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arXiv:1807.09804 · Physical Review A

Quantum state engineering with twisted photons via adaptive shaping of the pump beam

E. V. Kovlakov, Stanislav S. Straupe, S. P. Kulik

Published Dec 12, 2018• 72 citations
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Abstract

High-dimensional entanglement is a valuable resource for quantum communication, and photon pairs entangled in orbital angular momentum (OAM) are commonly used for encoding high-dimensional quantum states. However, methods for the preparation of maximally entangled states of arbitrary dimensionality are still lacking, and currently used approaches essentially rely on filtering and entanglement concentration. Here, we experimentally realize a method for the generation of high-dimensional maximally entangled OAM states of photon pairs which does not require any of these procedures. Moreover, the prepared state is restricted to the subspace of the specified dimensionality, thus requiring minimal spatial postselection.

DOI
10.1103/physreva.98.060301

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High-dimensional entanglement is a valuable resource for quantum communication, and photon pairs entangled in orbital angular momentum (OAM) are commonly used for encoding high-dimensional quantum states. However, methods for the preparation of maximally entangled states of arbitrary dimensionality are still lacking, and currently used approaches essentially rely on filtering and entanglement concentration. Here, we experimentally realize a method for the generation of high-dimensional maximally entangled OAM states of photon pairs which does not require any of these procedures. Moreover, the prepared state is restricted to the subspace of the specified dimensionality, thus requiring minimal spatial postselection.
Photon pairs generated from spontaneous parametric down-conversion are a well-established method to realize entangled bipartite photonic systems. Laguerre-Gaussian modes, which carry orbital angular momentum (OAM), are commonly exploited to engineer high-dimensional entangled quantum states. For Hilbert spaces with dimension d>2d>2d>2, maximally entangled states (MESs) help to improve the capacity and security of quantum communication protocols, among several other promising features. However, the direct generation of MESs in well-defined high-dimensional subspaces of the infinite OAM basis has remained a challenge. Here, we formalize how the spatial distribution of the pump beam and the nonlinear profile of the crystal can be simultaneously utilized to generate MESs without additional spatial filtering of OAM modes within a subspace. We illustrate our approach with maximally entangled qutrits (d=3)(d=3)(d=3) and ququints (d=5)(d=5)(d=5).
Maximally entangled photon pairs with a spatial degree of freedom is a potential way for realizing high-capacity quantum computing and communication. However, methods to generate such entangled states with high quality, high brightness, and good controllability are needed. Here, a scheme is experimentally demonstrated that generates spatially maximally entangled photon pairs with an orbital angular momentum degree of freedom via spontaneous parametric down-conversion in a nonlinear crystal. Compared with existing methods using postselection, the technique can directly modulate the spatial spectrum of down-converted photon pairs by engineering the input pump beam. In addition, the relative phase between spatially entangled photon pairs can be easily manipulated by preparing the relative phase of input pump states.
Cutting-edge quantum technologies lean on sources of high-dimensional entangled states (HDESs) that reliably prepare high-fidelity target states. The idea to overlap photon paths from distinct but indistinguishable sources was recently introduced for the creation of HDESs, known as entanglement by path identity. In this regard, the use of orbital angular momentum (OAM) modes is promising, as they offer a high-dimensional and discrete Hilbert space to encode information. While entanglement by path identity with OAM has been verified experimentally, a detailed investigation of how the OAM distribution of photon pairs can be engineered to maximize the entanglement is lacking. We address this gap and identify an optimal dimensionality for maximally entangled states (MESs) when the spatial engineering of the pump beam and the path-identity approach are combined. Our theoretical study reveals notable limitations for the fidelity of high-dimensional target states. We also establish the equivalence of entangled biphoton states pumped by a spatially engineered beam and generated via path identity. These findings constitute a valuable step toward the optimized preparation of MESs in high dimensions.
High-dimensional quantum entanglement is an important resource for fundamental research and quantum technological application. Recently, the orbital angular momentum (OAM) of light has received much attention to realize high-dimensional quantum systems, and generating and manipulating the OAM entanglement has become an important topic. Here, we propose an inverse design technique to manipulate the OAM entanglement of the photon pair generated through spontaneous parametric down-conversion. The proposed technique directly tailors the OAM states of entangled photon pairs by shaping the incident pump beam. In particular, we construct a physics-informed model by leveraging the backpropagation algorithm and adhering to the overlap integral relation. Our model automatically optimizes diffractive masks, allowing the structured pump to quantitatively modulate the OAM distribution. The generation of maximally entangled states in different dimensions is demonstrated numerically and experimentally.
High-dimensional maximally entangled orbital angular momentum (OAM) states are a promising resource for enhancing information capacity and robustness in quantum communication. However, it still lacks an effective method to increase the state dimensionality. Here, we theoretically propose an efficient scheme to generate maximally entangled OAM states of ultrahigh dimensionality by manipulating the radial components of a Laguerre-Gaussian (LG) pump beam. By optimizing the complex amplitudes of multiple radial modes of the LG pump light, one can feasibly achieve 101-dimensional OAM-based maximally entangled states. Our scheme has potential applications in high capacity quantum communication networks.
High-dimensional entangled states in an orbital-angular-momentum (OAM) basis offer several unique advantages for quantum information applications. However, for the optimal performance of a given application, one requires a generation technique for OAM entangled states that is completely postselection-free and fully controllable. Nonetheless, despite several efforts in the past, no such technique currently exists. In this paper, we propose just such a technique and experimentally demonstrate postselection-free generation of up to about 150-dimensional OAM entangled states. We report the generation accuracy, which is a measure of the control, to be more than $98\mathrm{%}$ for states with Gaussian and triangular OAM Schmidt spectra, and up to $90\mathrm{%}$ for the maximally entangled OAM states, which have rectangular spectra.