Jonathan L. Habif

Quantum key distribution at 1550 nm with twin superconducting single-photon detectors

The authors report on the full implementation of a superconducting detector technology in a fiber-based quantum key distribution (QKD) link. Nanowire-based superconducting single-photon detectors (SSPDs) offer infrared single-photon detection with low dark counts, low jitter, and short recovery times. These detectors are highly promising candidates for future high key rate QKD links operating at 1550nm. The authors use twin SSPDs to perform the BB84 protocol in a 1550nm fiber-based QKD link clocked at 3.3MHz. They exchange secure key over a distance of 42.5km in telecom fiber and demonstrate that secure key can be transmitted over a total link loss exceeding 12dB.

Optical codeword demodulation with error rates below the standard quantum limit using a conditional nulling receiver

Researchers experimentally demonstrate the first joint-detection receiver capable of performing a joint measurement over pulse-position-modulation codewords. This result — the largest improvement over the standard quantum limit reported to date — is accomplished by using a conditional nulling receiver, which uses optimized-amplitude coherent pulse nulling, single-photon detection and quantum feedforward.

Heralding of telecommunication photon pairs with a superconducting single photon detector

Experiments involving entangled photon pairs created via spontaneous parametric down conversion typically use wavelengths in the visible regime. The extension of a photonic quantum information link to a fiber optical network requires that entangled pairs be created at telecommunication wavelengths (1550nm), for which photon counting detector technology is inferior to visible detection, in particular, low coincidence detection rates of correlated-photon pairs. We demonstrate a correlated-photon pair measurement using the superconducting single photon detector in a heralding scheme that can be used to substantially improve the correlated-photon detection rate.

Quantum-secure covert communication on bosonic channels

Computational encryption, information-theoretic secrecy and quantum cryptography offer progressively stronger security against unauthorized decoding of messages contained in communication transmissions. However, these approaches do not ensure stealth—that the mere presence of message-bearing transmissions be undetectable. We characterize the ultimate limit of how much data can be reliably and covertly communicated over the lossy thermal-noise bosonic channel (which models various practical communication channels). We show that whenever there is some channel noise that cannot in principle be controlled by an otherwise arbitrarily powerful adversary—for example, thermal noise from blackbody radiation—the number of reliably transmissible covert bits is at most proportional to the square root of the number of orthogonal modes (the time-bandwidth product) available in the transmission interval. We demonstrate this in a proof-of-principle experiment. Our result paves the way to realizing communications that are kept covert from an all-powerful quantum adversary.Boulat A. Bash, Andrei H. Gheorghe, Monika Patel, Jonathan L. Habif, Dennis Goeckel, Don Towsley, and Saikat Guha School of Computer Science, University of Massachusetts, Amherst, Massachusetts, USA 01003, Quantum Information Processing Group, Raytheon BBN Technologies, Cambridge, Massachusetts, USA 02138, Amherst College, Amherst, Massachusetts, USA 01002, Electrical and Computer Engineering Department, University of Massachusetts, Amherst, Massachusetts, USA 01003 ∗

Approaching Helstrom limits to optical pulse-position demodulation using single photon detection and optical feedback

Optical pulse-position modulation (PPM) is one of the primary modulation formats being investigated for use in deep-space communications, as well as terrestrial fiber optical communications. We consider the problem of demodulating M-ary optical PPM waveforms, and propose a structured receiver whose mean probability of symbol error is smaller than all known receivers, and approaches the Helstrom (quantum) limit of the minimum probability of error (MPE) of discriminating between the coherent-state PPM signals. The receiver uses photodetection coupled with optimized phase-coherent optical feedback control through the PPM pulse slots and a phase-sensitive parametric amplifier. We present a general framework of optical receivers known as the ‘conditional pulse nulling receiver’, and present new results on ultimate limits and achievable regions of the trade-off space between the spectral versus photon efficiency of PPM, for the single-spatial-mode far-field pure-loss optical communication channel.We consider the problem of demodulating M-ary optical PPM (pulse-position modulation) waveforms, and propose a structured receiver whose mean probability of symbol error is smaller than all known receivers, and approaches the quantum limit. The receiver uses photodetection coupled with optimized phase-coherent optical feedback control and a phase-sensitive parametric amplifier. We present a general framework of optical receivers known as the conditional pulse nulling receiver, and present new results on ultimate limits and achievable regions of spectral versus photon efficiency tradeoffs for the single-spatial-mode pure-loss optical communication channel.

Quantum key distribution with high-speed superconducting single-photon detectors

We explore the potential of high-speed nanowire superconducting single-photon detectors for quantum key distribution in fiber, over long distances (at 1550 nm) and at high bit rates (at 850 nm).

PPM demodulation: On approaching fundamental limits of optical communications

We consider the problem of demodulating M-ary optical PPM (pulse-position modulation) waveforms, and propose a structured receiver whose mean probability of symbol error is smaller than all known receivers, and approaches the quantum limit. The receiver uses photodetection coupled with optimized phase-coherent optical feedback control and a phase-sensitive parametric amplifier. We present a general framework of optical receivers known as the conditional pulse nulling receiver, and present new results on ultimate limits and achievable regions of spectral versus photon efficiency tradeoffs for the single-spatial-mode pure-loss optical communication channel.

Single photon detector comparison in a quantum key distribution test

We provide a direct comparison between the InGaAs avalanche photodiode (APD) and the NbN superconducting single photon detector (SSPD) for applications in fiber-based quantum cryptography. The quantum efficiency and dark count rate were measured for each detector, and used to calculate the quantum bit error rate (QBER) and shared key rate for a QKD link. The results indicate that, despite low quantum efficiency, the speed of the SSPD makes it a superior detector for quantum information applications. Finally, we present results of an initial integration of an SSPD into a receiver node of the DARPA quantum network to perform quantum key distribution.

Polar coded optical communications with weak coherent states

We present results from an optical communications testbed demonstrating polar coded pulse position modulation transmitted to a direct detection receiver. Using weak coherent states we achieve a photon information efficiency of 4.8 bits per received photon.