Abstract Entanglement is a key resource in quantum information science and associated emerging technologies. Photonic systems offer a large range of exploitable entanglement degrees of freedom (DOF) such as frequency, time, polarization, and spatial modes. Hyperentangled photons exploit multiple DOF simultaneously to enhance the performance of quantum information protocols. Here, we report a fully guided-wave approach for generating polarization and energy-time hyperentangled photons at telecom wavelengths. Moreover, by demultiplexing the broadband emission spectrum of the source into five standard telecom channel pairs, we demonstrate compliance with fibre network standards and improve the effective bit rate capacity of the quantum channel up to one order of magnitude. In all channel pairs, we observe a violation of a generalised Bell inequality by more than 27 standard deviations, underlining the relevance of our approach.
Abstract Entanglement is a key resource in quantum information science and associated emerging technologies. Photonic systems offer a large range of exploitable entanglement degrees of freedom (DOF) such as frequency, time, polarization, and spatial modes. Hyperentangled photons exploit multiple DOF simultaneously to enhance the performance of quantum information protocols. Here, we report a fully guided-wave approach for generating polarization and energy-time hyperentangled photons at telecom wavelengths. Moreover, by demultiplexing the broadband emission spectrum of the source into five standard telecom channel pairs, we demonstrate compliance with fibre network standards and improve the effective bit rate capacity of the quantum channel up to one order of magnitude. In all channel pairs, we observe a violation of a generalised Bell inequality by more than 27 standard deviations, underlining the relevance of our approach.
Hyperentanglement, the simultaneous and independent entanglement of quantum particles in multiple degrees of freedom, is a powerful resource that can be harnessed for efficient quantum information processing. In photonic systems, the two degrees of freedom (DOFs) often used to carry quantum and classical information are polarization and frequency, thanks to their robustness in transmission, both in free space and in optical fibers. Telecom-band hyperentangled photons generated in optical fibers are of particular interest because they are compatible with existing fiber-optic infrastructure and can be distributed over fiber networks with minimal loss. Here, we experimentally demonstrate the generation of telecom-band biphotons hyperentangled in both the polarization and frequency DOFs using a periodically poled silica fiber and observe entanglement concurrences above 0.95 for both polarization and frequency DOFs. Furthermore, by concatenating a Hong-Ou-Mandel interference test for frequency entanglement and full state tomography for polarization entanglement in a single experiment, we can demonstrate simultaneous entanglement in both the polarization and frequency DOFs. The states produced by our hyperentanglement source can enable protocols such as dense coding and high-dimensional quantum key distribution.
The aim of this thesis was to develop photonic entanglement sources and study their implementation in the general field of quantum information technologies. To this end, a novel fully wave-guided, high performance photonic entanglement source is presented, able to generate hyper-entangled states in the observables of polarization and energy-time by means of a nonlinear Sagnac loop. The waveguide-based design makes it flexible, reliable, and adaptable to a wide spectral range, paving the way towards compact photonic entanglement generators, compatible with fiber-based communication systems and networks. This has been underlined by generating and distributing hyperentanglement in 5x2 dense wavelength division multiplexed channel telecom pairs, simultaneously, towards higher bit rates. The quality of the generated entanglement has been qualified by violating the Bell inequalities in a 16-dimension Hilbert space. Moreover, to adapt the wavelength of the entangled telecom photon pairs to the absorption wavelength of current quantum memory systems, a coherent wavelength converter is demonstrated. Furthermore, within the framework of quantum metrology, a new concept for a high-precision chromatic dispersion (CD) measurement in standard single mode fibers is introduced and demonstrated. In this demonstration, due to conceptual advantages enabled by quantum optics, an unprecedented 2.6 times higher accuracy on CD measurements is shown, compared to state-of-the-art techniques. In the same context, a new protocol for measuring two-photon phase shifts is performed using single photon detection only, promising scalable and potential real device applications with limited resources and simplified detection schemes. Finally, any potential application of quantum optics will be realized using small-scale devices. In this framework, an integrated on-chip heralded path entanglement generator is demonstrated, and shown to be adaptable to logic gate operations.
We report a versatile and practical approach for generating high-quality polarization entanglement in a fully guided-wave fashion. Our setup relies on a high-brilliance type-0 waveguide generator producing paired photon at a telecom wavelength associated with an advanced energy-time to polarisation transcriber. The latter is capable of creating any pure polarization entangled state, and allows manipulating single photon bandwidths that can be chosen at will over five orders of magnitude, ranging from tens of MHz to several THz. We achieve excellent entanglement fidelities for particular spectral bandwidths, i.e. 25 MHz, 540 MHz and 100 GHz, proving the relevance of our approach. Our scheme stands as an ideal candidate for a wide range of network applications, ranging from dense division multiplexing quantum key distribution to heralded optical quantum memories and repeaters.
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.
Quantum communication is based on the generation of quantum states and exploitation of quantum resources for communication protocols. Currently, photons are considered as the optimal carriers of information, because they enable long-distance transition with resilience to decoherence and they are relatively easy to create and detect. Entanglement is a fundamental resource for quantum communication and information processing, and it is of particular importance for quantum repeaters. Hyperentanglement, a state where parties are entangled with two or more degrees of freedom (DoFs) simultaneously, provides an important additional resource because it increases data rates and enhances error resilience. However, in photonics, the channel capacity, i.e., the ultimate throughput, is fundamentally limited when dealing with linear elements. We propose a technique for achieving higher transmission rates for quantum communication by using hyperentangled states, based on multiplexing multiple DoFs on a single photon, transmitting the photon, and eventually demultiplexing the DoFs to different photons at the destination, using Bell state measurements. Following our scheme, one can generate two entangled qubit pairs by sending only a single photon. The proposed transmission scheme lays the groundwork for novel quantum communication protocols with higher transmission rates and refined control over scalable quantum technologies.
Frequency-bin encoding is massively parallelizable and robust for optical fiber transmission. When coupled with an additional degree of freedom (DoF), the expansion of the Hilbert space allows for deterministic controlled operations between two DoFs within a single photon. Such capabilities, when combined with photonic hyperentanglement, are of great value for quantum communication protocols, including dense coding and single-copy entanglement distillation. In this talk, we present an all-fiber-coupled, ultrabroadband polarization–frequency hyperentangled source and conduct comprehensive quantum state tomography across multiple dense wavelength division multiplexing channels spanning the optical C+L-band (1530–1625 nm). In addition, we design and implement a high-fidelity controlled-NOT (cnot) operation between polarization and frequency DoFs by exploiting electro-optic phase modulation within a fiber Sagnac loop. Collectively, our hyperentangled source and two-qubit gate should unlock new opportunities for harnessing polarization–frequency resources in established telecommunication fiber networks for future quantum applications.