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Conference on Lasers and Electro-Optics

A two-channel source of spectrally degenerate polarization entangled states on chip

Linda Sansoni, Kai-Hong Luo, Raimund Ricken, Stephan Krapick, H. Herrmann, Christine Silberhorn

Published Jan 1, 2016• 1 citation
Publisher

Abstract

In the last years the increasing complexity of quantum information protocols triggered the development of integrated waveguide technology for quantum applications [1-3] such that stability is intrinsically obtained thanks to the combination of many optical elements on a monolithic chip and scalability is potentially achievable. In this context the realization of integrated sources of quantum light is of fundamental interest. Some experimental implementations using parametric down conversion in nonlinear materials [4], as well as in semiconductors [5] have been reported. However, in the landscape of integrated devices, the realization of one important source of entangled states is still missing: a source of spectrally indistinguishable photon pairs emitted on two different spatial modes, which share entanglement in the polarization degree of freedom without the need of post-selection. This source is widely adopted in bulk setups [6], indeed entanglement is a fundamental requirement for quantum information purposes and the polarization of photons is the most used degree of freedom for quantum information encoding. However the implementation of such a scheme in integrated waveguides is a challenging task due to the combination of orthogonal polarizations and the need of two different spatial modes.

DOI
10.1364/cleo_qels.2016.fm2n.2

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In the last years the increasing complexity of quantum information protocols triggered the development of integrated waveguide technology for quantum applications [1-3] such that stability is intrinsically obtained thanks to the combination of many optical elements on a monolithic chip and scalability is potentially achievable. In this context the realization of integrated sources of quantum light is of fundamental interest. Some experimental implementations using parametric down conversion in nonlinear materials [4], as well as in semiconductors [5] have been reported. However, in the landscape of integrated devices, the realization of one important source of entangled states is still missing: a source of spectrally indistinguishable photon pairs emitted on two different spatial modes, which share entanglement in the polarization degree of freedom without the need of post-selection. This source is widely adopted in bulk setups [6], indeed entanglement is a fundamental requirement for quantum information purposes and the polarization of photons is the most used degree of freedom for quantum information encoding. However the implementation of such a scheme in integrated waveguides is a challenging task due to the combination of orthogonal polarizations and the need of two different spatial modes.
Abstract Integrated optics provides the platform for the experimental implementation of highly complex and compact circuits for quantum information applications. In this context integrated waveguide sources represent a powerful resource for the generation of quantum states of light due to their high brightness and stability. However, the confinement of the light in a single spatial mode limits the realization of multi-channel sources. Due to this challenge one of the most adopted sources in quantum information processes, i.e. a source which generates spectrally indistinguishable polarization entangled photons in two different spatial modes, has not yet been realized in a fully integrated platform. Here we overcome this limitation by suitably engineering two periodically poled waveguides and an integrated polarization splitter in lithium niobate. This source produces polarization entangled states with fidelity of F=0.973±0.003{\mathcal F} \mathrm{=0.973}\pm 0.003F=0.973±0.003 F = 0.973 ± 0.003 and a test of Bell’s inequality results in a violation larger than 14 standard deviations. It can work both in pulsed and continuous wave regime. This device represents a new step toward the implementation of fully integrated circuits for quantum information applications.
High-dimensional degrees of photons – such as orbital angular momentum, spatial modes of frequency – provide a powerful mean to increase the density and security of quantum communication, and to enhance flexibility in quantum computing. In particular, the spatial degree of freedom is particularly suited to on-chip integration. Rapid progress has been made in recent years to develop integrated circuits achieving on-chip quantum interference, entanglement and gate operations on spatially encoded states, culminating in the demonstration of the Boson Sampling task on-chip [1]. While many of these demonstrations have relied on external sources to generate quantum states of light, which were then fed into a passive circuitry, a next step has been taken recently with the demonstration of active chips combining the generation and manipulation of spatially encoded states [2].
Integrated optical circuits enable stable and scalable realization of quantum logic devices, which can form a basis for the mass production of photonic chips for quantum communications and computations. An important challenge is the integration of single-photon sources, which should provide on-chip generation and preparation of quantum photon states. Spontaneous parametric down-conversion (SPDC) provides an attractive solution for experimental generation of correlated and entangled photon pairs [1,2]. However, internal losses in the waveguides may still be present, affecting nontrivially the emerging photon state [3].
As quantum optics grows and develops, new sophisticated optical schemes require an increasing number of optical elements. Integrated photonic circuits have the capacity to provide a solution to this challenge, as they are intrinsically scalable and interferometrically stable. Integrated realization of multi-photon entanglement, quantum factoring algorithms, and polarization entanglement has already been demonstrated. One of particularly interesting structures in integrated photonics is the waveguide array, which enables flexible shaping of optical beam and pulses in space. Recently waveguide arrays have been shown to generate unusual and strongly non-classical spatial correlations of photon pairs propagating in the regime of quantum walks [1].
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.
Generating nonclassical states of photons such as polarization-entangled states on a monolithic chip is a crucial step towards practical applications of optical quantum information processing such as quantum computing and quantum key distribution. Here we demonstrate two polarization-entangled photon sources in a single monolithic semiconductor waveguide. The first source is achieved through the concurrent utilization of two spontaneous parametric down-conversion (SPDC) processes, namely, type-0 and type-I SPDC processes. The chip can also generate polarization-entangled photons via the type-II SPDC process, enabling the generation of both copolarized- and cross-polarized polarization-entangled photons in the same device. In both cases, polarization entanglement is generated directly on the chip without the use of any off-chip compensation, interferometry, or bandpass filtering. This enables direct, chip-based generation of both Bell states $(|H,H\ensuremath{\rangle}+|V,V\ensuremath{\rangle})/\sqrt{2}$ and $(|H,V\ensuremath{\rangle}+|V,H\ensuremath{\rangle})/\sqrt{2}$ simultaneously utilizing the same pump source. In addition, based on compound semiconductors, this chip can be directly integrated with it own pump laser. This technique ushers an era of self-contained, integrated, electrically pumped, room-temperature polarization-entangled photon sources.