minimum von Neumann entropy at the output of a bosonic channel with additive thermal noise occurs for coherent-state inputs. Evidence in support of this minimum output entropy conjecture has been accumulated, but a rigorous proof has not been obtained. We propose a minimum output entropy conjecture that, if proved to be correct, will establish that the capacity region of the bosonic broadcast channel equals the inner bound achieved using a coherent-state encoding and optimum detection. We provide some evidence that supports this conjecture, but again a full proof is not available.

Ultimate channel capacity of free-space optical communications (Invited)

Feature Issue on Optical Wireless Communications (OWC) The ultimate classical information capacity of multiple-spatial-mode, wideband optical communications in vacuum between soft-aperture transmit and receive pupils is considered. The ultimate capacity is shown to be achieved by coherent-state encoding and joint measurements over entire code words. This capacity is compared with the capacities realized with the same encoding and homodyne or heterodyne detection, which are single-channel-use measurements. Realistic background spectral-radiance values are used to obtain tight bounds on the capacity of single-spatial-mode, narrowband 1.55 μm wavelength free-space communications in the presence of background light.

Gaussian-state quantum-illumination receivers for target detection

The signal half of an entangled twin beam, generated using spontaneous parametric downconversion, interrogates a region of space that is suspected of containing a target and has high loss and high (entanglement-breaking) background noise. A joint measurement is performed on the returned light and the idler beam that was retained at the transmitter. An optimal quantum receiver, whose implementation is not yet known, was shown to achieve 6 dB gain in the error-probability exponent relative to that achieved with a single coherent-state (classical) laser transmitter and the optimum receiver. We present two structured optical receivers that achieve up to 3 dB gain in the error exponent over that attained with the classical sensor. These are designs of quantum-optical sensors for target detection, which can be readily implemented in a proof-of-concept experiment, that appreciably outperform the best classical sensor in the low-signal-brightness, high-loss, and high-noise operating regime.

Capacity of the bosonic wiretap channel and the Entropy Photon-Number Inequality

Determining the ultimate classical information carrying capacity of electromagnetic waves requires quantum-mechanical analysis to properly account for the bosonic nature of these waves. Recent work has established capacity theorems for bosonic single-user and broadcast channels, under the presumption of two minimum output entropy conjectures. Despite considerable accumulated evidence that supports the validity of these conjectures, they have yet to be proven. In this paper, it is shown that the second conjecture suffices to prove the classical capacity of the bosonic wiretap channel, which in turn would also prove the quantum capacity of the lossy bosonic channel. The preceding minimum output entropy conjectures are then shown to be simple consequences of an entropy photon-number inequality (EPnl), which is a conjectured quantum-mechanical analog of the entropy power inequality (EPI) from classical information theory.

The Entropy Photon-Number Inequality and its consequences

Determining the ultimate classical information carrying capacity of electromagnetic waves requires quantum-mechanical analysis to properly account for the bosonic nature of these waves. Recent work has established capacity theorems for bosonic single-user, broadcast, and wiretap channels, under the presumption of two minimum output entropy conjectures. Despite considerable accumulated evidence that supports the validity of these conjectures, they have yet to be proven. Here we show that the preceding minimum output entropy conjectures are simple consequences of an entropy photon-number inequality, which is a conjectured quantum-mechanical analog of the entropy power inequality from classical information theory.

Optical link design and validation testing of the Optical Payload for Lasercomm Science (OPALS) system

The Optical Payload for Lasercomm Science (OPALS) system developed by the Jet Propulsion Laboratory, California Institute of Technology, will be used for optical telecommunications link experiments from the International Space Station (ISS) to a ground telescope located at Table Mountain, CA. The launch of the flight terminal is scheduled for late February 2014 with an initially planned 90-day operations period following deployment on the exterior of the ISS. The simple, low-cost OPALS system will downlink a pre-encoded video file at 50 Mb/s on a 1550 nm laser carrier using on-off key (OOK) modulation and Reed-Solomon forward error correction. A continuous wave (cw) 976 nm multibeam laser beacon transmitted from the ground to the ISS will initiate link acquisition and tracking by the flight subsystem. Link analysis along with pre-flight results of the end-to-end free-space testing of the OPALS link are presented.