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Journal of Experimental and Theoretical Physics Letters

Method for the Direct Generation of Multidimensional Biphoton States with Polarization–Frequency Quantum Entanglement

Maksim A. Smirnov, A. M. Smirnova, O. A. Ermishev, K. S. Melnik, Sergey A. Moiseev

Published Sep 1, 2025• 3 citations
Publisher

Abstract

An approach has been proposed to directly generate a multidimensional biphoton state that combines the polarization and frequency quantum entanglement of visible and infrared modes in a microstructured fiber. The possibility of controlling polarization states and equalizing the amplitudes of all generated spectral modes without a significant decrease in the generation rate has been shown. The prospects for the use of multidimensional biphoton states in optical quantum processing and quantum communications have been discussed.

DOI
10.1134/s0021364025607638

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An approach has been proposed to directly generate a multidimensional biphoton state that combines the polarization and frequency quantum entanglement of visible and infrared modes in a microstructured fiber. The possibility of controlling polarization states and equalizing the amplitudes of all generated spectral modes without a significant decrease in the generation rate has been shown. The prospects for the use of multidimensional biphoton states in optical quantum processing and quantum communications have been discussed.
Photon entanglement is essential for quantum communication, sensing, and computing. While research on frequency-degenerate entangled photons in the near-infrared (NIR) region has advanced significantly, entangled light spanning mid-infrared (MIR) and visible (VIS) frequencies remains under development [1]. For instance, in the MIR region, chiral collective lattice vibration modes that exhibit selectivity for circularly polarized light have been observed in two-dimensional layered materials [2], suggesting the existence of quantum entanglement with electronic states [3]. This entanglement could lead to the conservation of quantum states. For this purpose, it is necessary to generate quantum entangled states of photons in the MIR and VIS regions with high fidelity.
We demonstrate a chip-integrated semiconductor source that combines polarization and frequency entanglement, allowing the generation of entangled biphoton states in a hybrid degree of freedom without post-manipulation. Our AlGaAs device is based on type-II spontaneous parametric downconversion in a counterpropagating phase-matching scheme in which the modal birefringence lifts the degeneracy between the two possible nonlinear interactions. This allows the direct generation of polarization–frequency entangled photons at room temperature and telecom wavelength, and in two distinct spatial modes, offering enhanced flexibility for quantum information protocols. The state entanglement is quantified by a combined measurement of the joint spectrum and Hong–Ou–Mandel interference (raw visibility 70.1 % ± 1.1 % ) of the biphotons, allowing to reconstruct a restricted density matrix in the hybrid polarization–frequency space.
We consider a new method for the generation of polarization-frequency entangled states of photon pairs. We use a frequency-nondegenerate regime of spontaneous parametric down conversion where the photon pairs (biphotons) are produced with identical polarizations, propagate mostly in the same direction, but differ in frequency. Entanglement is achieved by a coherent superposition of pairs emitted from two nonlinear crystals, with the polarization of the biphotons from the first crystal being changed by a transformer placed between the two crystals. We show that this scheme allows the degree of entanglement to be controlled by the choice of biphoton frequencies.
Abstract A frequency-multiplexed entangled photon pair and a high-dimensional hyperentangled photon pair are useful to realize a high-capacity quantum communication. A biphoton frequency comb (BFC) with entanglement can be used to prepare both states. We demonstrate polarization entangled BFCs with over 1400 frequency modes, which is approximately two orders of magnitude larger than those of earlier entangled BFCs, by placing a singly resonant periodically poled LiNbO3 waveguide resonator within a Sagnac loop. The BFCs are demonstrated by measuring the joint spectral intensity, cross-correlation, and autocorrelation. Moreover, the polarization entanglement at representative groups of frequency modes is verified by quantum state tomography, where each fidelity is over 0.7. The efficient generation of a massive-mode entangled BFC is expected to accelerate the increase of capacity in quantum communication.