Entangled photons generation is an interesting field of research, since progress in this area will directly affect the development of photonic quantum technologies, including quantum computing, simulation and sensing. Several methods have been sifted to increase the performances of entangled photon sources and the integrated optics approach represents a promising strategy. In particular, integrated waveguide sources represent a robust tool, thanks to their stability and the enhancement of nonlinear light-crystal interaction provided by waveguide field confinement.
Here, we show the versatility of a hybrid approach, realizing an integrated optical source for the generation of entangled photon-pairs at telecom wavelength. The nonlinear active medium used is lithium niobate, while the routing and manipulation of the generated signal is performed in aluminum-borosilicate glass photonic circuits. The system is composed of three cascaded devices. First, a balanced directional coupler at the fundamental wavelength equally splits the pump in the lithium niobate waveguides, which generate single-photon pairs through type 0 spontaneous parametric down-conversion process. A third chip, encompassing directional couplers and waveplates, closes the interferometer and recombines the generated photons, thus giving access to different quantum states of light: path-entangled or polarization-entangled states. A thermal phase shifter, which controls the relative phase between the interferometer arms, gives an additional degree of freedom for engineering the output state of the presented photon pairs source. All these components are entirely fabricated by femtosecond laser micromachining, a direct and very versatile technique that allows to process different kind of materials and realize high quality optical circuits.
Quantum simulators are getting to the level of real devices, constituted by a quantum system which can be controlled in its preparation, evolution and measurement and whose dynamics can implement that of the target quantum system we want to simulate. In this context, photonics quantum technologies are expected to play an instrumental role in the realization of controlled quantum systems capable, in their evolution, to simulate a given complex system.
I will present some of the main results obtained in this field in our laboratory by using integrated waveguide optical circuits that represent the hardware of a quantum simulator. These systems are constituted by interferometer arrays of beam splitters and phase shifters fabricated on single integrated platforms by femtosecond laser writing technique and have the potential of speeding-up the evolution from lab systems to the next generation of quantum optical devices for real-world applications. Using the mobility of photons we are able to create arbitrary interconnections within these systems and to mimic the main features of quantum phenomena of increasing complexity.
Encoding many qubits in different degrees of freedom (DOFs) of single photons is one of the routes towards enlarging the Hilbert space spanned by a photonic quantum state. Hyperentangled photon states (i.e. states showing entanglement in multiple DOFs) have demonstrated significant implications for both fundamental physics tests and quantum communication and computation. Increasing the number of qubits of photonic experiments requires miniaturization and integration of the basic elements and functions to guarantee the set-up stability. This motivates the development of technologies allowing the control of different photonic DOFs on a chip. Femtosecond laser writing on a glass makes possible to use both path and polarization of photon states enabling precise control of both degrees of freedom. We demonstrate the contextual use of path and polarization qubits propagating within a laser written integrated quantum circuit and use them to engineer a four qubit hyperentangled cluster state. We also characterized the cluster state and exploited it to perform the Grover's search algorithm following the one-way quantum computation model. In addition, we tested the non-local properties of the cluster state by using multipartite non-locality tests.
The investigation of multi-photon quantum interference in symmetric multi-port splitters has both fundamental and applicative interest. Destructive quantum interference in devices with specific symmetry leads to the suppression of a large number of possible output states, generalizing the Hong-Ou-Mandel effect; simple suppression laws have been developed for interferometers implementing the Fourier or the Hadamard transform over the modes. In fact, these enhanced interference features in the output distribution can be used to assess the indistinguishability of single-photon sources, and symmetric interferometers have been envisaged as benchmark or validation devices for Boson-Sampling machines. In this work we devise an innovative approach to implement symmetric multi-mode interferometers that realize the Fourier and Hadamard transform over the optical modes, exploiting integrated waveguide circuits. Our design is based on the optical implementations of the Fast-Fourier and Fast-Hadamard transform algorithms, and exploits a novel three-dimensional layout which is made possible by the unique capabilities of femtosecond laser waveguide writing. We fabricate devices with m = 4 and m = 8 modes and we let two identical photons evolve in the circuit. By characterizing the coincidence output distribution we are able to observe experimentally the known suppression laws for the output states. In particular, we characterize the robustness of this approach to assess the photons' indistinguishability and to rule out alternative non-quantum states of light. The reported results pave the way to the adoption of symmetric multiport interferometers as pivotal tools in the diagnostics and certification of quantum photonic platforms.
We experimentally observed in an optical setup and using full tomography process the so-called weak non-Markovian dynamics of a qubit . This was done implementing the collisional model proposed in  to investigate the non- Markovian dynamics of an open quantum system interacting with a carefully controlled environment state. We also observed the transition from weak to strong (essentially) non-Markovianity. In our all-optical setup, a single photon system, initially entangled in polarization with an ancilla, is made to interact with a sequence of liquid crystal retarders driven by proper electric pulses, which simulates the environment. Depending on how the voltage is applied on each liquid crystal, it will work as a half-wave plate with different orientations. Then, by changing properly the parameters of the qubit-environment interactions, the system dynamics can suffer a transition from weak to strong non-Markovianity. In the strong regime, the full reconstruction of the entangled state was made by single entanglement witness between system and ancilla, showing a backflow of information, while, in the weak regime, given the contractive unital map feature, we can only measure the dynamics by a full process tomography analysis, searching for the violation of the divisibility completely positive map criterion, what was done successfully.
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.
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 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.
Cluster states of two photons and four qubits, built on the double entanglement of two photons in the degrees
of freedom of polarization and linear momentum, have been used in the realization of a complete set of basic
operations of one-way quantum computation. Basic computation algorithms, namely, the Grover's search and
the Deutsch's algorithm, have been realized by using these states. Hyperentangled states of increasing size are
of paramount importance for the realization of even more complex algorithms and can be extended to a lager
number of degrees of freedom of the photons. Some recent results obtained with entangled states of two photons
and six qubits are presented.
Quantum states of two photons simultaneously entangled in polarization and linear momentum, namely hyper-entangled
or cluster states, allow to operate in a larger Hilbert space, since we can associate four qubits to two
photons. We describe how these states are generated, characterized and manipulated by linear optics technique.
Some recent results verifying that the ratio between the quantum and classical prediction grows with the size of
the Hilbert space are also presented in this work. Finally, we show the efficient realization of a C-NOT gate by
using two photon cluster states operating in the one-way model of quantum computation.
High fidelity, high count rate four qubit linear cluster states have been realized by using two photons that are
entangled both in polarization and linear momentum. Their properties have been investigated by evaluating the
entanglement witness and carrying out a "stronger two observer all versus nothing" test of quantum nonlocality.
The experimental results concerning the realization of single qubit rotations demonstrates the feasibility of
one-way quantum computation by using these states.
Two photon states entangled in polarization and momentum, hyper-entangled, have been generated by using linear optics and a single Type I nonlinear crystal. These states have been completely characterized and their nonlocal behaviour have been verified by independent Bell's inequalities tests performed in the two degrees of freedom of entanglement and by an "all versus nothing" test of local realism. The manipulation of these states may represent a useful control in quantum state engineering and Bell state measurements and, more in general, in Quantum Information applications.
Maximally entangled states, Werner state and maximally entangled mixed states (MEMS) have been created and fully characterized by a novel high brilliance universal source of entangled photon pairs with striking spatial characteristics. Mixed states of any structure, spanning a 2 x 2 Hilbert space may be created by this source. The non local properties of the generated entanglement have been tested by standard Bell measurements. Tunable Werner states and Maximally Entangled Mixed States (MEMS) have been created by an original patchwork technique and investigated by quantum tomography. The entropic and nonlocal properties of these states have been also undertaken.
A novel Mach-Zehnder interferometer terminated at two different frequencies which realizes for a single photon quantum state (qubit) the nonlinear frequency conversion has been realized. The information-preserving character of the nonlinear process allows to transfer the coherence of the input to the output state. The results of this experiment can have relevant applications in quantum information technology.
The process of two-dipole superradiance has been investigated by femtosecond excitation of two ensembles of dye molecules, located at a mutual distance R on the symmetry plane of a microcavity. In these conditions, superradiant coupling between the two objects can be established, giving rise to emission correlation effects, which have been investigated in the space-time domain.
Some ultrafast phenomena occurring in an active microcavity have been investigated. This device can behave as an efficient source of non-classical light, when a small number of molecules inside are excited by a femtosecond laser.In this way single photon n > states are generated with anon-classical sub-Poissonian distribution. Multiple excitations of a larger number of molecules can give rise to collective phenomena because of the strong super-radiant ultrafast coupling within the transverse region of the microcavity electromagnetic field. This process has been experimentally studied by means of a high efficiency, single photon, femtosecond non-linear optical gate.
The process of the spontaneous emission (SpE) from an active microscopic cavity (microcavity) is shown with emphasis on mirror separation of the order of the optical wavelength. The relevant effects of SpE enhancement and inhibition, non-exponential decay, and emission anisotropy are outlined for a cavity terminated by mirrors bearing either metal -- or semiconductor -- multilayered coatings. Finally, an experiment regarding the possibility of detecting the field distribution within the cavity of the emission wavelength is shown.