Two-photon Interference Visibility Research Articles

Microresonator-enhanced quantum dot single-photon emission in GaAs-on-insulator platform

Abstract Single-photon emitters play a crucial role in integrated photonic quantum systems across various material platforms. In this study, we present a cavity-enhanced quantum dot single-photon source in a high-index contrast material platform (GaAs-on-insulator). A quantum dot is embedded in a microring resonator with a quality factor of approximately 9000, where the cavity-induced light enhancement results in a spontaneous emission acceleration factor of around 2.6, and we achieve a g ( 2 ) ( 0 ) as low as 0.03 ± 0.02 and post-selected two-photon interference visibility of 51.7 % ± 3.2 % for the single-photon source. This demonstration shows the potential of the GaAs-on-Insulator platform for scalable on-chip quantum information processing.

InGaP χ(2) integrated photonics platform for broadband, ultra-efficient nonlinear conversion and entangled photon generation

Nonlinear optics plays an important role in many areas of science and technology. The advance of nonlinear optics is empowered by the discovery and utilization of materials with growing optical nonlinearity. Here we demonstrate an indium gallium phosphide (InGaP) integrated photonics platform for broadband, ultra-efficient second-order nonlinear optics. The InGaP nanophotonic waveguide enables second-harmonic generation with a normalized efficiency of 128, 000%/W/cm2 at 1.55 μm pump wavelength, nearly two orders of magnitude higher than the state of the art in the telecommunication C band. Further, we realize an ultra-bright, broadband time-energy entangled photon source with a pair generation rate of 97 GHz/mW and a bandwidth of 115 nm centered at the telecommunication C band. The InGaP entangled photon source shows high coincidence-to-accidental counts ratio CAR > 104 and two-photon interference visibility > 98%. The InGaP second-order nonlinear photonics platform will have wide-ranging implications for non-classical light generation, optical signal processing, and quantum networking.

Quantum Light Generation Based on GaN Microring toward Fully On-Chip Source.

An integrated quantum light source is increasingly desirable in large-scale quantum information processing. Despite recent remarkable advances, a new material platform is constantly being explored for the fully on-chip integration of quantum light generation, active and passive manipulation, and detection. Here, for the first time, we demonstrate a gallium nitride (GaN) microring based quantum light generation in the telecom C-band, which has potential toward the monolithic integration of quantum light source. In our demonstration, the GaN microring has a free spectral range of 330GHz and a near-zero anomalous dispersion region of over 100nm. The generation of energy-time entangled photon pair is demonstrated with a typical raw two-photon interference visibility of 95.5±6.5%, which is further configured to generate a heralded single photon with a typical heralded second-order autocorrelation g_{H}^{(2)}(0) of 0.045±0.001. Our results pave the way for developing a chip-scale quantum photonic circuit.

Quantum interference of identical photons from remote GaAs quantum dots

Photonic quantum technology provides a viable route to quantum communication1,2, quantum simulation3 and quantum information processing4. Recent progress has seen the realization of boson sampling using 20 single photons3 and quantum key distribution over hundreds of kilometres2. Scaling the complexity requires architectures containing multiple photon sources, photon counters and a large number of indistinguishable single photons. Semiconductor quantum dots are bright and fast sources of coherent single photons5-9. For applications, a roadblock is the poor quantum coherence on interfering single photons created by independent quantum dots10,11. Here we demonstrate two-photon interference with near-unity visibility (93.0 ± 0.8)% using photons from two completely separate GaAs quantum dots. The experiment retains all the emission into the zero phonon line-only the weak phonon sideband is rejected; temporal post-selection is not employed. By exploiting quantum interference, we demonstrate a photonic controlled-not circuit and an entanglement with fidelity of (85.0 ± 1.0)% between photons of different origins. The two-photon interference visibility is high enough that the entanglement fidelity is well above the classical threshold. The high mutual coherence of the photons stems from high-quality materials, diode structure and relatively large quantum dot size. Our results establish a platform-GaAs quantum dots-for creating coherent single photons in a scalable way.

High-Performance Hyperentanglement Generation and Manipulation Based on Lithium Niobate Waveguides

Here, we demonstrate the generation and manipulation of hyperentanglement in polarization and time-energy degrees of freedom. The hyperentangled photon pairs are generated by cascaded second-harmonic generation and spontaneous parametric down-conversion processes in a high-efficiency periodically poled lithium niobate (PPLN) waveguide in a fiber Sagnac loop. The states achieve the maximum value of Clauser-Horne-Shimony-Holt Bell inequality of $S=2.76\ifmmode\pm\else\textpm\fi{}0.03$ with two-photon interference visibility of higher than 99% for polarization entanglement and the Franson-type two-photon interference visibility of 98.5% for time-energy entanglement. In addition, the polarization maximally entangled states can be fast switched to each other by using a PPLN on insulator chip through the electro-optic effect. After the entanglement manipulation, the two-photon interference visibility for polarization entanglement is still higher than 97% while keeping the quantum characteristics of time-energy degrees of freedom maintained. Our system paves a way toward reducing the complexity of quantum systems and has potential applications in many quantum-information tasks like fast superdense coding and teleportation.

Ultrabroadband Entangled Photons on a Nanophotonic Chip.

The development of quantum technologies on nanophotonic platforms has seen momentous progress in the past decade. Despite that, a demonstration of time-frequency entanglement over a broad spectral width is still lacking. Here we present an efficient source of ultrabroadband entangled photon pairs on a periodically poled lithium niobate nanophotonic waveguide. Employing dispersion engineering, we demonstrate a record-high 100THz (1.2 μm-2 μm) generation bandwidth with a high efficiency of 13 GHz/mW and excellent noise performance with the coincidence-to-accidental ratio exceeding 10^{5}. We also measure strong time-frequency entanglement with over 98% two-photon interference visibility.

High-performance quantum entanglement generation via cascaded second-order nonlinear processes

In this paper, we demonstrate the generation of high-performance entangled photon-pairs in different degrees of freedom from a single piece of fiber pigtailed periodically poled LiNbO3 (PPLN) waveguide. We utilize cascaded second-order nonlinear optical processes, i.e., second-harmonic generation (SHG) and spontaneous parametric downconversion (SPDC), to generate photon-pairs. Previously, the performance of the photon-pairs is contaminated by Raman noise photons. Here by fiber-integrating the PPLN waveguide with noise-rejecting filters, we obtain a coincidence-to-accidental ratio (CAR) higher than 52,600 with photon-pair generation and detection rate of 52.36 kHz and 3.51 kHz, respectively. Energy-time, frequency-bin, and time-bin entanglement is prepared by coherently superposing correlated two-photon states in these degrees of freedom, respectively. The energy-time entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.71 ± 0.02 with two-photon interference visibility of 95.74 ± 0.86%. The frequency-bin entangled two-photon states achieve fidelity of 97.56 ± 1.79% with a spatial quantum beating visibility of 96.85 ± 2.46%. The time-bin entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.60 ± 0.04 and quantum tomographic fidelity of 89.07 ± 4.35%. Our results provide a potential candidate for the quantum light source in quantum photonics.

Triggered emission of indistinguishable photons from an organic dye molecule

Single molecules in solid state matrices have been proposed as sources of single photon Fock states back 20 years ago. Their success in quantum optics and in many other research fields stems from the simple recipes used in the preparation of samples, with hundreds of nominally identical and isolated molecules. Main challenges as of today for their application in photonic quantum technologies are the optimization of light extraction and the on-demand emission of indistinguishable photons. We here present Hong–Ou–Mandel (HOM) experiments with photons emitted by a single molecule of dibenzoterrylene in an anthracene nanocrystal at 3 K, under continuous wave and also pulsed excitation. A detailed theoretical model is applied, which relies on independent measurements for most experimental parameters, hence allowing for an analysis of the different contributions to the two-photon interference visibility, from residual dephasing to spectral filtering. A HOM interference visibility of more than 75% is reported, which, according to the model, is limited by the residual dephasing present at the operating temperature.

A bright and fast source of coherent single photons.

A single-photon source is an enabling technology in device-independent quantum communication1, quantum simulation2,3, and linear optics-based4 and measurement-based quantum computing5. These applications employ many photons and place stringent requirements on the efficiency of single-photon creation. The scaling on efficiency is typically an exponential function of the number of photons. Schemes taking full advantage of quantum superpositions also depend sensitively on the coherence of the photons, that is, their indistinguishability6. Here, we report a single-photon source with a high end-to-end efficiency. We employ gated quantum dots in an open, tunable microcavity7. The gating provides control of the charge and electrical tuning of the emission frequency; the high-quality material ensures low noise; and the tunability of the microcavity compensates for the lack of control in quantum dot position and emission frequency. Transmission through the top mirror is the dominant escape route for photons from the microcavity, and this output is well matched to a single-mode fibre. With this design, we can create a single photon at the output of the final optical fibre on-demand with a probability of up to 57% and with an average two-photon interference visibility of 97.5%. Coherence persists in trains of thousands of photons with single-photon creation at a repetition rate of 1 GHz.

Coherence in single photon emission from droplet epitaxy and Stranski–Krastanov quantum dots in the telecom C-band

The ability of two photons to interfere lies at the heart of many photonic quantum networking concepts and requires that the photons are indistinguishable with sufficient coherence times to resolve the interference signals. However, for solid-state quantum light sources, this can be challenging to achieve as they are in constant interaction with noise sources in their environment. Here, we investigate the noise sources that affect InAs/InP quantum dots emitting in the telecom C-band by comparing their behavior on a wetting layer for Stranski–Krastanov grown quantum dots with a nearly wetting layer-free environment achieved with the droplet epitaxy growth mode. We show that the droplet epitaxy growth mode is beneficial for a quiet environment, leading to 96% of exciton transitions having a coherence time longer than the typical detector resolution of 100 ps, even under non-resonant excitation. We also show that the decay profile indicates the presence of slow dephasing processes, which can be compensated for experimentally. We finally conduct Hong–Ou–Mandel interference measurements between subsequently emitted photons and find a corrected two-photon interference visibility of 98.6 ± 1.6% for droplet-epitaxy grown quantum dots. The understanding of the influence of their surroundings on the quantum optical properties of these emitters is important for their optimization and use in future quantum networking applications.

Indistinguishable Photons from a Single Molecule under Pulsed Excitation

Quantum light sources are crucial for the future of quantum photonic technologies and, among them, single photons on-demand are key resources in quantum communications and information processing. Ideal quantum emitters providing indistinguishable photons in a clocked manner, negligible decoherence and spectral diffusion, and with potential for scalability are today still a major challenge. We report on photostable and indistinguishable single photon emission from dibenzoterrylene molecules isolated in anthracene nanocrystals (DBT:Ac NCs) at 3K. The visibility of two-photon interference is preserved even when they are separated more than thirty times the excited-state lifetime, or ten fluorescence cycles. One of the advantages of organic molecules is the low-cost mass production of nominally identical emitters, that also allow for on-chip integration. These aspects combined with high spectral stability and coherence make them promising for applications and future quantum technologies.

Mid-infrared quantum optics in silicon.

Applied quantum optics stands to revolutionise many aspects of information technology, provided performance can be maintained when scaled up. Silicon quantum photonics satisfies the scaling requirements of miniaturisation and manufacturability, but at 1.55 µm it suffers from problematic linear and nonlinear loss. Here we show that, by translating silicon quantum photonics to the mid-infrared, a new quantum optics platform is created which can simultaneously maximise manufacturability and miniaturisation, while reducing loss. We demonstrate the necessary platform components: photon-pair generation, single-photon detection, and high-visibility quantum interference, all at wavelengths beyond 2 µm. Across various regimes, we observe a maximum net coincidence rate of 448 ± 12 Hz, a coincidence-to-accidental ratio of 25.7 ± 1.1, and, a net two-photon quantum interference visibility of 0.993 ± 0.017. Mid-infrared silicon quantum photonics will bring new quantum applications within reach.

Development of site-controlled quantum dot arrays acting as scalable sources of indistinguishable photons

We report on the realization of an array of 28 × 28 mesas with site-controlled InGaAs quantum dots acting as single-photon sources for potential applications in photonic quantum technology. The site-selective growth of quantum dots is achieved by using the buried stressor approach where an oxide aperture serves as the nucleation site in the center of each mesa. Spectroscopic maps demonstrate the positioning of quantum dots with an inhomogeneous broadening of the ensemble emission of only 15.8 meV. Individual quantum dots are characterized by clean single-quantum-dot spectra with narrow exciton, biexciton, and trion lines, with a best value of 27 μeV and an ensemble average of 120 μeV. Beyond that, Hanbury Brown and Twiss and Hong-Ou-Mandel measurements validate the quantum nature of emission in terms of high single-photon purity and photon indistinguishability with a g(2)(0) value of (0.026 ± 0.026) and a post-selected two-photon interference visibility V = (87.1 ± 9.7)% with an associated coherence time of τc = (194 ± 7) ps.

Deterministically fabricated quantum dot single-photon source emitting indistinguishable photons in the telecom O-band

In this work, we develop and study single-photon sources based on InGaAs quantum dots (QDs) emitting in the telecom O-band. Quantum devices are fabricated using in situ electron beam lithography in combination with thermocompression bonding to realize a backside gold mirror. Our structures are based on InGaAs/GaAs heterostructures, where the QD emission is redshifted toward the telecom O-band at 1.3 μm via a strain-reducing layer. QDs pre-selected by cathodoluminescence mapping are embedded into mesa structures with a backside gold mirror for enhanced photon-extraction efficiency. Photon-autocorrelation measurements under pulsed non-resonant wetting-layer excitation are performed at temperatures up to 40 K, showing pure single-photon emission, which makes the devices compatible with stand-alone operation using Stirling cryocoolers. Using pulsed p-shell excitation, we realize single-photon emission with a high multi-photon suppression of g(2)(0) = 0.027 ± 0.005, an as-measured two-photon interference visibility of (12 ± 4)%, a post-selected visibility of (96 ± 10)%, and an associated coherence time of (212 ± 25) ps. Moreover, the structures show an extraction efficiency of ∼5%, which is comparable to values expected from numeric simulations of this photonic structure. Further improvements of our devices will enable implementations of quantum communication via optical fibers.

Quantum Beat between Sunlight and Single Photons.

We demonstrate fourth-order quantum beat between sunlight and single photons from a quantum dot. With a fast time-resolved detection system, we observed high-visibility quantum beat between the independent photons of different frequencies from the two astronomically separated light sources. The temporal dynamics of the beat oscillation indicate the coherent behavior of the interfering photons, and the raw visibility of two-photon interference shows violation of the classical limit with a frequency mismatch of three-times the line width.

Carbon Nanotube Color Centers in Plasmonic Nanocavities: A Path to Photon Indistinguishability at Telecom Bands.

Indistinguishable single photon generation at telecom wavelengths from solid-state quantum emitters remains a significant challenge to scalable quantum information processing. Here we demonstrate efficient generation of "indistinguishable" single photons directly in the telecom O-band from aryl-functionalized carbon nanotubes by overcoming the emitter quantum decoherence with plasmonic nanocavities. With an unprecedented single-photon spontaneous emission time down to 10 ps (from initially 0.7 ns) generated in the coupling scheme, we show a two-photon interference visibility at 4 K reaching up to 0.79, even without applying post selection. Cavity-enhanced quantum yields up to 74% and Purcell factors up to 415 are achieved with single-photon purities up to 99%. Our results establish the capability to fabricate fiber-based photonic devices for quantum information technology with coherent properties that can enable quantum logic.

Overcoming correlation fluctuations in two-photon interference experiments with differently bright and independently blinking remote quantum emitters

As a fundamental building block for quantum computation and communication protocols, the correct verification of the two-photon interference (TPI) contrast between two independent quantum light sources is of utmost importance. Here, we experimentally demonstrate how frequently present blinking dynamics and changes in emitter brightness critically affect the Hong-Ou-Mandel-type (HOM) correlation histograms of remote TPI experiments measured via the commonly utilized setup configuration. We further exploit this qualitative and quantitative explanation of the observed correlation dynamics to establish an alternative interferometer configuration, which is overcoming the discussed temporal fluctuations, giving rise to an error-free determination of the remote TPI visibility. We prove full knowledge of the obtained correlation by reproducing the measured correlation statistics via Monte-Carlo simulations. As exemplary system, we make use of two pairs of remote semiconductor quantum dots, however, the same conclusions apply for TPI experiments with flying qubits from any kind of remote solid state quantum emitters.

Phonon-Assisted Two-Photon Interference from Remote Quantum Emitters.

Photonic quantum technologies are on the verge of finding applications in everyday life with quantum cryptography and quantum simulators on the horizon. Extensive research has been carried out to identify suitable quantum emitters and single epitaxial quantum dots have emerged as near-optimal sources of bright, on-demand, highly indistinguishable single photons and entangled photon-pairs. In order to build up quantum networks, it is essential to interface remote quantum emitters. However, this is still an outstanding challenge, as the quantum states of dissimilar “artificial atoms” have to be prepared on-demand with high fidelity and the generated photons have to be made indistinguishable in all possible degrees of freedom. Here, we overcome this major obstacle and show an unprecedented two-photon interference (visibility of 51 ± 5%) from remote strain-tunable GaAs quantum dots emitting on-demand photon-pairs. We achieve this result by exploiting for the first time the full potential of a novel phonon-assisted two-photon excitation scheme, which allows for the generation of highly indistinguishable (visibility of 71 ± 9%) entangled photon-pairs (fidelity of 90 ± 2%), enables push-button biexciton state preparation (fidelity of 80 ± 2%) and outperforms conventional resonant two-photon excitation schemes in terms of robustness against environmental decoherence. Our results mark an important milestone for the practical realization of quantum repeaters and complex multiphoton entanglement experiments involving dissimilar artificial atoms.

Screening Nuclear Field Fluctuations in Quantum Dots for Indistinguishable Photon Generation.

A semiconductor quantum dot can generate highly coherent and indistinguishable single photons. However, intrinsic semiconductor dephasing mechanisms can reduce the visibility of two-photon interference. For an electron in a quantum dot, a fundamental dephasing process is the hyperfine interaction with the nuclear spin bath. Here, we directly probe the consequence of the fluctuating nuclear spins on the elastic and inelastic scattered photon spectra from a resident electron in a single dot. We find the in-plane component of the nuclear Overhauser field leads to detuned Raman scattered photons, broadened over experimental time scales by field fluctuations, which are distinguishable from both the elastic and incoherent components of the resonance fluorescence. This significantly reduces two-photon interference visibility. However, we demonstrate successful screening of the nuclear spin noise, which enables the generation of coherent single photons that exhibit high visibility two-photon interference.

Spectral properties of broadband biphotons generated from PPMgSLT under a type-II phase-matching condition

We report the spectral properties of broadband biphotons generated by pulsed type-II spontaneous parametric down-conversion based on a periodically poled MgO-doped stoichiometric LiTaO3 (PPMgSLT). Spectroscopic and interferometric measurements were used to characterize the biphoton source. For the spectroscopic measurements, we used conventional spectroscopy and joint spectral intensity measurements. For the interferometric measurements, sum- and difference-frequency two-photon interference experiments were performed to estimate the spectral correlation and to ensure the broadband nature of biphotons, respectively. When using a picosecond Ti:sapphire as a pump source, the spectral width of biphotons was over 40 nm. Finally, we demonstrated the relationship between the two-photon interference visibility and the symmetricity of the joint spectral distribution.