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Optics Letters

Arbitrarily rotated optical axis waveguide induced by a trimming line

Ying-De Wang, Zhiyuan Zhang, Yang Chen, Yi‐Ke Sun, Yichun Li, Zhen‐Nan Tian, Xi‐Feng Ren, Qi‐Dai Chen, Guang‐Can Guo

Published May 18, 2023• 7 citations
Publisher

Abstract

Rotated optical axis waveguides can facilitate on-chip arbitrary wave-plate operations, which are crucial tools for developing integrated universal quantum computing algorithms. In this paper, we propose a unique technique based on femtosecond laser direct writing technology to fabricate arbitrarily rotated optical axis waveguides. First, a circular isotropic main waveguide with a non-optical axis was fabricated using a beam shaping method. Thereafter, a trimming line was used to create an artificial stress field near the main waveguide to induce a rotated optical axis. Using this technique, we fabricated high-performance half- and quarter-wave plates. Subsequently, high-fidelity (97.1%) Pauli-X gate operation was demonstrated via quantum process tomography, which constitutes the basis for the full manipulation of on-chip polarization-encoded qubits. In the future, this work is expected to lead to new prospects for polarization-encoded information in photonic integrated circuits.

DOI
10.1364/ol.493410

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Rotated optical axis waveguides can facilitate on-chip arbitrary wave-plate operations, which are crucial tools for developing integrated universal quantum computing algorithms. In this paper, we propose a unique technique based on femtosecond laser direct writing technology to fabricate arbitrarily rotated optical axis waveguides. First, a circular isotropic main waveguide with a non-optical axis was fabricated using a beam shaping method. Thereafter, a trimming line was used to create an artificial stress field near the main waveguide to induce a rotated optical axis. Using this technique, we fabricated high-performance half- and quarter-wave plates. Subsequently, high-fidelity (97.1%) Pauli-X gate operation was demonstrated via quantum process tomography, which constitutes the basis for the full manipulation of on-chip polarization-encoded qubits. In the future, this work is expected to lead to new prospects for polarization-encoded information in photonic integrated circuits.
The application of integrated photonic technologies to quantum optics has recently enabled a wealth of breakthrough experiments in several quantum information areas. In particular, femtosecond laser written optical circuits revealed to be the ideal tool for investigating the features of polarization encoded qubits. However, the difficulty of integrating half and quarter wave plates in such circuits avoids the possibility to perform arbitrary rotations of the polarization state of photons on chip. Femtosecond laser written waveguides intrinsically exhibit a certain degree of birefringence and thus they could be exploited as integrated waveplates. In practice, the direction of the birefringence axes of the waveguides is the same of the propagation direction of the writing femtosecond laser beam, namely perpendicular to the substrate surface. Its fine rotation in a controlled fashion, preserving the accuracy of the positioning of the laser focal spot required by the fabrication process, is extremely challenging. In order to achieve this goal, we combine a high NA (1.4) focusing objective partially filled with a reduced diameter writing beam. In this way, the translation of the beam with respect to the objective center produces a rotation of the focusing direction, without altering the focal spot position. With this method we are able to tilt the birefringence axes of the waveguides up to 45°, and thus to use them as integrated light polarization rotators. In order to demonstrate the effectiveness of these components, we developed a fully integrated device capable to perform the quantum tomography of an arbitrary two-photon polarization state.
Chip-based photonic quantum computing is an emerging technology that promises much speedup over conventional computers at small integration volumes. Particular interest is thereby given to polarisation-encoded photonic qubits, and many protocols have been developed for this encoding. However, arbitrary wave plate operation on chip are not available so far, preventing from the implementation of integrated universal quantum computing algorithms. In our work we close this gap and present Hadamard, Pauli-X, and rotation gates of high fidelity for photonic polarisation qubits on chip by employing a reorientation of the optical axis of birefringent waveguides. The optical axis of the birefringent waveguide is rotated due to the impact of an artificial stress field created by an additional modification close to the waveguide. By adjusting this length of the defect along the waveguide, the retardation between ordinary and extraordinary field components is precisely tunable including half-wave plate and quarter-wave plate operations. Our approach demonstrates the full range control of orientation and strength of the induced birefringence and thus allows arbitrary wave plate operations without affecting the degree of polarisation or introducing additional losses to the waveguides. The implemented gates are tested with classical and quantum light.
Controlling and manipulating the polarization state of a light beam is crucial in applications ranging from optical sensing to optical communications, both in the classical and quantum regime, and ultimately whenever interference phenomena are to be exploited. In addition, many of these applications present severe requirements of phase stability and greatly benefit from a monolithic integrated-optics approach. However, integrated devices that allow arbitrary transformations of the polarization state are very difficult to produce with conventional lithographic technologies. Here we demonstrate waveguide-based optical waveplates, with arbitrarily rotated birefringence axis, fabricated by femtosecond laser pulses. To validate our approach, we exploit this component to realize a compact device for the quantum state tomography of two polarization-entangled photons. This work opens perspectives for integrated manipulation of polarization-encoded information with relevant applications ranging from integrated polarimetric sensing to quantum key distribution. Integrated photonic circuits with arbitrary control over the light polarization state are important in quantum information applications. Corrielli et al. realize compact quantum state tomography of polarization-entangled photons using waveguide-integrated waveplates fabricated by femtosecond laser inscription.
Photonics is a powerful framework for testing in experiments quantum information ideas, which promise significant advantages in computation, cryptography, measurement and simulation tasks. Linear optics is in principle sufficient to achieve universal quantum computation, but stability requirements become severe when experiments have to be implemented with bulk components. Integrated photonic circuits, on the contrary, due to their compact monolithic structure, easily overcome stability and size limitations of bench-top setups. Anyway, for quantum information applications, they have been operated so far only with fixed polarization states of the photons. On the other hand, many important quantum information processes and sources of entangled photon states are based on the polarization degree of freedom. In our work we demonstrate femtosecond laser fabrication of novel integrated components which are able to support and manipulate polarization entangled photons. The low birefringence and the unique possibility of engineering three-dimensional circuit layouts, allow femtosecond laser written waveguides to be eminently suited for quantum optics applications. In fact, this technology enables to realize polarization insensitive circuits which have been employed for entangled Bell state filtration and implementation of discrete quantum walk of entangled photons. Polarization sensitive devices can also be fabricated, such as partially polarizing directional couplers, which have enabled on-chip integration of quantum logic gates reaching high fidelity operation.