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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Discrete, tunable color-entanglement

Sven Ramelow, L. Ratschbache, Alessandro Fedrizzi, Nathan K. Langford, Anton Zeilinger

Published Jun 1, 2009• 6 citations
Publisher

Abstract

Color, or frequency, is one of the most familiar degrees of freedom (DOFs) of light and has been routinely analyzed in spectroscopy for centuries. However, it has been used relatively little for quantum technologies. Entangled qubits are a key ingredient in many such technologies. For other optical degrees of freedom discretely entangled states have been extensively investigated, e.g. [1–5]. In contrast, discrete frequency entanglement has not yet been unambiguously demonstrated, despite potentially interesting applications such as enhanced clock synchronization beyond the classical limit [6,7] and improved quantum communication in noisy channels [8]. Moreover, quantum information encoded in discrete, tunable frequency bins of photons would be an ideal mediator between stationary qubits with different energy levels. For many of those applications it is essential, that the color-entangled stated can be directly created - in contrast to spectrally post-selecting them from broadband continuous spectral entanglement [9,10]. So far, no experiment was able to achieve this.

DOI
10.1109/cleoe-eqec.2009.5192629

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Color, or frequency, is one of the most familiar degrees of freedom (DOFs) of light and has been routinely analyzed in spectroscopy for centuries. However, it has been used relatively little for quantum technologies. Entangled qubits are a key ingredient in many such technologies. For other optical degrees of freedom discretely entangled states have been extensively investigated, e.g. [1–5]. In contrast, discrete frequency entanglement has not yet been unambiguously demonstrated, despite potentially interesting applications such as enhanced clock synchronization beyond the classical limit [6,7] and improved quantum communication in noisy channels [8]. Moreover, quantum information encoded in discrete, tunable frequency bins of photons would be an ideal mediator between stationary qubits with different energy levels. For many of those applications it is essential, that the color-entangled stated can be directly created - in contrast to spectrally post-selecting them from broadband continuous spectral entanglement [9,10]. So far, no experiment was able to achieve this.
Although frequency multiplexing of information has revolutionized the field of classical communications, the color degree of freedom (DOF) has been used relatively little for quantum applications. We experimentally demonstrate a new hybrid quantum gate that transfers polarization entanglement of nondegenerate photons onto the color DOF. We create, for the first time, high-quality, discretely color-entangled states (with energy band gap up to 8.4 THz) without any spectrally selective filtering, and unambiguously verify and quantify the amount of entanglement (tangle, 0.611 +/- 0.009) by reconstructing a restricted density matrix; we generate a range of maximally entangled states, including a set of mutually unbiased bases for an encoded qubit space. The technique can be generalized to transfer polarization entanglement onto other photonic DOFs, like orbital angular momentum.
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.
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.
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.
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].
Quantum entanglement is a fundamental resource for secure information processing and communications. The canonical optical quantum information processing encodes a single qubit per photon, with remarkable demonstrations of secure quantum communications, linear optical quantum computing amongst others. Often the polarization qubit, a discrete variable with entangled Bell states, is used. Continuous variables such as energy-time modes and spatial modes, however, present a dramatically larger Hilbert space for quantum information processing. The high-dimensional entanglement capture more qubits per photon, enabling dramatically higher secure key rates over longer distances and with better error resilience. Here I will describe our results on high-dimensional entanglement in quantum frequency combs for dense quantum communications. We first demonstrate revival of the Hong-Ou-Mandel interferences, long postulated by theorists more than a decade ago, up to 19-dimensions and with visibilities up to 96.5%. The mode-locked two-photon state in high-dimensions is further witnessed through a stabilized Franson interferometer, as a generalized Bell’s inequality test and hyperentangled through multiple degrees-of-freedom. Entanglement revivals of the non-local interference at discrete time-bins are uncovered, up to 97.8% visibility, as a fundamental resource for dense secure information processing.