Integrated photonic circuits with many input and output modes are essential in applications ranging from conventional optical telecommunication networks, to the elaboration of photonic qubits in the integrated quantum information framework. In particular, the latter field has been object in the recent years of an increasing interest: the compactness and phase stability of integrated waveguide circuits are enabling experiments unconceivable with bulk-optics set-ups. Linear photonic devices for quantum information are based on quantum and classical interference effects: the desired circuit operation can be achieved only with tight fabrication control on both power repartition in splitting elements and phase retardance in the various paths. Here we report on a novel three-dimensional circuit architecture, made possible by the unique capabilities of femtosecond laser waveguide writing, which enables us to realize integrated multimode devices implementing arbitrary linear transformations. Networks of cascaded directional couplers can be built with independent control on the splitting ratios and the phase shifts in each branch. In detail, we show an arbitrarily designed 5×5 integrated interferometer: characterization with one- and two-photon experiments confirms the accuracy of our fabrication technique. We exploit the fabricated circuit to implement a small instance of the boson-sampling experiments with up to three photons, which is one of the most promising approaches to realize phenomena hard to simulate with classical computers. We will further show how, by studying classical and quantum interference in many random multimode circuits, we may gain deeper insight into the bosonic coalescence phenomenon.
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.
The ability to manipulate quantum states of light by integrated devices may open new perspectives both for
fundamental tests of quantum mechanics and for novel technological applications. The technology for handling
polarization-encoded qubits, the most commonly adopted approach, was still missing in quantum optical circuits
until the ultrafast laser writing (ULW) technique was adopted for the first time to realize integrated devices able
to support and manipulate polarization encoded qubits.1 Thanks to this method, polarization dependent and independent
devices can be realized. In particular the maintenance of polarization entanglement was demonstrated
in a balanced polarization independent integrated beam splitter1 and an integrated CNOT gate for polarization
qubits was realized and carachterized.2 We also exploited integrated optics for quantum simulation tasks: by
adopting the ULW technique an integrated quantum walk circuit was realized3 and, for the first time, we investigate
how the particle statistics, either bosonic or fermionic, influences a two-particle discrete quantum walk.
Such experiment has been realized by adopting two-photon entangled states and an array of integrated symmetric
directional couplers. The polarization entanglement was exploited to simulate the bunching-antibunching
feature of non interacting bosons and fermions. To this scope a novel three-dimensional geometry for the waveguide
circuit is introduced, which allows accurate polarization independent behaviour, maintaining a remarkable
control on both phase and balancement of the directional couplers.
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.
The emerging strategy to overcome the limitations of bulk quantum optics consists of taking advantage of the
robustness and compactness achievable by the integrated waveguide technology. Here we report the realization
of a directional coupler, fabricated by femtosecond laser waveguide writing, acting as an integrated beam splitter
able to support polarization encoded qubits. This maskless and single step technique allows to realize circular
transverse waveguide profiles able to support the propagation of Gaussian modes with any polarization state.
Using this device, we demonstrate the quantum interference with polarization entangled states.
The orbital angular momentum carried by single photons represents a promising resource in the quantum information
field. In this paper we report the characterization in the quantum regime of a recently introduced
optical device, known as q-plate. Exploiting the spin-orbit coupling that takes place in the q-plate, it is possible
to transfer coherently the information from the polarization to the orbital angular momentum degree of freedom,
and viceversa. Hence the q-plate provides a reliable bi-directional interface between polarization and orbital
angular momentum. As a first paradigmatic demonstration of the q-plate properties, we have carried out the
first experimental Hong-Ou-mandel effect purely observed in the orbital angular momentum degree of freedom.