Chuang Liang

Generation of high-purity telecom-band entangled photon pairs in dispersion-shifted fiber

We generate entangled photon-pairs from dispersion-shifted fiber at various temperatures. Two-photon interference with > 98% visibility and Bellpsilas inequality violation by > 8 standard deviations are observed at 77 K, without subtracting background Raman photons.

Quantum-noise randomized data encryption for wavelength-division-multiplexed fiber-optic networks

We demonstrate high-rate randomized data-encryption through optical fibers using the inherent quantum-measurement noise of coherent states of light. Specifically, we demonstrate 650 Mbit/s data encryption through a 10 Gbit/s data-bearing, in-line amplified 200-km-long line. In our protocol, legitimate users (who share a short secret key) communicate using an M-ry signal set while an attacker (who does not share the secret key) is forced to contend with the fundamental and irreducible quantum-measurement noise of coherent states. Implementations of our protocol using both polarization-encoded signal sets as well as polarization-insensitive phase-keyed signal sets are experimentally and theoretically evaluated. Different from the performance criteria for the cryptographic objective of key generation (quantum key-generation), one possible set of performance criteria for the cryptographic objective of data encryption is established and carefully considered.

Fiber-based telecom-band degenerate-frequency source of entangled photon pairs

We demonstrate the generation of polarization-entangled photon pairs of degenerate frequency for the first time, to the best of our knowledge, in standard optical fiber using a novel dual-pump, counterpropagating configuration. Two-photon interference with >97% visibility is obtained. The purity of the photon source, as characterized by the ratio of coincidence to accidental-coincidence counts, is shown to be as high as 116 under suitable operating conditions.

Characterization of fiber-generated entangled photon pairs with superconducting single-photon detectors

We demonstrate the suitability of fiber-generated entangled photon pairs for practical quantum communications in the telecom band by measuring their properties with superconducting single-photon detectors that produce negligible dark counts. The photon pairs are created in approximately 5-ps duration windows at 50 MHz rate while the detectors are operated in ungated free running mode. We obtain a coincidence to accidental-coincidence ratio >80 with raw photon-counting data, i.e., without making any post-measurement corrections. Using a previously demonstrated counter-propagating scheme we also produce polarization-entangled photon pairs at 50-MHz rate, which in coincidence detection directly yield two-photon interference with a fringe visibility >98%.

Quantum noise protected data encryption in a WDM network

We demonstrate fully streaming optical data encryption over a 250-km-long fiber-optic wavelength-division-multiplexing (WDM) line at OC-12 data rate. Our scheme, based on the fundamental and irreducible quantum noise of laser light, allows for optical amplification and is shown to be compatible with todays high-speed WDM telecommunications infrastructure.

High-speed data encryption over 25 km of fiber by two-mode coherent-state quantum cryptography

We demonstrate high-speed (250 Mbps) data encryption over 25 km of telecommunication fiber by use of coherent states. For the parameter values used in the experiment, the demonstration is secure against individual ciphertext-only eavesdropping attacks near the transmitter with ideal detection equipment. Whereas other quantum-cryptographic schemes require the use of fragile quantum states and ultrasensitive detection equipment, our protocol is loss tolerant, uses off-the-shelf components, and is optically amplifiable.

Ultra stable all-fiber telecom-band entangled photon-pair source for turnkey quantum communication applications

We demonstrate a novel alignment-free all-fiber source for generating telecom-band polarization-entangled photon pairs. Polarization entanglement is created by injecting two relatively delayed, orthogonally polarized pump pulses into a piece of dispersion-shifted fiber, where each one independently engages in four-photon scattering, and then removing any distinguishability between the correlated photon-pairs produced by each pulse at the fiber output. Our scheme uses a Michelson-interferometer configuration with Faraday mirrors to achieve practically desirable features such as ultra-stable performance and turnkey operation. Up to 91.7% two-photon-interference visibility is observed without subtracting the accidental coincidences that arise from background photons while operating the source at room temperature.

Distribution of Fiber-Generated Polarization Entangled Photon-Pairs over 100 km of Standard Fiber in OC-192 WDM Environment

We demonstrate distribution of polarization entanglement over 100 km in a WDM environment. Two-photon interference with visibility >90% is observed in coincidence detection of photon-pairs that have traveled with classical data signals, one on adjacent channel.

High data rate quantum noise protected encryption over long distances

We report on a new physical layer optical encryption approach based on quantum noise limited optical signals and M-ary optical phase shift keying that operates at high data rates. In contrast to established encryption methods that rely solely on deterministic algorithms, this system utilizes quantum noise to realize a randomized cipher. Keyed M-ary optical phase modulation is used to encrypt quantum-noise limited mesoscopic signals (50k photons/bit) that are compatible with current directions in optical networking: the physically encrypted signals may be optically amplified, routed through optical switches, and can propagate over long distances approaching 1000 km. We describe the approach in detail and report on results of experiments in which these encrypted signals were transmitted over an 850 km network at 622 Mb/s. Bit-error rate measurements were performed under varying network conditions. Our paper concludes with the engineering challenges for extending this approach to 2.5 Gb/s and beyond and how these are being addressed

Exploiting quantum and classical noises for securing high-speed optical communication networks

We will describe keyed communication in quantum noise (KCQ) and how it can be used for either data encryption or key generation. Specifically, we will focus on the AlphaEta protocol for data encryption where the role of quantum noise will be discussed. Additionally, the potential of using classical noise to enhance security via deliberate signal randomization (DSR) will be investigated. We will also investigate the effect of unwanted impairments, such as nonlinearities in a wavelength-division-multiplexed fiber transmission system, and how they affect the ultimate allowable propagation distance. Our simulations and experiments suggest that AlphaEta-protocol based physical-layer encryption is compatible with long-haul optical transmission systems operating at Gb/s data rates.