Abstract Distributing entangled photon pairs over noisy channels is an important task for various quantum information protocols. Encoding an entangled state in a decoherence-free subspace (DFS) formed by multiple photons is a promising way to circumvent the phase fluctuations and polarisation rotations in optical fibres. Recently, it has been shown that the use of a counter-propagating coherent light as an ancillary photon enables us to faithfully distribute entangled photon with success probability proportional to the transmittance of the optical fibres. Several proof-of-principle experiments have been demonstrated, in which entangled photon pairs from a sender side and the ancillary photon from a receiver side originate from the same laser source. In addition, bulk optics have been used to mimic the noises in optical fibres. Here, we demonstrate a DFS-based entanglement distribution over 1 km optical fibre using DFS formed by using fully independent light sources at the telecom band, and obtain a high-fidelity entangled state. This shows that the DFS-based scheme protects the entanglement against collective noise in 1 km optical fibre. In the experiment, we utilise an interference between asynchronous photons from continuous wave pumped spontaneous parametric down conversion (SPDC) and mode-locked coherent light pulse. After performing spectral and temporal filtering, the SPDC photons and light pulse are spectrally indistinguishable. This property allows us to observe high-visibility interference without performing active synchronisation between fully independent sources.
Abstract Distributing entangled photon pairs over noisy channels is an important task for various quantum information protocols. Encoding an entangled state in a decoherence-free subspace (DFS) formed by multiple photons is a promising way to circumvent the phase fluctuations and polarisation rotations in optical fibres. Recently, it has been shown that the use of a counter-propagating coherent light as an ancillary photon enables us to faithfully distribute entangled photon with success probability proportional to the transmittance of the optical fibres. Several proof-of-principle experiments have been demonstrated, in which entangled photon pairs from a sender side and the ancillary photon from a receiver side originate from the same laser source. In addition, bulk optics have been used to mimic the noises in optical fibres. Here, we demonstrate a DFS-based entanglement distribution over 1 km optical fibre using DFS formed by using fully independent light sources at the telecom band, and obtain a high-fidelity entangled state. This shows that the DFS-based scheme protects the entanglement against collective noise in 1 km optical fibre. In the experiment, we utilise an interference between asynchronous photons from continuous wave pumped spontaneous parametric down conversion (SPDC) and mode-locked coherent light pulse. After performing spectral and temporal filtering, the SPDC photons and light pulse are spectrally indistinguishable. This property allows us to observe high-visibility interference without performing active synchronisation between fully independent sources.
Embedding a quantum state in a decoherence-free subspace (DFS) formed by multiple photons is one of the promising methods for robust entanglement distribution of photonic states over collective noisy channels. In practice, however, such a scheme suffers from a low efficiency proportional to transmittance of the channel to the power of the number of photons forming the DFS. The use of a counter-propagating coherent pulse can improve the efficiency to scale linearly in the channel transmission, but it achieves only protection against phase noises. Recently, it was theoretically proposed [Phys. Rev. A 87, 052325(2013)] that the protection against bit-flip noises can also be achieved if the channel has a reciprocal property. Here we experimentally demonstrate the proposed scheme to distribute polarization-entangled photon pairs against a general collective noise including the bit flip noise and the phase noise. We observed an efficient sharing rate scaling while keeping a high quality of the distributed entangled state. Furthermore, we show that the method is applicable not only to the entanglement distribution but also to the transmission of arbitrary polarization states of a single photon.
Recent advances in guided-wave optics bring quantum key distribution (QKD) to a new level where less complex, yet more robust, schemes have been proposed. Through continuous research in this field, newer QKD systems achieve higher bit rate, distance, and reliability. Entanglement-based QKD receives enormous attention from the scientific community, in order to increase the communication distance and to go to device-independent systems. In this framework, photons are generally the preferred qubit carrier, taking advantage of low-loss fibre optical networks, and high-performance standard telecom components. Future QKD networks will demand sources exhibiting high brightness, reliability, and photons in the standard telecom ITU channels for compatibility and easy integration with current technology. One of the most exploited approaches towards genera- ting photonic entanglement is based on the process of spontaneous parametric down-conversion (SPDC) in non-linear crystals. Usually employed entanglement observables are energy-time and polarization. The latter one is, without doubt, the easiest to analyze thanks to interferometer-free setups. Polarization entanglement is conveniently generated via the so-called type-II SPDC interaction, possibly in a waveguide non-linear structure for increasing the efficiency. Nevertheless, with a type-II source, there are some precautions that must be taken for achieving high entanglement qualities. In particular, the generated paired photons need to be spectrally and temporally indistinguishable. Typically, this requires spectral filtering, and temporal walk-off compensation stages. So far, these issues have been addressed by using bulk optics arrangements. We demonstrate a fully fibred approach based on a type-II periodically poled lithium niobate waveguide (PPLN/W) generating paired photons at 1540 nm (telecom C-band wavelength). We highlight the use of a standard polarization maintaining fibre (PMF) as an essentially loss-free, guided-wave solution to the temporal walk-off problem. By using standard telecom DWDMs, we perform, at the same time, the tasks of spectral filtering and deterministic photon pair separation in two ITU channels. By the violation of the Bell's inequalities with more than 100 standard deviations, we demonstrate excellent entanglement qualities and high coincidence rates. Our fully fibred approach offers very low losses, high reliability, and would be ready for QKD field tests.
We study distribution schemes for a polarization entangled photon pair based on a decoherence-free subspace over lossy quantum channels and propose an efficient scheme that is robust against not only collective phase noises but also general collective noises for two qubits. While the proposed scheme employs two photons to build the decoherence-free subspace, the success probability is proportional to the channel transmittance of a single photon with the aid of a counter-propagating weak coherent light pulse. The key ingredient in the scheme is found to be the reciprocity of the channel. The proposed scheme shares the rest of the properties with the previously proposed schemes; i.e., it can be realized by linear optical elements and it is robust against the fluctuations in the optical circuits used by the two parties.
Most quantum communication protocols rely on the distribution of entangled photon pairs. If the transmission channel is an optical fiber both photons should have a wavelength around 1550nm. We present a source of polarisation entanglement based on spontaneous parametric down-conversion (SPDC) at telecom wavelengths. Using fiber-pigtailed ppLN crystals we can profit from the clean χ2SPDC process, as opposed to four-wave mixing techniques, and also from a stable and efficient coupling to single mode fibers. Using a cw-pump, a tight energy correlation between the entangled photons is achieved. This fact is used here to distribute the entanglement through multiple channels employing only passive wavelength multiplexing. We demonstrate a possible application scenario with a single central source distributing entanglement to four users simultaneously. The source can also be used for creating tri-partite entanglement using the recently demonstrated cascaded SPDC scheme.
Quantum cryptography with entangled photon pairs can be more powerful than protocols based onsingle photons or weak coherent pulses: they can tolerate higher losses and thus allow thedistribution of quantum secret keys (QKD) over longer distances [1], they also provide a waytowards device-independent quantum cryptography [2]. However, in order to enable a wide use ofentangled photon pairs in future quantum telecommunication systems, further developments areneeded to demonstrate performant sources that can be easily fabricated and integrated into Telecomfiber networks.Here we present a source consisting of an aluminium gallium arsenide waveguide generatingphoton pairs in the Telecom band by type II spontaneous parametric down-conversion [3]. Such adevice has already been proven to work under electrical pumping [4]. Thanks to the very smallbirefringence of the guided modes, the pairs are directly generated in a polarization-entangled Bellstate, without the need for any post-compensation. Moreover, as the photons are emitted over alarge bandwidth (about 100 nm) with a joint spectrum that exhibits frequency anticorrelation, thesame source can be used to simultaneously distribute keys among multiple pairs of users by usingstandard Telecom wavelength demultiplexers [5]. Here, we experimentally show the distribution ofquantum secret keys with the BBM92 QKD protocol [6] between four different pairs of users with acommercial 100 GHz demultiplexer (0.8 nm channel width and spacing). Under CW pumpingconditions, using free-running InGaAs single-photon detectors, we achieve a secret key rate of 0.21bits/s and a qubit error rate (QBER) of 6.9% over 50 km of standard optical fiber.Our results, obtained with a robust and simple experimental set-up, open the way towards theimplementation of practical device-independent quantum communication protocols.
Abstract Quantum key distribution (QKD) based on entangled photon pairs holds the potential for repeater-based quantum networks connecting clients over long distance. We demonstrate long-distance entanglement distribution by means of polarisation-entangled photon pairs through two successive deployed 96 km-long telecommunications fibres in the same submarine cable. One photon of each pair was detected directly after the source, while the other travelled the fibre cable in both directions for a total distance of 192 km and attenuation of 48 dB. The observed two-photon Bell state exhibited a fidelity 85 ± 2% and was stable over several hours. We employed neither active stabilisation of the quantum state nor chromatic dispersion compensation for the fibre.