type-II Optical Parametric Oscillator Research Articles

Generation of continuous-variable high-dimensional entanglement with three degrees of freedom and multiplexing quantum dense coding

High-dimensional entanglement is a critical foundation for the growing demand for information capacity to implement the high-capacity quantum task. Here, we report continuous-variable high-dimensional entanglement with three degrees of freedom (frequency, polarization, and orbital angular momentum) directly generated with a single type-II optical parametric oscillator (OPO) cavity. By compensating both for dispersion in frequency modes and astigmatism in higher-order transverse modes, the OPO is capable of oscillating simultaneously and outputting thousands of entanglement pairs. The three degrees of freedom high-dimensional entanglement are verified simultaneously possessing frequency comb, spin, and orbital angular momentum entanglement via 14 pairs of Hermite–Gaussian mode correlations measurement. Then, the “space-frequency” multiplexing quantum dense coding communication is also demonstrated by using the entanglement resource. It shows the great superiority of high-dimensional entanglement in implementing the high-capacity quantum task. Apart from an increased channel capacity, it is possible to conduct deterministic high-dimensional quantum protocols, quantum imaging, and especially quantum computing.

Type-II PPRKTP optical parametric oscillators in the 2 µm spectral range

We experimentally explored the design parameter space for periodically poled R b : K T i O P O 4 (PPRKTP) structures in type-II optical parametric oscillators (OPOs) pumped at 1.064 µm and operating in the 1.8–2.5 µm range. This study identifies a set of Sellmeier expansions and thermooptic coefficients that can be used to accurately design the type-II PPRKTP structures in this spectral range. The performance of this OPO regime is investigated by employing different periodicities of PPRKTP, and the effective nonlinear coefficient is determined. Output at the mJ level with a bandwidth of better than 10 MHz is readily generated without optical damage. The temperature tuning rate of about 8 GHz/K of the central OPO frequency is substantially lower than in similar periodically poled lithium niobate devices and can be exploited for precise frequency tuning and easier thermal stabilization. The stimulated backward polariton scattering in the OPO cavities is also investigated and discussed.

SLM operation of a high repetition rate BBO Optical Parametric Oscillator pumped by DPSSL at 355 nm

Single longitudinal mode (SLM) operation of a high repetition rate (1 kHz) nanosecond Type-II BBO optical parametric oscillator (OPO), pumped by the third harmonic (355 nm) of a Diode Pumped Solid State Laser (DPSSL), is demonstrated using a single intra-cavity etalon. Implementation of the cylindrical focusing configuration of the pump beam along with its double passing geometry through the nonlinear OPO crystal has facilitated a low threshold (~0.7 mJ/pulse @355 nm) operation with narrow spectral linewidth (~150 GHz) output. Operations of the OPO at such a low threshold has enabled to maneuver insertion of an intra-cavity etalon for further reducing the spectral linewidth, which allows the OPO to provide SLM lasing output over the spectral range 500–600 nm. The spectral characteristics of the SLM OPO are studied in detail. The time-averaged spectral linewidth of the SLM OPO is measured to be less than 250 MHz.

Fine-tuning of orbital angular momentum in an optical parametric oscillator

We investigate the dynamics of a type-II optical parametric oscillator pumped by a structured light beam carrying orbital angular momentum. Different operating regimes are theoretically derived and experimentally demonstrated, depending on the magnitude of the anisotropy imposed by the nonlinear medium. In a weakly anisotropic regime, fine-tuning of the orbital angular momentum transfer is obtained by adjusting the crystal parameters, such as temperature and orientation. Under strong anisotropy, orbital angular momentum transfer is not possible and the crystal tuning leads to an abrupt switching between different Hermite–Gauss transverse modes.

Conditions for optical parametric oscillation with a structured light pump

We investigate the transverse mode structure of the down-converted beams generated by a type-II optical parametric oscillator (OPO) driven by a structured pump. Our analysis focus on the selection rules imposed by the spatial overlap between the transverse modes of the three fields involved in the non-linear interaction. These rules imply a hierarchy of oscillation thresholds that determine the possible transverse modes generated by the OPO, as remarkably confirmed with experimental results.

Different operational meanings of continuous variable Gaussian entanglement criteria and Bell inequalities

Entanglement, one of the most intriguing aspects of quantum mechanics, marks itself into different features of quantum states. For this reason different criteria can be used for verifying entanglement. In this paper we review some of the entanglement criteria casted for continuous variable states and link them to peculiar aspects of the original debate on the famous Einstein–Podolsky–Rosen (EPR) paradox. We also provide a useful expression for valuating Bell-type non-locality on Gaussian states. We also present the experimental measurement of a particular realization of the Bell operator over continuous variable entangled states produced by a sub-threshold type-II optical parametric oscillators (OPOs).

Survival of continuous variable entanglement over long distances

Any quantum system, in interacting with the external classical environment, slowly loses its quantumness and entanglement degrades. In this paper, we experimentally found that, even in the presence of a strong decoherence, the state generated by a sub-threshold type-II optical parametric oscillator never disentangles, keeps breaking the quantum limit for the discord and, as a resource for quantum teleportation of a coherent state, would allow us, in principle, to realize quantum teleportation over an infinitely long Gaussian channel.

Experimental analysis of decoherence in continuous-variable bipartite systems

Quantum properties are soon subject to decoherence once the quantum system interacts with the classical environment. In this paper we experimentally test how propagation losses, in a Gaussian channel, affect the bipartite Gaussian entangled state generated by a subthreshold type-II optical parametric oscillator. Experimental results are discussed in terms of different quantum markers, as teleportation fidelity, quantum discord, and mutual information, and continuous-variable (CV) entanglement criteria. To analyze state properties we have retrieved the composite system covariance matrix by a single homodyne detector. We experimentally found that, even in the presence of a strong decoherence, the generated state never disentangles and keeps breaking the quantum limit for the discord. This result proves that the class of CV entangled states discussed in this paper would allow, in principle, to realize quantum teleportation over an infinitely long Gaussian channel.

A new method for locking the signal-field phase difference in a type-II optical parametric oscillator above threshold

We propose and demonstrate a new method for phaselocking the signal fields emitted above threshold by a nondegenerate, type-II optical parametric oscillator (OPO). This method is based on the observation that amplitude modulation of the pump beam produces a related modulation of the frequency difference of the OPO signals via the temperature-tuning of the index of refraction in the nonlinear crystal. We successfully use pump modulation as a correction for phase-difference locking of the OPO signals and observe a 1 kHz beat note stable over more than 10 s, both figures solely limited by the measurement time. This method eliminates the need for applying electronic phase-correction signals directly to the nonlinear crystal which caused crystal damage in a previous phaselocking technique.

Characterization of bipartite Gaussian states from OPO

In a recent paper by D'Auria et al (2009 Phys. Rev. Lett.102 020502) we reported the full experimental reconstruction of Gaussian entangled states generated by a type-II optical parametric oscillator (OPO) below threshold using a single homodyne detector. Here we investigate more deeply the Gaussian character of the OPO output addressing the homodyne traces. More precisely, we apply a suitable normality test to check the actual Gaussian distribution of the homodyne data and then we perform the full reconstruction of the signal.

Tunable and coherent nanosecond 7.2–12.2 μm mid-infrared generation based on difference frequency mixing in ZnGeP2 crystal

A coherent mid-infrared laser source, which can be tuned from 7.2 μm to 12.2 μm based on the type-II phase-matched difference frequency generation (DFG) in an uncoated ZnGeP2 (ZGP) crystal, is reported. The two pump waves are from a type-II phase-matched dual-wavelength KTP optical parametric oscillator (OPO) of which the signal and idler waves are tuned during 1.85–1.96 μm (extraordinary wave) and 2.5–2.33 μm (ordinary wave), respectively. The maximum energy of the generated mid-infrared laser is 10 μJ at 9.22 μm, corresponding to the peak power of 2.2 kW.

Quantum characterization of bipartite Gaussian states

Gaussian bipartite states are basic tools for the realization of quantum information protocols with continuous variables. Their complete characterization is obtained by the reconstruction of the corresponding covariance matrix. Here we describe in details and experimentally demonstrate a robust and reliable method to fully characterize bipartite optical Gaussian states by means of a single homodyne detector. We have successfully applied our method to the bipartite states generated by a sub-threshold type-II optical parametric oscillator which produces a pair of thermal cross-polarized entangled CW frequency degenerate beams. The method provide a reliable reconstruction of the covariance matrix and allows to retrieve all the physical information about the state under investigation. These includes observable quantities, as energy and squeezing, as well as non observable ones as purity, entropy and entanglement. Our procedure also includes advanced tests for Gaussianity of the state and, overall, represents a powerful tool to study bipartite Gaussian state from the generation stage to the detection one.

Dynamics of a self-phase-locked nondegenerate optical parametric oscillator with nonsymmetric feedback loops

We discuss the evolution of two-mode states of a self-phase-locked type-II nondegenerate optical parametric oscillator with homodyne-mediated quantum feedback. It is shown that different strengths of the feedback loops can simulate the effect of a squeezed thermal environment and lead additionally to the process of the parametric-down conversion for each mode. In the steady state regime all initial density matrices evolve towards a Gaussian equilibrium state. It is shown that the nonsymmetric feedback can further increase entanglement of the equilibrium state. Dynamical behaviour of intracavity entanglement for a number of initial states is investigated, and entanglement sudden death and birth phenomena, and a delay birth of entanglement are reported.

Full Characterization of Gaussian Bipartite Entangled States by a Single Homodyne Detector

We present the full experimental reconstruction of Gaussian entangled states generated by a type-II optical parametric oscillator below threshold. Our scheme provides the entire covariance matrix using a single homodyne detector and allows for the complete characterization of bipartite Gaussian states, including the evaluation of purity, entanglement, and nonclassical photon correlations, without a priori assumptions on the state under investigation. Our results show that single homodyne schemes are convenient and robust setups for the full characterization of optical parametric oscillator signals and represent a tool for quantum technology based on continuous variable entanglement.

Transfer of orbital angular momentum in a multimode parametric oscillator

We investigate the transfer of orbital angular momentum (OAM) in a type-II optical parametric oscillator (OPO). We produce different kinds of transverse modes on the downconverted beams by pumping the OPO with a first-order Laguerre-Gaussian beam well above threshold. Different operation regimes are attained showing a rich variety of transverse modes bearing OAM.

Bright filter-free source of indistinguishable photon pairs

We demonstrate a high-brightness source of pairs of indistinguishable photons based on a type-II phase-matched doubly-resonant optical parametric oscillator operated far below threshold. The cavityenhanced down-conversion output of a PPKTP crystal is coupled into two single-mode fibers with a mode coupling efficiency of 58%. The high degree of indistinguishability between the photons of a pair is demonstrated by a Hong-Ou-Mandel interference visibility of higher than 90% without any filtering at an instantaneous coincidence rate of 450,000 pairs/s per mW of pump power per nm of down-conversion bandwidth. For the degenerate spectral mode with a linewidth of 7 MHz at 795 nm a rate of 70 pairs/(s mW MHz) is estimated, increasing the spectral brightness for indistinguishable photons by two orders of magnitude compared to similar previous sources.

Experimental demonstration of frequency-degenerate bright EPR beams with a self-phase-locked OPO

We report the first experimental observation of bright EPR beams produced by a type-II optical parametric oscillator operating above threshold at frequency degeneracy. The degenerate operation is obtained by introducing a birefringent plate inside the cavity resulting in phase locking. After filtering the pump noise, which plays a critical role, continuous-variable EPR correlations between the orthogonally polarized signal and idler beams are demonstrated.

Continuous-wave phase-sensitive parametric image amplification

We study experimentally parametric amplification in the continuous regime using a transverse-degenerate type-II Optical Parametric Oscillator operated below threshold. We demonstrate that this device is able to amplify either in the phase insensitive or phase sensitive way first a single mode beam, then a multimode image. Furthermore the total intensities of the amplified image projected on the signal and idler polarizations are shown to be correlated at the quantum level.

Non-linear and quantum optics of a type II OPO containing a birefringent element

We describe theoretically the quantum properties of a type-II Optical Parametric Oscillator containing a birefringent plate which induces a linear coupling between the orthogonally polarized signal and idler beams and results in phase locking between these two beams. As in a standard OPO, the signal and idler waves show large quantum correlations which can be measured experimentally due to the phase locking between the two beams. We study the influence of the waveplate on the various criteria characterizing quantum correlations. We show in particular that the quantum correlations can be maximized by using optimized quadratures.

Non-linear and quantum optics of a type II OPO containing a birefringent element

We describe theoretically the quantum properties of a type-II Optical Parametric Oscillator containing a birefringent plate which induces a linear coupling between the orthogonally polarized signal and idler beams and results in phase locking between these two beams. As in a standard OPO, the signal and idler waves show large quantum correlations which can be measured experimentally due to the phase locking between the two beams. We study the influence of the waveplate on the various criteria characterizing quantum correlations. We show in particular that the quantum correlations can be maximized by using optimized quadratures.