We demonstrate a chip-integrated semiconductor source that combines polarization and frequency entanglement, allowing the generation of entangled biphoton states in a hybrid degree of freedom without postmanipulation. Our AlGaAs device is based on type-II spontaneous parametric down-conversion (SPDC) in a counterpropagating phase-matching scheme, in which the modal birefringence lifts the degeneracy between the two possible nonlinear interactions. This allows the direct generation of polarization-frequency entangled photons, at room temperature and telecom wavelength, and in two distinct spatial modes, offering enhanced flexibility for quantum information protocols. The state entanglement is quantified by a combined measurement of the joint spectrum and Hong-ou-Mandel interference of the biphotons, allowing to reconstruct a restricted density matrix in the hybrid polarization-frequency space.

High-dimensional degrees of photons – such as orbital angular momentum, spatial modes of frequency – provide a powerful mean to increase the density and security of quantum communication, and to enhance flexibility in quantum computing. In particular, the spatial degree of freedom is particularly suited to on-chip integration. Rapid progress has been made in recent years to develop integrated circuits achieving on-chip quantum interference, entanglement and gate operations on spatially encoded states, culminating in the demonstration of the Boson Sampling task on-chip [1]. While many of these demonstrations have relied on external sources to generate quantum states of light, which were then fed into a passive circuitry, a next step has been taken recently with the demonstration of active chips combining the generation and manipulation of spatially encoded states [2].

In this Letter, we report on the fabrication and characterization of a monolithic III–V semiconductor photonic chip, designed to perform nonlinear parametric optical processes for frequency conversion and non-classical state generation. This chip co-integrates an AlGaAs microdisk that is evanescently coupled to two distinct suspended waveguides designed for light injection and collection around 1600 nm and 800 nm, respectively. Quasi-phase matching provided by the resonator geometry and material symmetry, resonant field enhancement, and confinement ensure efficient nonlinear interactions. We demonstrate second-harmonic generation efficiency of 5 % W − 1 and a biphoton generation rate of 1.2 kHz/µW through spontaneous down-conversion.

We show that the characterization of the quantum correlations generated by a photon-pair source can be directly performed via a classical measurement leading to an unprecedented spectral resolution and a shorter integration time.

In recent years, great efforts have been devoted to the miniaturization of quantum information technology on semiconductor chips. In the context of photon pair sources, the bi-exciton cascade of a quantum dot and the four wave mixing in a Silicon waveguide have been used to demonstrate the generation of entangled states. Spontaneous parametric down-conversion in III-V semiconductor waveguides combines the advantages of room temperature and telecom wavelength operation, while keeping open the possibility of electrically pumping of the device. Here we present a source consisting of a multilayer AlGaAs waveguide grown on a GaAs substrate and then chemically etched to achieve lateral confinement in a ridge. The structure design is such that a pump beam (around 759 nm), impinging on the waveguide surface with an incidence angle theta generates two counterpropagating orthogonally polarized beams (around 1518 nm). The waveguide core is surrounded by distributed Bragg reflectors to enhance the pump field within the device. We demonstrate the direct emission of polarization entangled photons by pumping the device at two symmetric angles of incidence corresponding to frequency degeneracy and performing a quantum tomography measurement. Most common entanglement witnesses are satisfied and a raw fidelity of F = 0.86 to a Bell state is obtained. These results open the route to the demonstration of other interesting features of our device such as the generation of hyper-entangled states via the control of the frequency correlation degree through the spatial and spectral pump beam profile, leading to a new generation of completely integrated devices for quantum information.

We demonstrate the generation of polarization entangled Bell states at room temperature and telecom wavelength on a 3-5 semiconductor chip. A theoretical model provides ways to understand and control the amount of entanglement.

The miniaturization of quantum information technology is a subject attracting a growing attention. The exploitation of spontaneous parametric down conversion in AlGaAs waveguides to generate photon pairs presents several advantages: high nonlinear susceptibility, room-temperature operation and high emission directionality in the telecom range. In this work we will present our recent results on three different kinds of AlGaAs devices: a selectively oxidized source based on form birefringence, a waveguide based on modal phase matching and a microcavity-based source based on counterpropagating phase matching. We will discuss and compare the figures of merit characterizing the three devices for quantum communication applications.

In the last few years considerable effort has been devoted to the miniaturization of quantum information technology on semiconductor chips; in addition, recent developments in quantum information theory have roused a growing interest in 'generalized' states of frequency correlation. Parametric generation in semiconductor waveguides allows roomtemperature operation in the telecom range. We propose and compare some microcavity-based schemes for the generation of counterpropagating photon pairs and we experimentally demonstrate a bright source emitting 1.2 × 10-11pairs/pump photon for a 1.8 mm long waveguide. The indistiguishability of the photons of the pair is measured via a Hong-Ou-Mandel two-photon interference experiment showing a visibility of 85 %. The versatility of the source to control the generated two-photon state is also discussed.

S. Barbieri, P. Gellie, M. Ravaro, G. Santarelli, Lu Ding, W. Maineult, J-F. Lampin, P. Filloux, C. Manquest, C. Sirtori, R. Colombelli, S. P. Khanna, E. H. Linfield, A. G. Davies, H. Beere, D. Ritchie, Laboratoire MPQ, Universite Paris 7and CNRS UMR 7162, 75205 Paris, France LNE-SYRTE, CNRS, UPMC, Observatoire de Paris, 75014 Paris, France IEMN, UMR CNRS 8520 Universite de Lille 1, 59652 Villeneuve d’Ascq, France Laboratoire IEF, Universite Paris Sud and CNRS, UMR 8622, 91405 Orsay, France School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, UK Cavendish Laboratory, Cambridge CB3 0HE, UK

We experimentally demonstrate an integrated semiconductor ridge microcavity source of counterpropagating twin photons at room temperature in the telecom range. Based on type II parametric down conversion with a counterpropagating phase-matching, pump photons generate photon pairs with an efficiency of about 10(-11) and a spectral linewidth of 0.3 nm for a 1 mm long sample. The indistiguishability of the photons of the pair is measured via a Hong-Ou-Mandel two-photon interference experiment showing a visibility of 85 %. This work opens a route towards new guided-wave semiconductor quantum devices.

Terahertz pulse generation is demonstrated by a resonant femtosecond interband excitation of the miniband of a quantum-cascade-laser. The laser gain is subsequently used to amplify the terahertz pulse generated as it propagates through the cavity.

Terahertz time-domain spectroscopy is used to determine the gain of terahertz quantum-cascade lasers. Gain clamping and spectral gain narrowing are observed above threshold, giving insight into subband alignment as a function of the applied voltage.

We report on microwave impedance measurements of metal-metal ridge-waveguide terahertz quantum cascade lasers. Experimental data, recorded at 4 K in the 100 MHz–55 GHz range, are well reproduced by distributed-parameter transmission-line simulations, showing that the modulation cutoff is limited by the propagation losses that increase for higher microwave frequencies, yielding a 3 dB modulation bandwidth of ∼70 GHz for a 1 mm-long ridge. By using a shunt-stub matching we demonstrate amplitude modulation of a 2.3 THz QCL up to 24 GHz.

The amplification of spontaneous emission is used to initiate laser action. As the phase of spontaneous emission is random, the phase of the coherent laser emission (the carrier phase) will also be random each time laser action begins. This prevents phase-resolved detection of the laser field. Here, we demonstrate how the carrier phase can be fixed in a semiconductor laser: a quantum cascade laser (QCL). This is performed by injection seeding a QCL with coherent terahertz pulses, which forces laser action to start on a fixed phase. This permits the emitted laser field to be synchronously sampled with a femtosecond laser beam, and measured in the time domain. We observe the phase-resolved buildup of the laser field, which can give insights into the laser dynamics. In addition, as the electric field oscillations are directly measured in the time domain, QCLs can now be used as sources for time-domain spectroscopy.

At Terahertz (THz) frequencies metals are still excellent materials to guide and confine electromagnetic radiation with relatively low losses. Therefore the concepts developed in the microwave range to design efficient waveguides and resonators can be successfully transferred up to this frequency region. A successful example of such "technology transfer" is the so-called metal-metal resonator, effectively used as a waveguide for THz Quantum Cascade Lasers (QCLs). This type of resonator is essentially a downscaled version of a microstrip waveguide, widely used at microwave frequencies. In this work we report on microwave impedance measurements of metal-metal ridge-waveguide THz QCLs. Experimental data, recorded at 4K in the 100MHz-55GHz range, are well reproduced by distributed-parameter transmission-line simulations, showing that the modulation cutoff is limited by the propagation losses that increase for higher microwave frequencies, yielding a 3dB modulation bandwidth of ~70GHz for a 1mm-long ridge. By using a shunt-stub matching we demonstrate amplitude modulation of a 2.3THz QCL up to 24GHz. In the last part of this work we discuss the experimental evidence of a feedback-coupling between the intracavity THz field and the microwave field generated by the beating of the Fabry-Perot longitudinal modes above the lasing threshold.

We report on the high frequency modulation of Terahertz (THz) quantum cascade lasers (QCLs). By resonantly enhancing the frequency response of the RF biasing circuit we show that 4mm and 1.5mm-long metal-metal waveguide 2.3THz QCLs, can be modulated up to 21 and 24GHz respectively.

A 2.8 THz metal-metal quantum cascade laser operating with a microtransverse-electromagnetic-horn antenna has been demonstrated. By comparing the far-field radiation pattern to a standard ridge cavity with a cleaved facet, a striking improvement of the directionality is observed. The effectiveness of the horn antenna is critically dependent on the control of the lateral modes in the laser cavity.

The spectral gain of bound-to-continuum terahertz quantum cascade lasers (QCLs) is measured as a function of current density using terahertz time-domain spectroscopy. During lasing action the full width at half maximum (FWHM) of the gain is found to monotonically decrease with increasing current density until lasing action stops at which point the FWHM reaches a minimum (0.22 THz for a laser operating at 2.1 THz). Band structure calculations show that the spectral gain narrowing is due to the alignment and misalignment of the injector with the active region as a function of the applied bias field.