Polarization-entangled Photon Pairs Research Articles

Direct generation of hybrid entangled photon pairs in waveguides

In this paper, we propose a scheme for direct generation of hybrid spatial modal–polarization entangled photon pairs using a type-II spontaneous parametric down-conversion process and the electro-optic effect in a domain engineered potassium titanyl phosphate nonlinear waveguide. Such hybrid entangled states must find applications in quantum information processing using integrated photonic circuits.

High-fidelity entanglement swapping and generation of three-qubit GHZ state using asynchronous telecom photon pair sources

We experimentally demonstrate a high-fidelity entanglement swapping and a generation of the Greenberger-Horne-Zeilinger (GHZ) state using polarization-entangled photon pairs at telecommunication wavelength produced by spontaneous parametric down conversion with continuous-wave pump light. While spatially separated sources asynchronously emit photon pairs, the time-resolved photon detection guarantees the temporal indistinguishability of photons without active timing synchronizations of pump lasers and/or adjustment of optical paths. In the experiment, photons are sufficiently narrowed by fiber-based Bragg gratings with the central wavelengths of 1541 nm & 1580 nm, and detected by superconducting nanowire single-photon detectors with low timing jitters. The observed fidelities of the final states for entanglement swapping and the generated three-qubit state were 0.84 ± 0.04 and 0.70 ± 0.05, respectively.

Equivalence principle and quantum mechanics: quantum simulation with entangled photons.

Einstein's equivalence principle (EP) states the complete physical equivalence of a gravitational field and corresponding inertial field in an accelerated reference frame. However, to what extent the EP remains valid in non-relativistic quantum mechanics is a controversial issue. To avoid violation of the EP, Bargmann's superselection rule forbids a coherent superposition of states with different masses. Here we suggest a quantum simulation of non-relativistic Schrödinger particle dynamics in non-inertial reference frames, which is based on the propagation of polarization-entangled photon pairs in curved and birefringent optical waveguides and Hong-Ou-Mandel quantum interference measurement. The photonic simulator can emulate superposition of mass states, which would lead to violation of the EP.

Fiber entangled photon pair source connecting telecom to quantum memories

A fiber optic based source of polarization entangled photon pairs is proposed to connect and entangle a telecom quantum memory to a solid state quantum memory based on single-qubits operating in the near infrared. To span this large wavelength difference while matching the narrow spectral bandwidth of the quantum memories, the photon pairs are generated by four wave mixing in a highly birefringent fiber. Raman noise is expected to be strongly suppressed under these conditions.

Single electron-photon pair creation from a single polarization-entangled photon pair

Quantum entanglement between different forms of qubits is an indication of the universality of quantum mechanics. Entanglement transfer between light and matter, especially photon and spin, has long been studied as the central concept, but it remains technically challenging for single photons and spins. In this paper, we show paired generation of a single electron in a GaAs quantum dot and a single photon from a single polarization-entangled photon pair. We measure temporal coincidence between the single photo-electron detection and the single photon detection. Considering a single photon polarization is converted to an electron spin via an optical selection rule, the present result indicates the capability of photon to spin entanglement transfer. This may be useful to explore the physics of entanglement transfer and also for applications to quantum teleportation based quantum communication.

Distribution of entangled photon pairs over few-mode fibers

Few-mode fibers (FMFs) have been recently employed in classical optical communication to increase the data transmission capacity. Here we explore the capability of employing FMF for long distance quantum communication. We experimentally distribute photon pairs in the forms of time-bin and polarization entanglement over a 1-km-long FMF. We find the time-bin entangled photon pairs maintain their high degree of entanglement, no matter what type of spatial modes they are distributed in. For the polarization entangled photon pairs, however, the degree of entanglement is maintained when photon pairs are distributed in LP01 mode but significantly declines when photon pairs are distributed in LP11 mode due to a mode coupling effect in LP11 mode group. We propose and test a remedy to recover the high degree of entanglement. Our study shows, when FMFs are employed as quantum channels, selection of spatial channels and degrees of freedom of entanglement should be carefully considered.

Correlations and Entanglement of Microwave Photons Emitted in a Cascade Decay.

We use a three-level artificial atom in the ladder configuration as a source of correlated, single microwave photons of different frequency. The artificial atom, a transmon-type superconducting circuit, is driven at the two-photon transition between ground and second-excited state, and embedded into an on-chip switch that selectively routes different-frequency photons into different spatial modes. Under continuous driving, we measure power cross-correlations between the two modes and observe a crossover between strong antibunching and superbunching, typical of cascade decay, and an oscillatory pattern as the drive strength becomes comparable to the radiative decay rate. By preparing the source in a superposition state using an excitation pulse, we achieve deterministic generation of entangled photon pairs, as demonstrated by nonvanishing phase correlations and more generally by joint quantum state tomography of the two itinerant photonic modes.

Polarization-entangled photons from an InGaAs-based quantum dot emitting in the telecom C-band

We demonstrate the emission of polarization-entangled photons from a single semiconductor quantum dot in the telecom C-band (1530 nm–1565 nm). To reach this telecommunication window, the well-established material system of InAs quantum dots embedded in InGaAs barriers is utilized with an additional insertion of an InGaAs metamorphic buffer to spectrally shift the system to the desired wavelengths. For the observation of polarization-entangled photon pairs, the biexciton-exciton cascade of a quantum dot displaying an intrinsically low fine-structure splitting is investigated by means of polarization-dependent cross-correlation measurements. A complete set of tomography measurements enables us to reconstruct the two-photon density matrix and therefore to calculate a corresponding fidelity f+ to the maximally entangled Bell state Ψ+ of 0.61 ± 0.07, a concurrence of 0.74 ± 0.11, a tangle of 0.55 ± 0.14, and a negativity of 0.63 ± 0.12, clearly proving the entanglement of the states. Finally, the development of the concurrence is studied in dependency of the post-selected time-gate of the emission events and the progression of the time-delay dependent fidelity to distinct Bell states is displayed.

Coherent Perfect Absorption in Metamaterials with Entangled Photons

Quantum nonlocality, i.e., the presence of strong correlations in spatially separated systems that are forbidden by local realism, lies at the heart of quantum communications and quantum computing. Here, we use polarization-entangled photon pairs to demonstrate a nonlocal interaction of light with a plasmonic structure. Through the detection of one photon with a polarization-sensitive device, we can prevent or allow absorption of a second, remotely located photon. We demonstrate this with pairs of entangled photons in polarization, one of which is coupled into a plasmon of a thin metamaterial absorber in the path of a standing wave of an interferometer. Thus, we realize a quantum eraser experiment using photons and plasmonic resonances from metamaterials that promises opportunities for probabilistic quantum gating and controlling plasmon–photon conversion and entanglement. Moreover, by using the so-called coherent perfect absorption effect, we can expect near-perfect interaction.

Benefit of birefringence for the direct generation of polarization-entangled photon pairs.

Generating polarization-entangled photon pairs on chip is generally complicated by the birefringence of waveguides. In this work, we propose a technique that uses waveguide birefringence and lends itself to simple device designs. The technique relies on two orthogonal spontaneous four-wave mixing processes. We employ the full quantum optics theory and dispersion analysis, and show that the technique can produce highly entangled states, with concurrence as high as 0.976 and covering the entire C-band.

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.

Two-photon interference of polarization-entangled photons in a Franson interferometer

We present two-photon interference experiments with polarization-entangled photon pairs in a polarization-based Franson-type interferometer. Although the two photons do not meet at a common beamsplitter, a phase-insensitive Hong-Ou-Mandel type two-photon interference peak and dip fringes are observed, resulting from the two-photon interference effect between two indistinguishable two-photon probability amplitudes leading to a coincidence detection. A spatial quantum beating fringe is also measured for nondegenerate photon pairs in the same interferometer, although the two-photon states have no frequency entanglement. When unentangled polarization-correlated photons are used as an input state, the polarization entanglement is successfully recovered through the interferometer via delayed compensation.

Experimental demonstration of robust entanglement distribution over reciprocal noisy channels assisted by a counter-propagating classical reference light

Embedding a quantum state in a decoherence-free subspace (DFS) formed by multiple photons is one of the promising methods for robust entanglement distribution of photonic states over collective noisy channels. In practice, however, such a scheme suffers from a low efficiency proportional to transmittance of the channel to the power of the number of photons forming the DFS. The use of a counter-propagating coherent pulse can improve the efficiency to scale linearly in the channel transmission, but it achieves only protection against phase noises. Recently, it was theoretically proposed [Phys. Rev. A 87, 052325(2013)] that the protection against bit-flip noises can also be achieved if the channel has a reciprocal property. Here we experimentally demonstrate the proposed scheme to distribute polarization-entangled photon pairs against a general collective noise including the bit flip noise and the phase noise. We observed an efficient sharing rate scaling while keeping a high quality of the distributed entangled state. Furthermore, we show that the method is applicable not only to the entanglement distribution but also to the transmission of arbitrary polarization states of a single photon.

On-demand source of maximally entangled photon pairs using the biexciton-exciton radiative cascade

We perform full time resolved tomographic measurements of the polarization state of pairs of photons emitted during the radiative cascade of the confined biexciton in a semiconductor quantum dot. The biexciton was deterministically initiated using a $\pi$-area pulse into the biexciton two-photon absorption resonance. Our measurements demonstrate that the polarization states of the emitted photon pair are maximally entangled. We show that the measured degree of entanglement depends solely on the temporal resolution by which the time difference between the emissions of the photon pair is determined. A route for fabricating an on demand source of maximally polarization entangled photon pairs is thereby provided.

Solid-state ensemble of highly entangled photon sources at rubidium atomic transitions

Semiconductor InAs/GaAs quantum dots grown by the Stranski–Krastanov method are among the leading candidates for the deterministic generation of polarization-entangled photon pairs. Despite remarkable progress in the past 20 years, many challenges still remain for this material, such as the extremely low yield, the low degree of entanglement and the large wavelength distribution. Here, we show that with an emerging family of GaAs/AlGaAs quantum dots grown by droplet etching and nanohole infilling, it is possible to obtain a large ensemble of polarization-entangled photon emitters on a wafer without any post-growth tuning. Under pulsed resonant two-photon excitation, all measured quantum dots emit single pairs of entangled photons with ultra-high purity, high degree of entanglement and ultra-narrow wavelength distribution at rubidium transitions. Therefore, this material system is an attractive candidate for the realization of a solid-state quantum repeater—among many other key enabling quantum photonic elements.

Bright nanoscale source of deterministic entangled photon\xa0pairs violating Bell\u2019s 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 (SCHSH > 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.

High-Brightness Semiconductor Laser-Pumped $1.56~\mu$ m Polarization-Entangled Photon Pairs

In this paper, a semiconductor laser-pumped high-brightness $1.56~\mu \text{m}$ polarization-entangled photon pairs with a 30-mm-length PPKTP down-converter inside a Sagnac-type interferometer is firstly demonstrate. Large active-area InGaAs/InP-based single photon detectors and step-index multi-mode fiber collection are attempted to evaluate the brightness and entanglement quality. The maximum spectral brightness is about $1.33\times 10^{5}$ pairs( $\text{s}\ast $ mW $\ast $ nm) $^{-1}$ with a pump power of 200 mW and a spectral bandwidth of 1.34 nm. Under the condition of maximum entanglement brightness, polarization visibility of 96.08%±1.62% is obtained in the diagonal orthogonal basis. The quantum state tomography also shows a high fidelity of 97% for the ideal Bell states. Meanwhile, the CHSH-type Bell inequality is violated with a value of 2.8196±0.0098 ( $96~\sigma$ ). Both the performance in brightness and entanglement quality indicate that the current $1.56~\mu \text{m}$ polarization-entangled source has great potential in free-space quantum key distribution or quantum communication.

偏振纠缠光子对产生中的相位补偿

偏振纠缠光子对产生中的相位补偿

Thermal poling of silica optical fibers using liquid electrodes.

Thermal poling is a well-known technique for inducing second-order nonlinearities in centrosymmetric silica optical fibers. However, some 25years since its discovery, there still remain a number of issues that prevent the realization of very long length, highly efficient all-fiber nonlinear device applications that include frequency conversion or sources of polarization-entangled photon pairs. In this Letter, we report a thermal poling method that utilizes a novel range of liquid metal and aqueous electrodes embedded into the optical fibers. We demonstrate that it is possible to pole samples that are potentially meters in length, characterized by very low losses for efficient second-harmonic generation processes. The maximum estimated effective value of χ(2) (0.12pm/V) obtained using mercury electrodes is the highest reported in periodically poled silica fibers.

Generation of Narrow-Band Polarization-Entangled Photon Pairs at a Rubidium D1 Line

Using the process of cavity-enhanced spontaneous parametric down-conversion (SPDC), we generate a narrow-band polarization-entangled photon pair resonant on the rubidium (Rb) D1 line (795 nm). The degenerate single-mode photon pair is selected by multiple temperature controlled etalons. The linewidth of generated polarization-entangled photon pairs is 15 MHz which matches the typical atomic memory bandwidth. The measured Bell parameter for the polarization-entangled photons S=2.73+-0.04 which violates the Bell-CHSH inequality by ~18 standard deviations. The presented entangled photon pair source could be utilized in quantum communication and quantum computing based on quantum memories in atomic ensemble.