Using the process of spontaneous parametric down-conversion in a two-crystal geometry, we have generated a source of polarization-entangled photon pairs that is more than ten times brighter, per unit of pump power, than previous sources, with another factor of 30 to 75 expected to be readily achievable. We have measured a high level of entanglement between photons emitted over a relatively large collection angle, and over a 10-nm bandwidth. As a demonstration of the source capabilities, we obtained a 242-\ensuremath{\sigma} violation of Bell's inequalities in less than three minutes, and observed near-perfect photon correlations when the collection efficiency was reduced. In addition, both the degree of entanglement and the state purity should be readily tunable.
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 θ generates two counterpropagating orthogonally polarized beams (around 1518 nm). The waveguide core is surrounded by distributed Bragg reectors 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 propose a compact high-intensity room-temperature source of entangled photons based on the efficient second-order process of two-photon spontaneous emission from electrically pumped semiconductor quantum wells in a photonic microcavity. Two-photon emission rate in room-temperature semiconductor devices is determined solely by the carrier density, regardless of the residual one-photon emission. The microcavity selects two-photon emission for a specific signal and idler wavelengths and at a preferred direction without modifying the overall rate. Pair-generation rate in
GaAs∕AlGaAs quantum well structure is estimated using a 14-band model to be 3 orders of magnitude higher than for traditional broadband parametric down-conversion sources.
Abstract The typical approaches to generate heralded single photons rely on parametric processes, with the advantage of generating highly entangled states at the price of a random pair emission. To overcome this limit, degenerate spontaneous Four-Wave-Mixing is a reliable technique which combines two pump photons into a pair of signal and idler photons via Kerr nonlinear optical effect. By exploiting the intrinsic small confinement volume and thermally tuning the resonances of a 20 μ m-long Photonic Crystal cavity, we efficiently generate time-energy entangled photon pairs and heralded single photons at a large maximum on-chip rate of 22 MHz, using 36 μ W of pump power. We measure time-energy entanglement with net visibility up to 96.6 % using 1 second integration time constant. Our measurements demonstrate the viability of Photonic Crystal cavities to act as an alternative and efficient photon pair source for quantum photonics.
On-chip generation of non-classical states of light is a fundamental building block for the development of quantum information [1]. A convenient method harnesses resonantly enhanced parametric processes, since they operate at room temperature. We consider here spontaneous Four-Wave-Mixing (FWM). The generation of time correlated, entangled photons pairs and heralded single photons via FWM has been demonstrated on silicon and on different III-V semiconductor ring resonators with very large efficiency [2], [3]. Here, an alternative type of cavity is used: a Photonic Crystal (PhC) cavity. These periodic structures offer the possibility to reduce the mode volume V in all-dielectric structure, with a micrometer-scale footprint, which strongly increases the efficiency of the nonlinear process
(∝Q3/V2)if high quality factor Q is preserved. We achieve it with a bichromatic Photonic Crystal cavity depicted in fig. 1.a). The cavity is located between two lines of holes with reduced period
a′compared to the rest of the holes with period
a; while light propagates in a waveguide created with a missing line of holes ended by a reflector [4]. This design imposes an effective parabolic potential for the optical field, that ultimately leads to equispaced resonances in frequency, which is a strict condition for efficient resonant enhancement of the interaction imposed by the energy conservation of FWM. The cavity is pumped with a CW laser with pump power level of
P0transferred to the cavity. A self thermal tuning technique is used to compensate for any residual frequency mismatch of the resonance triplet [4] after fabrication. After the chip, the generated photon pairs encounter an unbalanced Mach-Zehnder “Franson” interferometer [5]. At the output, they are time-energy entangled. They travel through a filtering stage to suppress any residual pump photons before being detected by superconducting nanowire single-photon detectors. The efficiency of our source is measured by bypassing the Franson interferometer. Taking into account the losses between the cavity and the detectors, it is estimated to be 16 GHz/mW2and is comparable to the state-of-the-art of semiconductor based resonators for photon pair generation (20 GHz/mW2, fig. 1.b)) [2]; while our average Q-factor (2.1 × 105) is five times smaller for this sample but could be larger (up to 7 × 105). Comparable efficiency is achieved because the resonator volume is at least one order of magnitude smaller than in ring resonators. Consequently, the footprint is very small
(<70μm2,≈600and 300 m2in [2] and [3]), which is an important asset for scalability. The entangled nature of the photon pair is quantified with two-photon interferences depicted in fig. 1.c). For pump power of 15
μW, large net visibility is measured up to 96.6 ± 4.2 %. This work shows that PhC cavities are novel and promising sources for integrated quantum optics.