Femtosecond laser writing shows great potential for novel 3D photonic architectures and high quality NV- quantum emitters in the bulk of diamond. However, the direct writing method cannot achieve nanometric placement of NV- centers near the surface of diamond, which is required for certain quantum sensing tasks. We will demonstrate a hybrid approach where the advantages of 3D optical waveguides by femtosecond laser writing and precise and shallow placement of NV- centers by ion implantation will be combined to form an integrated quantum sensor with record high performance.
Diamond’s nitrogen-vacancy (NV) center has been shown as a promising candidate for sensing applications and quantum computing because of its long electron spin coherence time and its ability to be found, manipulated and read out optically. An integrated photonics platform in diamond would be useful for NV-based magnetometry and quantum computing, in which NV centers are optically linked for long-range quantum entanglement due to the integration and stability provided by monolithic optical waveguides. Surface microchannels in diamond would be a great benefit for sensing applications, where NV centers could be used to probe biomolecules.
In this work, we applied femtosecond laser writing to form buried 3D optical waveguides in diamond. By engineering the geometry of the type II waveguide, we obtained single mode guiding from visible to the infrared wavelengths. Further, we demonstrate the first Bragg waveguide in bulk diamond with narrowband reflection. We show the formation of single, high quality NV centers on demand in ultrapure diamond using a single pulse from a femtosecond laser. With these building blocks in place, we fabricated an integrated quantum photonic circuit containing optical waveguides coupled to NV centers deterministically placed within the waveguide. The single NVs were excited and their emission collected by the optical waveguides, allowing easy interfacing to standard optical fibers. We also report high aspect ratio surface microchannels, which we will integrate with laser-written NVs and waveguides, paving the way for ultrasensitive, nanoscale resolution biosensors.
We present the experimental investigation of the coherence properties of the light produced by parametric down conversion in the macroscopic regime, also including pump depletion. In particular, we compare the results obtained in very similar geometric conditions by using two nonlinear crystals having different lengths. We observe that the number of generated photons, the size of spatio-spectral coherence areas, and the number of modes in the photon number statistics exhibit a similar behavior in the two crystals as a function of pump mean power, even if we notice that the absolute values are different. The available theory of parametric down conversion cannot account for these differences.
It is well known that optical twin-beam states (TWB) generated by spontaneous parametric down-conversion (PDC) exhibit spatial and spectral correlations, which can appear in single-shot images obtained by using an imaging spectrometer to resolve emission angles and wavelengths simultaneously. By analyzing series of single-shot images recorded by an EMCCD camera at different powers of the pump beam, we studied the evolution of several quantities characterizing the generated TWB. In particular, we demonstrated that correlation widths in spectrum and space increase monotonically at low pump powers and then start decreasing at higher powers due to the onset of pump depletion. In a complementary way, the Fedorov ratio decreases and then increases again. At the same time, the number of modes evaluated from photon statistics follows a complementary behavior to correlation widths that can be interpreted in terms of the evolution of the number of Schmidt modes in the field.
In this paper we report about the experimental investigation of the non factorable spatio-temporal correlation of
twin beams generated in parametric down conversion (PDC) at the crystal output. We present the correlation
features to be reconstructed by means of the inverse process of PDC, that is sum frequency generation, in a
scheme based on achromatic imaging. In particular we show the ultra-narrow temporal localization (6fs) observed
thanks to the huge spectral bandwidth detected in the near field of the crystal. We illustrate the deteriorating
effects of imperfect imaging conditions or spatial modes selection on the temporal correlation, giving evidence of
the interdependence of spatial and temporal degrees of freedom in PDC as claimed by the theory. Throughout
the paper we shall discuss about the characteristics of the experimental set-up being used for the investigation
of the twin beam correlation in both the temporal and spatial domain, highlighting the important features for
the success of the experiment and the demonstration of the X-shaped structure of the space-time correlation,
already emerging from preliminary results.
We investigate the spatio-temporal structure of the biphoton entanglement in Parametric Down Conversion
(PDC). In particular we study the biphoton amplitude at the output face of the nonlinear crystal (near-field)
and we demonstrate its X-shaped geometry in the space-time dimensions, i.e. the non-factorability of the state
with respect to spatial and temporal variables. Our analysis provides a precise and quantitative characterization
of this structure in various regimes and types of phase matching of PDC. The key elements of novelty emerging
from our analysis are the non-factorability of the state with respect to spatial and temporal variables, and the
extreme relative localization of the entangled photons, both in space (few microns) and time (few femtoseconds).
This extreme localization is connected to our ability to resolve the photon positions in the source near-field. The
non factorability opens the possibility of tailoring the temporal entanglement by acting on the spatial degrees of
freedom of twin photons.
We review and compare the results of quantum spatial correlation measurements in parametric down-conversion
in the high-gain pulsed regime, with preliminary measurements performed in the low-gain cw-pumped regime.
The diagnostics is based on a high quantum efficiency CCD camera, and in the second case the radiation pattern
is recorded after temporal integration of the "single-photon" spatial distribution. The effect of the detected
number of temporal modes on the accessibility of the sub-shot noise regime is discussed, together with the
identification of suitable regimes for weak image detection.
We propose an imaging scheme based on the quantum spatial correlation of twin beams generated by PDC,
and we show that it provides a substantial enhancement of the signal-to-noise ratio with respect to classical
schemes.
We propose an imaging scheme based on the quantum spatial correlation of twin beams generated by PDC,
and we show that it provides a substantial enhancement of the signal-to-noise ratio with respect to classical
schemes.
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