Direct Bell states generation on a III-V semiconductor chip at room temperature

Steady Bell State Generation via Magnon-Photon Coupling

We show that parity-time (PT) symmetry can be spontaneously broken in the recently reported energy level attraction of magnons and cavity photons. In the PT-broken phase, the magnon and photon form a high-fidelity Bell state with maximum entanglement. This entanglement is steady and robust against the perturbation of the environment, which is in contrast to the general wisdom that expects instability of the hybridized state when the symmetry is broken. This anomaly is further understood by the compete of non-Hermitian evolution and particle number conservation of the hybrid system. As a comparison, neither PT-symmetry breaking nor steady magnon-photon entanglement is observed inside the normal level repulsion case. Our results may open an exciting window to utilize magnon-photon entanglement as a resource for quantum information science.

Photonic quantum information processing: a review

Photonic quantum technologies represent a promising platform for several applications, ranging from long-distance communications to the simulation of complex phenomena. Indeed, the advantages offered by single photons do make them the candidate of choice for carrying quantum information in a broad variety of areas with a versatile approach. Furthermore, recent technological advances are now enabling first concrete applications of photonic quantum information processing. The goal of this manuscript is to provide the reader with a comprehensive review of the state of the art in this active field, with a due balance between theoretical, experimental and technological results. When more convenient, we will present significant achievements in tables or in schematic figures, in order to convey a global perspective of the several horizons that fall under the name of photonic quantum information.

Atom-mediated spontaneous parametric down-conversion in periodic waveguides

We propose the concept of atom-mediated spontaneous parametric down-conversion, in which photon-pair generation can take place only in the presence of a single two-level emitter, relying on the bandgap evanescent modes of a nonlinear periodic waveguide. Using a guided signal mode, an evanescent idler mode, and an atom-like emitter with the idler's transition frequency embedded in the structure, we find a heralded excitation mechanism, in which the detection of a signal photon outside the structure heralds the excitation of the embedded emitter. We use a rigorous Green's function quantization method to model this heralding mechanism in a 1D periodic waveguide and determine its robustness against losses.

Integrated sources of photon quantum states based on nonlinear optics

The ability to generate complex optical photon states involving entanglement between multiple optical modes is not only critical to advancing our understanding of quantum mechanics but will play a key role in generating many applications in quantum technologies. These include quantum communications, computation, imaging, microscopy and many other novel technologies that are constantly being proposed. However, approaches to generating parallel multiple, customisable bi- and multi-entangled quantum bits (qubits) on a chip are still in the early stages of development. Here, we review recent advances in the realisation of integrated sources of photonic quantum states, focusing on approaches based on nonlinear optics that are compatible with contemporary optical fibre telecommunications and quantum memory platforms as well as with chip-scale semiconductor technology. These new and exciting platforms hold the promise of compact, low-cost, scalable and practical implementations of sources for the generation and manipulation of complex quantum optical states on a chip, which will play a major role in bringing quantum technologies out of the laboratory and into the real world.

Polarization-entangled photon pair sources based on spontaneous four wave mixing assisted by polarization mode dispersion

Photonic-based qubits and integrated photonic circuits have enabled demonstrations of quantum information processing (QIP) that promises to transform the way in which we compute and communicate. To that end, sources of polarization-entangled photon pair states are an important enabling technology. However, such states are difficult to prepare in an integrated photonic circuit. Scalable semiconductor sources typically rely on nonlinear optical effects where polarization mode dispersion (PMD) degrades entanglement. Here, we directly generate polarization-entangled states in an AlGaAs waveguide, aided by the PMD and without any compensation steps. We perform quantum state tomography and report a raw concurrence as high as 0.91 ± 0.01 observed in a 1,100-nm-wide waveguide. The scheme allows direct Bell state generation with an observed maximum fidelity of 0.90 ± 0.01 from another (800-nm-wide) waveguide. Our demonstration paves the way for sources that allow for the implementation of polarization-encoded protocols in large-scale quantum photonic circuits.

Semiconductor devices for entangled photon pair generation: a review

Entanglement is one of the most fascinating properties of quantum mechanical systems; when two particles are entangled the measurement of the properties of one of the two allows the properties of the other to be instantaneously known, whatever the distance separating them. In parallel with fundamental research on the foundations of quantum mechanics performed on complex experimental set-ups, we assist today with bourgeoning of quantum information technologies bound to exploit entanglement for a large variety of applications such as secure communications, metrology and computation. Among the different physical systems under investigation, those involving photonic components are likely to play a central role and in this context semiconductor materials exhibit a huge potential in terms of integration of several quantum components in miniature chips. In this article we review the recent progress in the development of semiconductor devices emitting entangled photons. We will present the physical processes allowing the generation of entanglement and the tools to characterize it; we will give an overview of major recent results of the last few years and highlight perspectives for future developments.

Temporally versatile polarization entanglement from Bragg reflection waveguides

Bragg reflection waveguides emitting broadband parametric downconversion (PDC) have been proven to be well suited for the on-chip generation of polarization entanglement in a straightforward fashion [Sci. Rep.3, 2314 (2013)SRWSDA2045-232210.1038/srep02314]. Here, we investigate how the properties of the created states can be modified by controlling the relative temporal delay between the pair of photons created via PDC. Our results offer an easily accessible approach for changing the coherence of the polarization entanglement, in other words, to tune the phase of the off-diagonal elements of the density matrix. Furthermore, we provide valuable insight into the engineering of these states directly at the source.

Bright nanoscale source of deterministic entangled photon pairs violating Bell's inequality

Global, secure quantum channels will require efficient distribution of entangled photons. Long distance, low-loss interconnects can only be realized using photons as quantum information carriers. However, a quantum light source combining both high qubit fidelity and on-demand bright emission has proven elusive. Here, we show a bright photonic nanostructure generating polarization-entangled photon pairs that strongly violates Bell’s inequality. A highly symmetric InAsP quantum dot generating entangled photons is encapsulated in a tapered nanowire waveguide to ensure directional emission and efficient light extraction. We collect ~200 kHz entangled photon pairs at the first lens under 80 MHz pulsed excitation, which is a 20 times enhancement as compared to a bare quantum dot without a photonic nanostructure. The performed Bell test using the Clauser-Horne-Shimony-Holt inequality reveals a clear violation (S_CHSH > 2) by up to 9.3 standard deviations. By using a novel quasi-resonant excitation scheme at the wurtzite InP nanowire resonance to reduce multi-photon emission, the entanglement fidelity (F = 0.817 ± 0.002) is further enhanced without temporal post-selection, allowing for the violation of Bell’s inequality in the rectilinear-circular basis by 25 standard deviations. Our results on nanowire-based quantum light sources highlight their potential application in secure data communication utilizing measurement-device-independent quantum key distribution and quantum repeater protocols.

Generation of Counterpropagating Path-Entangled Photon Pairs in a Single Periodic Waveguide

We propose the use of nonlinear periodic waveguides for direct and fully integrated generation of counterpropagating photon pairs by spontaneous parametric down-conversion. Using the unique properties of Bloch modes in such periodic structures, we furthermore show that two counterpropagating phase-matching conditions can be fulfilled simultaneously, allowing for the generation of path-entangled Bell states in a single periodic waveguide. To demonstrate the feasibility of our proposal, we design a photonic crystal slab waveguide made of lithium niobate and numerically demonstrate Bell-state generation.

Monolithic semiconductor chips as a source for broadband wavelength-multiplexed polarization entangled photons

Generating entangled photons from a monolithic chip is a major milestone towards real-life applications of optical quantum information processing including quantum key distribution and quantum computing. Ultrabroadband entangled photons are of particular interest to various applications such as quantum metrology and multi-party entanglement distribution. In this work, we demonstrate the direct generation of broadband wavelength-multiplexed polarization entangled photons from a semiconductor chip for the first time. Without the use of any off-chip compensation or interferometry, entangled photons with a signal-idler separation as large as 95 nm in the telecom band were observed. The highest concurrence of 0.98±0.01 achieved in this work is also the highest, to the best of our knowledge, comparing to all previously demonstrated semiconductor waveguide sources. This work paves the way for fully integrated, ultrabroadband sources of polarization entangled photons.

Energy correlations of photon pairs generated by a silicon microring resonator probed by Stimulated Four Wave Mixing

Compact silicon integrated devices, such as micro-ring resonators, have recently been demonstrated as efficient sources of quantum correlated photon pairs. The mass production of integrated devices demands the implementation of fast and reliable techniques to monitor the device performances. In the case of time-energy correlations, this is particularly challenging, as it requires high spectral resolution that is not currently achievable in coincidence measurements. Here we reconstruct the joint spectral density of photons pairs generated by spontaneous four-wave mixing in a silicon ring resonator by studying the corresponding stimulated process, namely stimulated four wave mixing. We show that this approach, featuring high spectral resolution and short measurement times, allows one to discriminate between nearly-uncorrelated and highly-correlated photon pairs.

Correlated photon pair generation in AlGaAs nanowaveguides via spontaneous four-wave mixing

We demonstrate a source of correlated photon pairs which will have applications in future integrated quantum photonic circuits. The source utilizes spontaneous four-wave mixing (SFWM) in a dispersion-engineered nanowaveguide made of AlGaAs, which has merits of negligible two-photon absorption and low spontaneous Raman scattering (SpRS). We observe a coincidence-to-accidental (CAR) ratio up to 177, mainly limited by propagation losses. Experimental results agree well with theoretical predictions of the SFWM photon pair generation and the SpRS noise photon generation. We also study the effects from the SpRS, propagation losses, and waveguide lengths on the quality of our source.

Coherent dynamics of a telecom-wavelength entangled photon source

Quantum networks can interconnect remote quantum information processors, allowing interaction between different architectures and increasing net computational power. Fibre-optic telecommunications technology offers a practical platform for routing weakly interacting photonic qubits, allowing quantum correlations and entanglement to be established between distant nodes. Although entangled photons have been produced at telecommunications wavelengths using spontaneous parametric downconversion in nonlinear media, as system complexity increases their inherent excess photon generation will become limiting. Here we demonstrate entangled photon pair generation from a semiconductor quantum dot at a telecommunications wavelength. Emitted photons are intrinsically anti-bunched and violate Bell's inequality by 17 standard deviations High-visibility oscillations of the biphoton polarization reveal the time evolution of the emitted state with exceptional clarity, exposing long coherence times. Furthermore, we introduce a method to evaluate the fidelity to a time-evolving Bell state, revealing entanglement between photons emitted up to 5 ns apart, exceeding the exciton lifetime.

Effect of scattering loss on connections between classical and quantum processes in second-order nonlinear waveguides

We show that a useful connection exists between spontaneous parametric downconversion (SPDC) and sum frequency generation in nonlinear optical waveguides with arbitrary scattering loss, while the same does not hold true for SPDC and difference frequency generation. This result refines the relationship between these quantum and classical second-order nonlinear optical processes in waveguides, and identifies the most accurate characterization of a waveguide's quantum performance in the presence of loss based solely on classical measurements.

High-visibility two-photon interference of frequency-time entangled photons generated in a quasi-phase-matched AlGaAs waveguide

We demonstrate experimentally the frequency-time entanglement of photon pairs produced in a CW-pumped quasi-phased-matched AlGaAs superlattice waveguide. A visibility of 96.0±0.7% without background subtraction has been achieved, which corresponds to the violation of the Bell inequality by 52 standard deviations.

An on-chip coupled resonator optical waveguide single-photon buffer

Integrated quantum optical circuits are now seen as one of the most promising approaches with which to realize single-photon quantum information processing. Many of the core elements for such circuits have been realized, including sources, gates and detectors. However, a significant missing function necessary for photonic quantum information processing on-chip is a buffer, where single photons are stored for a short period of time to facilitate circuit synchronization. Here we report an on-chip single-photon buffer based on coupled resonator optical waveguides (CROW) consisting of 400 high-Q photonic crystal line-defect nanocavities. By using the CROW, a pulsed single photon is successfully buffered for 150 ps with 50-ps tunability while maintaining its non-classical properties. Furthermore, we show that our buffer preserves entanglement by storing and retrieving one photon from a time-bin entangled state. This is a significant step towards an all-optical integrated quantum information processor.

Photonic circuits are a promising route to developing scalable quantum technologies, if all the necessary components can be built. Using coupled resonator optical waveguides, Takesue et al. present an on-chip single-photon buffer that can delay one photon from a pair for 150 ps while preserving entanglement.

Second-harmonic generation in AlGaAs microdisks in the telecom range

We report on second-harmonic generation in whispering-gallery-mode AlGaAs microcavities suspended on a GaAs pedestal. Frequency doubling of a 1.58 μm pump is observed with 7×10(-4)   W(-1) conversion efficiency. This device can be integrated in a monolithic photonic chip for classical and quantum applications in the telecom band.

Electrically injected photon-pair source at room temperature

One of the main challenges for future quantum information technologies is the miniaturization and integration of high performance components in a single chip. In this context, electrically driven sources of nonclassical states of light have a clear advantage over optically driven ones. Here we demonstrate the first electrically driven semiconductor source of photon pairs working at room temperature and telecom wavelengths. The device is based on type-II intracavity spontaneous parametric down-conversion in an AlGaAs laser diode and generates pairs at 1.57  μm. Time-correlation measurements of the emitted pairs give an internal generation efficiency of 7×10(-11)  pairs/injected electron. The capability of our platform to support the generation, manipulation, and detection of photons opens the way to the demonstration of massively parallel systems for complex quantum operations.

Inherent polarization entanglement generated from a monolithic semiconductor chip

Creating miniature chip scale implementations of optical quantum information protocols is a dream for many in the quantum optics community. This is largely because of the promise of stability and scalability. Here we present a monolithically integratable chip architecture upon which is built a photonic device primitive called a Bragg reflection waveguide (BRW). Implemented in gallium arsenide, we show that, via the process of spontaneous parametric down conversion, the BRW is capable of directly producing polarization entangled photons without additional path difference compensation, spectral filtering or post-selection. After splitting the twin-photons immediately after they emerge from the chip, we perform a variety of correlation tests on the photon pairs and show non-classical behaviour in their polarization. Combined with the BRW's versatile architecture our results signify the BRW design as a serious contender on which to build large scale implementations of optical quantum processing devices.

Direct measurement of the biphoton Wigner function through two-photon interference

The Hong-Ou-Mandel (HOM) experiment was a benchmark in quantum optics, evidencing the non–classical nature of photon pairs, later generalized to quantum systems with either bosonic or fermionic statistics. We show that a simple modification in the well-known and widely used HOM experiment provides the direct measurement of the Wigner function. We apply our results to one of the most reliable quantum systems, consisting of biphotons generated by parametric down conversion. A consequence of our results is that a negative value of the Wigner function is a sufficient condition for non-gaussian entanglement between two photons. In the general case, the Wigner function provides all the required information to infer entanglement using well known necessary and sufficient criteria. The present work offers a new vision of the HOM experiment that further develops its possibilities to realize fundamental tests of quantum mechanics using simple optical set-ups.

Stimulated emission tomography

We identify a relation between the number of photon pairs generated by parametric fluorescence, through either spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing, and the number generated by the corresponding stimulated process, respectively, either difference-frequency generation or stimulated four-wave mixing. On the basis of this very general result, we show that the characterization of SPDC sources of two-photon states in a given system can be performed solely by studying stimulated emission. We call this technique stimulated emission tomography (SET). We show that the number of photons detected in SET can be 9 orders of magnitude larger than the average number of coincidence counts in two-photon quantum state tomography. These results open the way to the study of sources of quantum-correlated photon pairs with unprecedented precision and unparalleled resolution.

Multi-photon absorption limits to heralded single photon sources

Single photons are of paramount importance to future quantum technologies, including quantum communication and computation. Nonlinear photonic devices using parametric processes offer a straightforward route to generating photons, however additional nonlinear processes may come into play and interfere with these sources. Here we analyse spontaneous four-wave mixing (SFWM) sources in the presence of multi-photon processes. We conduct experiments in silicon and gallium indium phosphide photonic crystal waveguides which display inherently different nonlinear absorption processes, namely two-photon (TPA) and three-photon absorption (ThPA), respectively. We develop a novel model capturing these diverse effects which is in excellent quantitative agreement with measurements of brightness, coincidence-to-accidental ratio (CAR) and second-order correlation function g⁽²⁾(0), showing that TPA imposes an intrinsic limit on heralded single photon sources. We build on these observations to devise a new metric, the quantum utility (QMU), enabling further optimisation of single photon sources.

Tunable quantum dot parametric source

We report on the modeling of an electrically pumped nonlinear source for spontaneous parametric down-conversion in an AlGaAs single-sided Bragg waveguide. Laser emission from InAs quantum dots embedded in the waveguide core is designed to excite a Bragg pump mode at 950 nm. This mode is phase matched with two cross-polarized total-internal-reflection fundamental signal and idler modes around 1900 nm. Besides numerically evaluating the source efficiency, we discuss the crucial role played by the quantum dots in the practical implementation of the phase-matching condition along with the tuning capabilities of this promising active device.