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.
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.
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.
The field of high-dimensional quantum photonics involves the use of multimode photonic degrees-of-freedom such as the spatial, temporal, or spectral structure of light to encode multi-level quantum states. Recent years have seen rapid progress in the development of methods to generate, manipulate, and distribute such quantum states of light and their use in a range of quantum technology applications that offer practical advantages over conventional qubit-based approaches. High-dimensional quantum states of light encoded in photonic time-bins, frequency-bins, transverse-spatial modes, waveguide paths, and temporal modes have enabled noise-robust fundamental tests of quantum mechanics, error-resilient and high-capacity quantum communication protocols, andas well as efficient approaches for quantum information processing, to name just a few examples. However, research in this field has progressed fairly independently, with little exchange across different photonic degrees-of-freedom or between experiment and theory and no comprehensive comparison between degrees-of-freedom. This roadmap aims to bridge this gap by surveying progress in each area and identifying shared challenges and opportunities that cut across two or more photonic degrees-of-freedoms. We review early work and state-of-the-art experimental techniques under development for high-dimensional quantum states encoded in single and entangled photons, as well as theoretical tools for their measurement and certification. We outline the main outstanding challenges for theory and each experimental degree-of-freedom, identifying promising future directions of research that may enable these to be overcome. We end by discussing interconnections and shared challenges centered around their distribution, measurement, and manipulation, with a view towards their integration into next-generation quantum technology platforms and applications.
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.
Summary form only given. Entanglement is a fundamental resource for the quantum processing of information and to study the intrinsic nature of quantum correlations. Entangling higher dimensional bipartite systems (qudits) has been shown to give more insight on the nature of entanglement compared to the simplest entanglement system composed of two two level systems (two qubits) [1, 2]. For quantum key distribution, increasing the dimension of the alphabet by using qudits increases the effective bit rate of the protocol, still being secure [3]. Experimentally, higher dimensional entanglement in photonic systems have been demonstrated for various degrees of freedom of light, including time-bins [4,5], transverse momentum [6, 7] or orbital angular momentum modes [8].We demonstrate here a new way to encode qudits in the energy spectrum of the entangled photons by defining frequency bins within their spectra. Broadband entangled photons are generated by continuous wave spontaneous down-conversion and detected in coincidence by sum frequency generation. By means of experimental methods used for shaping fs-laser pulses, frequency bins can be addressed in the photons spectra. Furthermore, controlling each frequency component individually allows to manipulate and characterize the quantum states. The dimension of the generated states is in practice limited by the optical resolution of the setup only.
The time-frequency degree of freedom is a powerful resource for implementing high-dimensional quantum information processing. In particular, field-orthogonal pulsed temporal modes offer a flexible framework compatible with both long-distance fibre networks and integrated waveguide devices. In order for this architecture to be fully utilised, techniques to reliably generate diverse quantum states of light and accurately measure complex temporal waveforms must be developed. To this end, nonlinear processes mediated by spectrally shaped pump pulses in group-velocity engineered waveguides and crystals provide a capable toolbox. In this review, we examine how tailoring the phasematching conditions of parametric downconversion and sum-frequency generation allows for highly pure single-photon generation, flexible temporal-mode entanglement, and accurate measurement of time-frequency photon states. We provide an overview of experimental progress towards these goals, and summarise challenges that remain in the field.
Multidimensional entangled photon states represent a useful resource for large-scale quantum communication networks. By leveraging multiple degrees of freedom (DoFs) of photons, the dimensionality of the Hilbert space can be expanded without increasing the number of particles, enhancing the efficiency of quantum information processing. Specifically, hyperentanglement, i.e. simultaneous entanglement across multiple DoFs, is robust against decoherence, thus providing strengthened performance in noisy quantum communication protocols with respect to n-photon quantum states [1].