Timothy M. Yarnall

Experimental Violation of Bell's Inequality in Spatial-Parity Space

We report the first experimental violation of Bells inequality in the spatial domain using the Einstein-Podolsky-Rosen state. Two-photon states generated via optical spontaneous parametric down-conversion are shown to be entangled in the parity of their one-dimensional transverse spatial profile. Superpositions of Bell states are prepared by manipulation of the optical pumps transverse spatial parity-a classical parameter. The Bell-operator measurements are made possible by devising simple optical arrangements that perform rotations in the one-dimensional spatial-parity space of each photon of an entangled pair and projective measurements onto a basis of even-odd functions. A Bell-operator value of 2.389+/-0.016 is recorded, a violation of the inequality by more than 24 standard deviations.

Synthesis and Analysis of Entangled Photonic Qubits in Spatial-Parity Space

We present the novel embodiment of a photonic qubit that makes use of one continuous spatial degree of freedom of a single photon and relies on the parity of the photons transverse spatial distribution. Using optical spontaneous parametric down-conversion to produce photon pairs, we demonstrate the controlled generation of entangled-photon states in this new space. Specifically, two Bell states, and a continuum of their superpositions, are generated by simple manipulation of a classical parameter, the optical-pump spatial parity, and not by manipulation of the entangled photons themselves. An interferometric device, isomorphic in action to a polarizing beam splitter, projects the spatial-parity states onto an even-odd basis. This new physical realization of photonic qubits could be used as a foundation for future experiments in quantum information processing.

Angular and radial mode analyzer for optical beams

We describe an approach to determining both the angular and the radial modal content of a scalar optical beam in terms of optical angular momentum modes. A modified Mach-Zehnder interferometer that incorporates a spatial rotator to determine the angular modes and an optical realization of the fractional Hankel transform (fHT) to determine the radial modes is analyzed. Varying the rotation angle and the order of the fHT produces a two-dimensional (2D) interferogram from which we extract the modal coefficients by simple 2D Fourier analysis.

Generalized optical interferometry for modal analysis in arbitrary degrees of freedom

We generalize the traditional concept of temporal optical interferometry to any degree of freedom of a coherent optical field. By identifying the structure of a unitary optical transformation that we designate the generalized phase operator, we enable optical interferometry to be carried out in any modal basis describing a degree of freedom. The structure of the generalized phase operator is that of a fractional optical transform, thus establishing the connection between fractional transforms, optical interferometry, and modal analysis.

Demonstration of 2.1 photon-per-bit sensitivity for BPSK at 9.94-Gb/s with rate-½ FEC

Combining optical-phase-locked loop based coherent detection, interleaving, and powerful rate-½ FEC enabled the error-free transmission of BPSK waveforms at information rates of 9.94-Gb/s and 19.88-Gb/s with sensitivities of 2.1 photons-per-bit and 3.9 photons-per-bit, respectively.

Multi-aperture digital coherent combining for free-space optical communication receivers

Space-to-ground optical communication systems can benefit from reducing the size, weight, and power profiles of space terminals. One way of reducing the required power-aperture product on a space platform is to implement effective, but costly, single-aperture ground terminals with large collection areas. In contrast, we present a ground terminal receiver architecture in which many small less-expensive apertures are efficiently combined to create a large effective aperture while maintaining excellent receiver sensitivity. This is accomplished via coherent detection behind each aperture followed by digitization. The digitized signals are then combined in a digital signal processing chain. Experimental results demonstrate lossless coherent combining of four lasercom signals, at power levels below 0.1 photons/bit/aperture.

Phase-unlocked Hong-Ou-Mandel interferometry

There is a fundamental dimensional mismatch between the Hong-Ou-Mandel (HOM) interferometer and two-photon (2P) states: while the latter are represented using two temporal (or spectral) dimensions, the HOM interferometer allows access to only one temporal dimension. We introduce a linear 2P interferometer containing two independent delays spanning the 2P state. By “unlocking” the fixed phase relationship between the interfering 2P probability amplitudes in a HOM interferometer, one of these probability amplitudes now serves as a delay-free 2P reference against which the other beats, thereby resolving ambiguities in 2P state identification typical of HOM interferometry and extending its utility to a large family of 2P states.

Spatial coherence effects on second- and fourth-order temporal interference

We report the results of two experiments performed with two-photon light, produced via collinear degenerate optical spontaneous parametric downconversion (SPDC), in which both second-order (one-photon) and fourth-order (two-photon) interferograms are recorded in a Mach-Zehnder interferometer (MZI). In the first experiment, high-visibility fringes are obtained for both the second- and fourth-order interferograms. In the second experiment, the MZI is modified by the removal of a mirror from one of its arms; this leaves the fourth-order interferogram unchanged, but extinguishes the second-order interferogram. A theoretical model that takes into consideration both the temporal and spatial degrees-of-freedom of the two-photon state successfully explains the results. While the temporal interference in the MZI is independent of the spatial coherence of the source, that of the modified MZI is not.

A narrow-beam undersea laser communications field demonstration

We report a demonstration of narrow-beam laser communication through the waters of Narragansett Bay in Rhode Island, USA. The transmitter and receiver were mounted on an aluminum truss and placed in the water alongside a pier operated by the Naval Undersea Warfare Center. The transmitter consisted of a real-time modulator and encoder, a 515 nm wavelength commercial laser, collimating optics, and a steering mirror. The receiver included a steering mirror, a focal plane camera, a linear-mode avalanche photo-diode (APD), a photo-multiplier tube (PMT) single photon detector, a large area imaging camera, an iris to vary the field of view, optics to split the beam between the various detectors, and field-programmable gate array (FPGA) electronics for real-time demodulation and decoding. The PMT and APD detectors were used for communications demonstrations; the imaging and focal plane cameras were used for channel characterization measurements and system alignment. Communications and characterization data were collected through a variety of conditions over the five day field experiment, including day and night, calm and high winds, and flood and ebb tide. In the experiment, the transmit power, receiver field of view, and link distance were varied. The water transmissivity and volume scattering function were measured throughout the experiment to calibrate the results. Real-time communications demonstrations with the PMT were carried out between 1 megabit-per-second (Mbps) and 8.7 Mbps at 7.8 meters, which represented between 8 and 12 beam extinction lengths. With the APD, 125 Mbps were demonstrated at 4.8 meters, representing approximately 5 extinction lengths.

Turbid-harbor demonstration of transceiver technologies for wide dynamic range undersea laser communications

Undersea laser communications represent a promising area of research with a large set of applications. Wide dynamic range receivers are necessary to operate through a range of possible water qualities and link distances. In the signal-starved regime, photon-counting photomultiplier tubes (PMTs) are a key technology for high-sensitivity communications. When more signal is available, linear avalanche photodiodes (APDs) provide an opportunity for higher-rate communication. We have designed a receiver terminal employing both kinds of detectors to show robust operation over nearly two orders of magnitude in power and data rate. An optical link including this receiver terminal was submerged in Narragansett Bay, RI to demonstrate underwater optical communication over several days. The PMT receiver demonstrated robust, error-free performance over channel rates from 1.302 Mbaud to 10.416 Mbaud for received optical power levels ranging from -84.1 dBm to -75.3 dBm. The PMT link demonstrated an error-free user rate of 8.68 Mb/s. This corresponded to nearly-ideal detector efficiency on the order of one detected photon per bit. The PMT receiver was contained entirely within the submerged enclosure and demonstrated full real-time decoding, including strong forward error correction. A low-power transmitter was used to demonstrate a link with loss equivalent to 18 extinction lengths. With moderately-powered transmitters, this distance could be extended to 22.4 extinction lengths. The PMT receiver was capable of operating at near-theoretical limits during the day and night. Its multi-rate operation demonstrated the capability of trading sensitivity for data rate efficiently. With the same low-power transmitter, the APD receiver achieved a bit error rate less than 1×10-9 at 125 Mbaud. Furthermore, it achieved an error rate correctable by forward error correction for a link with loss equivalent to 9 extinction lengths.