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Conference on Lasers and Electro-Optics

Time-bin encoding for narrow-band single photons

Matthias Scholz, Nils Neubauer, Oliver J. Benson

Published May 1, 2008• 0 citations
Publisher

Abstract

Quantum networks require the reliable encoding of quantum information in narrow-band single photons which couple efficiently to transitions in atomic ensembles. We show time-bin encoding for photons of 10 MHz width via unbalanced Michelson interferometers and show the applicability of our scheme in a quantum key distribution experiment.
DOI
10.1109/cleo.2008.4552127

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Summary form only given. One of today's challenges to realize computing based on quantum mechanics is to reliably and scalably encode information in quantum systems. This is particularly crucial when dealing with promising approaches such as linear optics quantum computing (LOQC) in photonic circuits [1,2]. Although this method is in principle scalable, in practice it is limited by the probabilistic nature of spontaneous parametric down-conversion (SPDC) sources used to seed the circuits.Here, we demonstrate a novel tool to deterministically deliver quantum bits of information for LOQC circuits with the help of single photons emitted from a strongly coupled atom-cavity system (Fig. [1a]). Due to the ≈ 500ns coherence time of these photons [3] and our ability to arbitrarily shape their amplitude and phase profile we time-bin encode the information within one photon. Therefore, the preparation of a single photon sub-divided in d time bins allows for the delivery of arbitrary qu-d-its. RCPWe verify the fidelity of the quantum state preparation with time-resolved quantum-homodyne measurements [4], performed by sending single signal and reference photons into an elementary photonic circuit. Photon correlations monitored in a time-resolved manner then allow the state to be partially reconstructed. To obtain more insight into the time-bin encoding of information within one photon, it is helpful to define a `virtual' photonic circuit (Fig. [1b]) where temporal modes are displayed as spatial ones. Due to the photons being much longer than the detector time resolution, the absolute time of each photo-detection is used to associate the event with one of these virtual detectors. In this way, the respective number of cross-correlation signals for qubits and qutrits, respectively, can be found (Fig. [1c]). We obtain photon production efficiencies up to 85% and state-preparation fidelities of F=96% [4]. Due to the timebin encoding taking place upon the photon's generation process, our technique is inherently non-probabilistic, versatile, reconfigurable and not subject to systematic photon losses. Time-bins as an additional degree of freedom in LOQC experiments and an their deterministic photon-generation scheme can be regarded as a big step towards large-scale quantum computing in photonic networks.
Time-bin encoding of photons offers a robust kind of long-distance quantum communication over lossy channels. The diversity of material nodes in quantum networks operating at natural frequencies requires coherent frequency and wave-form conversion of information-carrying photons to provide an efficient quantum interface with optical fibers and various quantum memories, which has been extensively studied. However, the quantum frequency conversion with transfer of time-bin encoding between single photons of different wavelengths has not yet been explored. In this paper, we present a method for efficiently exchanging time-bin encoding in the conversion process between two photons propagating in cold tripod atoms driven by a strong laser field. The latter is designed to slow down photons and suppress their absorption due to electromagnetically induced transparency. Photons interact parametrically through modified atomic coherence, which is utilized also to achieve equal group velocities of photons. We demonstrate the ability of our model to generate entanglement between distant atoms that equally share the original quantum information stored in the ground-state polarization qubits of both atoms. The proposed method for frequency conversion based on modified atomic coherence is promising because it can simplify the implementation of reliable and high-speed quantum communication protocols based on single-photon entanglement.
In this study, we demonstrate the possibility of realizing the quantum key distribution system by temporal coding of time-bin qubits that are prepared and measured using unbalanced Mach-Zehnder interferometers. We also show that, to exclude a clear a priori identification of time-bin qubits with equal bases, their encoding with the symbols 0 and 1 can be accomplished by delaying one of them by half the clock interval. The uniqueness of measurements of qutrits in this algorithm is shown to be based on the ability of quantum particles in β-states to interfere with the probability amplitudes at the ports of the system interferometers, as well as the absence of collisions of signaling states of the qutrits in adjacent clock intervals. We describe a method for controlling such collisions, including those that are based on qubits prepared from quasi-one-photon coherent laser states.
Quantum key distribution (QKD) with deterministic single photon sources has been demonstrated over intercity fiber and free-space channels. The previous implementations relied mainly on polarization encoding schemes, which are susceptible to birefringence, polarization-mode dispersion and polarization-dependent loss in practical fiber networks. In contrast, time-bin encoding offers inherent robustness and has been widely adopted in mature QKD systems using weak coherent laser pulses. However, its feasibility in conjunction with a deterministic single-photon source has not yet been experimentally demonstrated. In this work, we construct a time-bin encoded QKD system employing a high-brightness quantum dot (QD) single-photon source operating at telecom wavelength. Our proof-of-concept experiment successfully demonstrates the possibility of secure key distribution over fiber link of 120 km, while maintaining extraordinary long-term stability over 6 h of continuous operation, that is highest secure key rate among the time-bin QKDs based on single-photon sources. This work provides the first experimental validation of integrating a QD single-photon source with time-bin encoding in a telecom-band QKD system. This development signifies a substantial advancement in the establishment of a robust and scalable QKD network based on solid-state single-photon technology.
Quantum key distribution (QKD) with deterministic single photon sources has been demonstrated over intercity fiber and free-space channels. The previous implementations relied mainly on polarization encoding schemes, which are susceptible to birefringence, polarization-mode dispersion and polarization-dependent loss in practical fiber networks. In contrast, time-bin encoding offers inherent robustness and has been widely adopted in mature QKD systems using weak coherent laser pulses. However, its feasibility in conjunction with a deterministic single-photon source has not yet been experimentally demonstrated. In this work, we construct a time-bin encoded QKD system employing a high-brightness quantum dot (QD) single-photon source operating at telecom wavelength. Our proof-of-concept experiment successfully demonstrates the possibility of secure key distribution over fiber link of 120 km, while maintaining extraordinary long-term stability over 6 h of continuous operation, that is highest secure key rate among the time-bin QKDs based on single-photon sources. This work provides the first experimental validation of integrating a QD single-photon source with time-bin encoding in a telecom-band QKD system. This development signifies a substantial advancement in the establishment of a robust and scalable QKD network based on solid-state single-photon technology.
Faithful transmission of quantum information over noisy channel is an essential requisite for quantum communication and distributed quantum computing. Here we experimentally demonstrate a noise-rejecting scheme for single-photon qubits transmission, resorting to neither ancillary photons nor globally entangled resources. Employing the unbalanced polarization interferometer, the polarization qubits are transformed to time-bin degrees of freedom. Meanwhile, the deterministic transmission of quantum states with high fidelity can be achieved. We find that the immunity of time-bin encoded qubits against collective noise paves the way to practical quantum error-correction and rejection in long-distance quantum communication.