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Tunable decoherence of quantum polarization states via birefringence-frequency coupling using liquid crystal

GYA PRASAD, Rajneesh Joshi

Published Oct 8, 2025• 0 citations
Publisher

Abstract

We theoretically investigate a novel mechanism for controlling the quantum degree of polarization of single- and multi-photon light fields through the combined effects of birefringence and dispersion in optical media. While birefringence alone introduces a unitary phase shift between horizontal (H) and vertical (V) polarization modes, the inclusion of dispersion produces frequency-dependent effects that couple polarization with spectral degrees of freedom, thereby inducing decoherence and transforming the quantum state into a mixed state. By employing an electro-optically controlled nematic liquid crystal as the birefringent medium, this decoherence process can be harnessed to achieve tunable control of the quantum degree of polarization. We model this voltage-dependent tunability theoretically and propose methods for experimental verification.

DOI
10.1364/opticaopen.30295615.v1

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We theoretically investigate a novel mechanism for controlling the quantum degree of polarization of single- and multi-photon light fields through the combined effects of birefringence and dispersion in optical media. While birefringence alone introduces a unitary phase shift between horizontal (H) and vertical (V) polarization modes, the inclusion of dispersion produces frequency-dependent effects that couple polarization with spectral degrees of freedom, thereby inducing decoherence and transforming the quantum state into a mixed state. By employing an electro-optically controlled nematic liquid crystal as the birefringent medium, this decoherence process can be harnessed to achieve tunable control of the quantum degree of polarization. We model this voltage-dependent tunability theoretically and propose methods for experimental verification.
We theoretically investigate a novel mechanism for controlling the quantum degree of polarization of single- and multi-photon light fields through the combined effects of birefringence and dispersion in optical media. While birefringence alone introduces a unitary phase shift between horizontal (H) and vertical (V) polarization modes, the inclusion of dispersion produces frequency-dependent effects that couple polarization with spectral degrees of freedom, thereby inducing decoherence and transforming the quantum state into a mixed state. By employing an electro-optically controlled nematic liquid crystal as the birefringent medium, this decoherence process can be harnessed to achieve tunable control of the quantum degree of polarization. We model this voltage-dependent tunability theoretically and propose methods for experimental verification.
The propagation of the quantum states of light in dispersive and anisotropic media is a fundamental problem in quantum optics. We present a unified theoretical framework for the propagation of the quantum states of light in voltage-controlled nematic liquid crystals, incorporating both material dispersion and electrically tunable birefringence. By treating photons as finite-bandwidth wave packets, we derive analytical expressions for group velocoity, temporal walk-off, and phase evolution of orthogonally polarized modes. The results demonstrate that nematic liquid crystals can serve as electrically tunable quantum photonic devices capable of manipulating photon arrival times, polarization correlations, and temporal indistinguishability of entangled photon pairs. These results show the direct relevance to quantum communication and photonic quantum information processing.
Entangled photons are a cornerstone of quantum technologies, enabling applications from secure communication to quantum computing. A longstanding challenge is to develop a compact source that would generate polarization-entangled photons with tunable quantum state on demand. The promising materials for such sources are ferroelectric nematic liquid crystals (FNLCs), due to their nonlinear optical properties and easily controllable configuration. In this work, it is demonstrated that the polarization state and the degree of entanglement of photon pairs generated within FNLCs can be changed in a controllable and reversible manner. First, tuning of the entanglement is demonstrated via sample geometry with twisted FNLC configurations in a sample of varying thickness. Secondly, by applying an electric field, the degree of entanglement can be tuned in real time. In both scenarios, the degree of entanglement can be adjusted from nearly entirely separate photons to fully entangled. These findings represent a significant step toward tunable quantum sources that can produce any desired polarization state on demand. In the future, by adding more electrodes, different parts of the sample could be controlled individually, allowing for the creation of pixelated quantum light sources.
Abstract Quantum entanglement is an integral part of quantum optics and has been exploited in areas such as computation, cryptography and metrology. The entanglement between photons can be present in various degrees of freedom (DOFs), and even the simplest bi-partite systems can occupy a large Hilbert space. Therefore, it is desirable to exploit this multi-dimensional space for various quantum applications by fully controlling the properties of the entangled photons in multiple DOFs. While current entangled-photon sources are capable of generating entanglement in one or more DOFs, there is currently a lack of practical techniques that can shape and control the entanglement properties in multiple DOFs. Here we show that cascading two or more entangled-photon sources with tunable linear media in between allows us to generate photon-pairs whose entanglement properties can be tailored and shaped in the frequency and polarisation domains. We first develop a quantum mechanical model to study the quantum state generated from the cascade structure with special considerations paid to the effects of pump temporal coherence, linear dispersion, and in-structure polarisation transformation applied between the entangled-photon sources. We then experimentally generate photon-pairs with tunable entanglement properties by manipulating the dispersion and birefringence properties of the linear medium placed in between two entangled-photon sources. This is done in an all-fibre, phase stable, and alignment-free configuration. Our results show that the cascade structure offers a great deal of flexibility in tuning the properties of entangled photons in multiple DOFs, opening up a new avenue in engineering quantum light sources.
Abstract Quantum information is often carried in the frequency and polarization degrees of freedom (DoFs) in single photons and entangled photons. We demonstrate an approach to couple and decouple the frequency and polarization DoFs of broadband biphotons. Our approach is based on a nonlinear interferometer consisting of a linear dispersive medium and a polarization controller in between the two biphoton sources (nonlinear media). When the two DoFs are decoupled, maximally polarization-entangled biphotons are observed in the polarization DoF, while interference fringes are observed in the spectrum of the biphotons. When the two DoFs are coupled, by adjusting the polarization controller, interference fringes disappear from the spectrum and instead appear in the degree of polarization entanglement, varying between 0 and 1, depending on the signal and idler frequencies. Our approach offers a convenient means of tuning the polarization entanglement and can be employed for arbitrary biphoton polarization state generation.
Structured optical materials provide a promising platform for photonic quantum information processing in free space. Beam splitters, a fundamental building block of photonic circuits, have recently been demonstrated in free space using geometric-phase optical elements. These devices coherently mix circularly polarized transverse modes of freely propagating optical fields, including modes carrying orbital angular momentum. In this work, we investigate liquid-crystal metagratings as electrically tunable beam splitters for transverse-momentum optical modes. By exploiting the voltage-controlled birefringence of liquid-crystal metasurfaces, we experimentally tune the splitting ratio of the device and thereby control the degree of two-photon interference between indistinguishable photons. At the output, photons are spatially resolved in different regions of a time-resolved single-photon-sensitive detector, enabling the reconstruction of coincidence maps in the Fourier plane. This approach is readily scalable and enables highly parallel coincidence measurements across a large number of optical modes.