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. Integrated photonic circuits have a strong potential to perform quantum information processing. Indeed, 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. However, the technology for handling polarization encoded
qubits, the most commonly adopted approach, is still missing in quantum optical circuits. Our research is focused on the realization of integrated photonic devices able to support and
manipulate polarization encoded qubits [
PRL105,
NatComm2]. This task has been achieved by means of the ultrafast laser writing technique: a femtosecond
laser is focused on a glass substrate and light-guiding structures are produced by translating the substrate with respect to the laser beam.
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The standard encoding process of quantum information adopting the methods of quantum optics is based on the two-dimensional
space of photon polarizations (``spin'' angular momentum). Very recently the orbital angular momentum (OAM) of light,
associated to the transverse amplitude profile, has been recognized as a new promising resource,
allowing the implementation of a higher-dimensional
quantum space, or a ``qu-dit'', encoded in a single photon. Our research topic is based on the study of new
optical devices able to couple the orbital and spinorial components of the angular momentum.
Such devices could allow to manipulate efficiently and deterministically
the orbital angular momentum degree of freedom, exploiting both the polarization and the OAM advantages
[
PRL103].
This can open interesting perspectives in the implementation of new quantum information protocols.
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The aim of quantum sensing is to develop methods to extract the maximum amount of information from a system with minimal disturbance upon it. In the case of optical interferometry,
the parameter to be estimated is an optical phase introduced by a sample. Within this context, it has been shown that the possibility of exploting quantum resources can increase the
achievable precision going beyond the semiclassical regime of operation. This approach can have wide applications for minimally invasive sensing methods in order to extract the
maximum amount of information from a system with minimal disturbance. When dealing with the practical implementation of quantum-enhanced phase estimation protocols, these approaches
present some limitations due to their extreme fragility with respect to losses and decoherence. Our aim is to develop and implement phase estimation protocols tailored for their
application in a lossy scenario [
PRL105,
arXiv:1107.3726].
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Cluster state are a particular class of multiqubit entangled states.
Cluster state are the fundamental resource for a new model of Quantum Computation, the so called
One-way model.
in this model the algorithms are simply realized by a sequence of single qubits measurements
and feed- forward operations.
In our laboratory, by starting from an
hyperentangled state, i.e. a two photon state with two
entangled degrees of freedom such as the polarization and the linear momentum, we generated a four qubits
cluster state [
PRL98].
We then realized some basic one-way algorithms with this state
[
PRL100,
PRA78].
The aim of our project is to increase the dimensionality of the generated quantum states by
using more degrees of freedom of the photons.
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Micro-Macro Entanglement
In recent years two fundamental aspects of quantum mechanics have attracted a great deal of interest,
namely the investigation on the irreducible nonlocal properties of Nature implied by quantum entanglement
and the physical realization of the "Schroedinger Cat". The last concept, by applying the nonlocality property
to a combination of microscopic and macroscopic systems, enlightens the concept of the quantum state,
the dynamics of large systems and ventures into the most intriguing philosophical problem, i.e.
the emergence of quantum mechanics in the real life. Recently, we reported the generation of a
Macro-state consisting of N=3.5x10^4 photons in a quantum superposition and which is entangled with a
far apart single-photon state (Micro-state). Precisely, an entangled photon pair was created by a nonlinear
optical process, then one photon of the pair was injected into an optical parametric amplifier (OPA)
operating for any input polarization state, i.e. into a phase covariant cloning machine.
Multimode Entanglement
Increasing the entanglement between two photon is an important resource in quantum physics. It allows to increment the power of many quantum
information protocols, such as dense coding and to enhance the violation of Bell inequalities.
We realized a source of two photon entangled in many optical paths. Precisely, each photon can be emitted by SPDC along one of four
directions of the SPDC cone. In the picture it is shown the device able to couple the SPDC radiation into 8 single
mode fibers
[PRL102].
