Integrated sources of entangled photons at telecom wavelength in femtosecond-laser-written circuits

Photon entanglement is an important state of light that is at the basis of many protocols in photonic quantum technologies, from quantum computing, to simulation and sensing. The capability to generate entangled photons in integrated waveguide sources is particularly advantageous due to the enhanced stability and more efficient light-crystal interaction. Here we realize an integrated optical source of entangled degenerate photons at telecom wavelength, based on the hybrid interfacing of photonic circuits in different materials, all inscribed by femtosecond laser pulses. We show that our source, based on spontaneous parametric down-conversion, gives access to different classes of output states, allowing to switch from path-entangled to polarization-entangled states with net visibilities above 0.92 for all selected combinations of integrated devices

Thermally-Reconfigurable Quantum Photonic Circuits at Telecom Wavelength by Femtosecond Laser Micromachining

The importance of integrated quantum photonics in the telecom band resides on the possibility of interfacing with the optical network infrastructure developed for classical communications. In this framework, femtosecond laser written integrated photonic circuits, already assessed for quantum information experiments in the 800 nm wavelength range, have great potentials. In fact these circuits, written in glass, can be perfectly mode-matched at telecom wavelength to the in/out coupling fibers, which is a key requirement for a low-loss processing node in future quantum optical networks. In addition, for several applications quantum photonic devices will also need to be dynamically reconfigurable. Here we experimentally demonstrate the high performance of femtosecond laser written photonic circuits for quantum experiments in the telecom band and we show the use of thermal shifters, also fabricated by the same femtosecond laser, to accurately tune them. State-of-the-art manipulation of single and two-photon states is demonstrated, with fringe visibilities greater than 95%. This opens the way to the realization of reconfigurable quantum photonic circuits on this technological platform

Machine learning for quantum metrology

Phase estimation represents a significant example to test the application of quantum theory for enhanced measurements of unknown physical parameters. Several recipes have been developed, allowing to define strategies to reach the ultimate bounds in the asymptotic limit of a large number of trials. However, in certain applications it is crucial to reach such bound when only a small number of probes is employed. Here, we discuss an asymptotically optimal, machine learning based, adaptive single-photon phase estimation protocol that allows us to reach the standard quantum limit when a very limited number of photons is employed

Experimental phase estimation enhanced by machine learning

Phase-estimation protocols provide a fundamental benchmark for the field of quantum metrology. The latter represents one of the most relevant applications of quantum theory, potentially enabling the capability of measuring unknown physical parameters with improved precision over classical strategies. Within this context, most theoretical and experimental studies have focused on determining the fundamental bounds and how to achieve them in the asymptotic regime where a large number of resources are employed. However, in most applications, it is necessary to achieve optimal precision by performing only a limited number of measurements. To this end, machine-learning techniques can be applied as a powerful optimization tool. Here, we implement experimentally single-photon adaptive phase-estimation protocols enhanced by machine learning, showing the capability of reaching optimal precision after a small number of trials. In particular, we introduce an approach for Bayesian estimation that exhibits best performance for a very low number of photons N. Furthermore, we study the resilience to noise of the tested methods, showing that the optimized Bayesian approach is very robust in the presence of imperfections. Application of this methodology can be envisaged in the more general multiparameter case, which represents a paradigmatic scenario for several tasks, including imaging or Hamiltonian learning

Hong–Ou–Mandel control through spectral shaping

Since its initial discovery, Hong-Ou-Mandel (HOM) interference has been a key tool in quantum-optical experiments, being at the basis of several quantum technology applications as well as being a relevant characterisation method for the quality of single photon sources. In the context of quantum interferometry, further applications can be enabled by improving the capability of tailoring such effect. Here, we report a proof-of-principle experiment on a novel HOM control approach, based on spectrally shaping photons generated by parametric down conversion sources. By means of a pulse shaper operating on the pump beam in the Fourier plane, selected frequencies can be cut off from the down conversion process, therefore shaping the HOM interference profile. We finally discuss the obtained results towards employing the method for practical application in interferometry

Quantum simulation of spin chain dynamics via integrated photonics

Summary form only given. Photonic circuits represent a promising platform to perform quantum simulation of several different physical phenomena. Indeed, large progresses have been achieved in the last few years due to the technological advances enabled by integrated photonics, which allowed to achieve a significant increase in the size of the implemented systems. Notable examples of observed phenomena in integrated circuits include Anderson localization [1] and transport mechanisms [2].[4]). We discuss the photonic simulation of spin chain dynamics after a quench in a 5-site system [3]. Such dynamics present the feature of entangling distant spins in pairs starting from the input (separable) Neel state, thus obtaining an amount of entanglement which is proportional to the number of sites present in the system (volume law [4]). The verification of such increase in the amount of the generated entanglement provides a useful resource for several quantum information protocols, including quantum teleportation and quantum networking

Quantum simulation of spin chain dynamics via integrated photonics

Summary form only given. Photonic circuits represent a promising platform to perform quantum simulation of several different physical phenomena. Indeed, large progresses have been achieved in the last few years due to the technological advances enabled by integrated photonics, which allowed to achieve a significant increase in the size of the implemented systems. Notable examples of observed phenomena in integrated circuits include Anderson localization [1] and transport mechanisms [2].[4]). We discuss the photonic simulation of spin chain dynamics after a quench in a 5-site system [3]. Such dynamics present the feature of entangling distant spins in pairs starting from the input (separable) Neel state, thus obtaining an amount of entanglement which is proportional to the number of sites present in the system (volume law [4]). The verification of such increase in the amount of the generated entanglement provides a useful resource for several quantum information protocols, including quantum teleportation and quantum networking