Hyper-entanglement between two or more photonic degrees of freedom (DOF) can enhance and enable new quantum protocols by allowing each DOF to perform the task it is optimally suited for. Here we demonstrate the generation of photon pairs hyper-entangled between pulse modes and frequency bins. The pulse modes are generated via parametric downconversion in a domain-engineered crystal and subsequently entangled to two frequency bins via a spectral mapping technique. The resulting hyper-entangled state is characterized and verified via measurement of its joint spectral intensity and non-classical two-photon interference patterns from which we infer its spectral phase. The protocol combines the robustness to loss, intrinsic high dimensionality and compatibility with standard fiber-optic networks of the energy-time DOF with the ability of hyper-entanglement to increase the capacity and efficiency of the quantum channel, already exploited in recent experimental applications in both quantum information and quantum computation.
Hyper-entanglement between two or more photonic degrees of freedom (DOF) can enhance and enable new quantum protocols by allowing each DOF to perform the task it is optimally suited for. Here we demonstrate the generation of photon pairs hyper-entangled between pulse modes and frequency bins. The pulse modes are generated via parametric downconversion in a domain-engineered crystal and subsequently entangled to two frequency bins via a spectral mapping technique. The resulting hyper-entangled state is characterized and verified via measurement of its joint spectral intensity and non-classical two-photon interference patterns from which we infer its spectral phase. The protocol combines the robustness to loss, intrinsic high dimensionality and compatibility with standard fiber-optic networks of the energy-time DOF with the ability of hyper-entanglement to increase the capacity and efficiency of the quantum channel, already exploited in recent experimental applications in both quantum information and quantum computation.
Quantum photonics is increasingly making use of the energy-time degree of freedom (DoF) to encode information. Its inherently high-dimensionality offers higher information capacity and robustness against noise. Most frequently, the energy-time DoF is exploited in the form of time bins and frequency bins, with direct applications in both quantum information processing tasks and quantum communication protocols. More recently, it has been used also in its pulse modes (or temporal modes) form, sets of states overlapping in time and frequency but kept orthogonal considering their temporal and spectral phase, whose difficult manipulation and characterisation made it less appealing for experimental applications. The production of states encoded in pulse modes has been demonstrated through sum frequency generation in waveguides [1] and parametric down conversion in bulk crystals [2], as well as the demonstration of schemes for creating hyper-entanglement among energy-time DoF and other high-dimensional DoFs [3], [4]. However, their implementation is still an area of continuous development and evolution, given the complexity of manipulating and characterising them.
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.
Quantum information is transforming modern society with concepts like entanglement, crucial for communication, computing and sensing. To mainstream these technologies, improved efficiency in encoding, transmission and information processing is required, especially in quantum key distribution (QKD). This method aims to securely distribute encryption keys while improving key rates and overcoming propagation loss and noise. Photons serve as ideal carriers due to their various properties for encoding information and minimal environmental interaction, making them vital for the global internet. High-dimensional and hyper-entanglement offer advantages over traditional two-mode encoding methods, including increase noise tolerance and information capacity. However, practical implementation is often limited by measurement device complexities and errors. Frequencies, or photon colours, present a promising approach to boost quantum communication thanks to their compatibility with existing telecommunications infrastructure and naturally occurs in photon pair generation. This thesis explores the frequency degree of freedom for applications in QKD, utilizing efficient all-fiber photon-pair sources to generate hyper-entangled states in polarization and frequency. The incorporation of fast modulators allows for high-speed operation while maintaining high entanglement fidelity. Reconfigurable spectral filters enable tailored spectro-temporal properties to fulfil requirements in multi-user networks and protocols beyond QKD, such as quantum secret sharing. To maximize quantum information capacity, methods for efficiently characterizing frequency entanglement are developed, achieving a certification of 35 modes, a record for the time and frequency degrees of freedom. Finally, estimates of secure key rates and a discussion of noise effects is shown. This foundation supports broadband and flexible quantum networks that effectively utilize frequency encoding in the future quantum internet.
Entanglement and encoding in discrete frequency bins – essentially a quantum analogue of wavelength-division multiplexing – represents a relatively new degree of freedom for quantum information with photons. In this talk I discuss biphoton frequency combs, generated either by spontaneous four-wave mixing (SFWM) from on-chip microring resonators or by spectral filtering of spontaneous parametric down conversion (SPDC) in second order nonlinear crystals. Potential advantages include generation of high dimensional units of quantum information (qudits), which can carry multiple qubits per photon, robust transmission over fiber, frequency parallelism and routing, and compatibility with on-chip implementations, as well as hyperentanglement with other photonic degrees, e.g., time-frequency hyperentanglement. Since the initial experiments less than two years ago, frequency bin quantum photonics has been advancing rapidly [1-8]. In this talk I will discuss a method using pulse shapers and phase modulators in order to mix different frequencies and perform two photon interference experiments for characterization of the frequency-bin entanglement. Similar components (pulse shapers and phase modulators) may be used to manipulate the frequency-encoded quantum states; this will be described in another talk by our collaborators at Oak Ridge National Labs. I will also discuss the use of time bins and frequency bins as independent degrees of freedom in a quantum encoding fashion to realize what we believe to be the first deterministic two qudit gates. Finally, I comment briefly on potential applications and on prospects for realization of integrated frequency bin quantum photonics.
Photonic quantum technologies rely on the deterministic preparation of quantum states encoded in single-photons degrees of freedom. While polarisation, orbital angular momentum and path have been routinely used since the early days of photonic quantum information, the last few years have seen an increasing interest in the frequency-time encoding [1, 2]. Indeed, the possibility of generating high-dimensional states, together with the compatibility of frequency modes and standard optical components, makes the spectral degree of freedom suitable for enhancing the efficiency of quantum protocols, spanning from quantum communication to optical quantum computation.
Abstract We demonstrate frequency-bin entanglement between ultra-narrowband photons generated by cavity enhanced spontaneous parametric down conversion. Our source generates photon pairs in widely non-degenerate discrete frequency modes, with one photon resonant with a quantum memory material based on praseodymium doped crystals and the other photon at telecom wavelengths. Correlations between the frequency modes are analyzed using phase modulators and narrowband filters before detection. We show high-visibility two photon interference between the frequency modes, allowing us to infer a coherent superposition of the modes. We develop a model describing the state that we create and use it to estimate optimal measurements to achieve a violation of the Clauser–Horne (CH) Bell inequality under realistic assumptions. With these settings we perform a Bell test and show a significant violation of the CH inequality, thus proving the entanglement of the photons. Finally we demonstrate the compatibility with a quantum memory material by using a spectral hole in the praseodymium (Pr) doped crystal as spectral filter for measuring high-visibility two-photon interference. This demonstrates the feasibility of combining frequency-bin entangled photon pairs with Pr-based solid state quantum memories.