Polarization-entangled Photon Pairs Research Articles

Emergence of the Pointer Basis through the Dynamics of Correlations

We use the classical correlation between a quantum system being measured and its measurement apparatus to analyze the amount of information being retrieved in a quantum measurement process. Accounting for decoherence of the apparatus, we show that these correlations may have a sudden transition from a decay regime to a constant level. This transition characterizes a nonasymptotic emergence of the pointer basis, while the system apparatus can still be quantum correlated. We provide a formalization of the concept of emergence of a pointer basis in an apparatus subject to decoherence. This contrast of the pointer basis emergence to the quantum to classical transition is demonstrated in an experiment with polarization entangled photon pairs.

Pulsed Sagnac source of polarization entangled photon pairs

Photonic quantum information experiments demand bright and highly entangled photon pair sources. The combination of periodic poling and collinear excitation geometry allows the use of considerably longer crystals for parametric down-conversion. We demonstrate a picosecond-pulsed laser pumped source of high quality polarization entangled photon pairs. The phase of the output biphoton state is affected by the relative phase of the two-color interferometer and the phase of the nonlinearly interacting Gaussian beams. We measure the influence of these onto the phase of the output state. The presented source is a promising candidate for a compact, semiconductor laser driven source of entangled photon pairs.

Generation of polarization-entangled photon pairs via concurrent spontaneous parametric downconversions in a single χ^(2) nonlinear photonic crystal

We propose a scheme for generating polarization-entangled photon pairs using a χ((2)) nonlinear photonic crystal, which is designed for enabling two concurrent quasi-phase-matched spontaneous parametric downconversion processes. Beamlike photon pairs produced from each process are collinear but noncollinear with the pump. Moreover, the source we design works in a postselection-free way and applies to both degenerate and nondegenerate cases. Combining possible waveguide technologies, our scheme may provide an integrated polarization entanglement source.

Quantum teleportation over 143 kilometres using active feed-forward

The quantum internet is predicted to be the next-generation information processing platform, promising secure communication and an exponential speed-up in distributed computation. The distribution of single qubits over large distances via quantum teleportation is a key ingredient for realizing such a global platform. By using quantum teleportation, unknown quantum states can be transferred over arbitrary distances to a party whose location is unknown. Since the first experimental demonstrations of quantum teleportation of independent external qubits, an internal qubit and squeezed states, researchers have progressively extended the communication distance. Usually this occurs without active feed-forward of the classical Bell-state measurement result, which is an essential ingredient in future applications such as communication between quantum computers. The benchmark for a global quantum internet is quantum teleportation of independent qubits over a free-space link whose attenuation corresponds to the path between a satellite and a ground station. Here we report such an experiment, using active feed-forward in real time. The experiment uses two free-space optical links, quantum and classical, over 143 kilometres between the two Canary Islands of La Palma and Tenerife. To achieve this, we combine advanced techniques involving a frequency-uncorrelated polarization-entangled photon pair source, ultra-low-noise single-photon detectors and entanglement-assisted clock synchronization. The average teleported state fidelity is well beyond the classical limit of two-thirds. Furthermore, we confirm the quality of the quantum teleportation procedure without feed-forward by complete quantum process tomography. Our experiment verifies the maturity and applicability of such technologies in real-world scenarios, in particular for future satellite-based quantum teleportation.

Plasmonic nanoantennas for efficient control of polarization-entangled photon pairs

We suggest a source of polarization-entangled photon pairs based on a cross-shaped plasmonic nanoantenna driven by a single quantum dot. The integration of the nanoantenna with a metal mirror overcomes the fundamental tradeoff between the spontaneous emission (SE) enhancement and the extraction efficiency typical of microcavity- and nanowire-based architectures. With a very high extraction efficiency of entangled photons ($\ensuremath{\approx}$$90%$) at $1.55$ $\ensuremath{\mu}$m and large SE enhancement ($\ensuremath{\approx}$90) over a broad 330-nm spectral range, the proposed design will pave the way toward reliable integrated sources of nonclassical light.

Experimental methods of post-growth-tuning of the excitonic fine structure splitting in semiconductor quantum dots.

Deterministic sources of polarization entangled photon pairs on demand are considered as important building blocks for quantum communication technology. It has been demonstrated that semiconductor quantum dots (QDs), which exhibit a sufficiently small excitonic fine structure splitting (FSS) can be used as triggered, on-chip sources of polarization entangled photon pairs. As-grown QDs usually do not have the required values of the FSS, making the availability of post-growth tuning techniques highly desired. This article reviews the effect of different post-growth treatments and external fields on the FSS such as thermal annealing, magnetic fields, the optical Stark effect, electric fields, and anisotropic stress. As a consequence of the tuning of the FSS, for some tuning techniques a rotation of the polarization of the emitted light is observed. The joint modification of polarization orientation and FSS can be described by an anticrossing of the bright excitonic states.

Experimental investigation in transmission performance of polarization-entangled photon-pairs generated by cascaded χ^(2) processes over standard single-mode optical fibers

In this paper we report experimental investigation in transmission performance over standard single-mode optical fibers (SMFs) of polarization-entangled photon-pairs in a 1.5-μm band generated by cascaded second-harmonic generation and spontaneous parametric down conversion (c-SHG/SPDC) from a periodically poled LiNbO(3) (PPLN) ridge-waveguide device. Clear two-photon interference fringes were observed even after the transmission over 140 km of the SMF spools, remaining small degradation in the visibilities of less than 3%. The performance was also investigated by using optical attenuators, instead of the SMF spools, to study the maximum reach of the distribution of the entanglement in terms of loss penalty. The results show that the quantum entanglement could be distributed even with 50 dB of the transmission loss with violation of Bell inequality by using the c-SHG/SPDC-based photon-pair source.

Bragg reflection waveguide as a source of wavelength-multiplexed polarization-entangled photon pairs

We put forward a new highly efficient source of paired photons entangled in polarization with an ultra-large bandwidth. The photons are generated by means of a conveniently designed spontaneous parametric down-conversion process in a semiconductor type-II Bragg reflection waveguide. The proposed scheme aims at being a key element of an integrated source of polarization-entangled photon pairs highly suitable for its use in a multi-user quantum-key-distribution system.

Direct Generation of Polarization-Entangled Photon Pairs in a Poled Fiber

We experimentally demonstrate the direct generation of polarization-entangled photon pairs in an optical fiber at room temperature by exploiting type-II phase-matched spontaneous parametric down-conversion. A second-order nonlinearity is artificially induced in the 17-cm-long weakly birefringent step-index fiber through the process of thermal poling, and quasi-phase-matching allows for the generation of entangled photons in the 1.5-micron telecom band when the fiber is pumped at 775 nm. A greater-than 80:1 coincidence-to-accidental ratio is achieved, limited mainly by multiphoton pair generation. Without the need to subtract accidentals or to compensate for walk-off, the raw two-photon interference visibility is found to be better than 95%, and violation of Bell's inequality is observed by more than 18 standard deviations. This makes for a truly alignment-free, plug-and-play source of polarization-entangled photon pairs.

THE GENERATION OF POLARIZATION-ENTANGLED PHOTON PAIRS USING GRATING STRUCTURES OF Ti-DIFFUSED LITHIUM NIOBATE WAVEGUIDES IN INTEGRATED OPTICS

Entangled photon source is an essential tool for quantum information experiments, such as tests of Bell inequalities, quantum teleportation, and entanglement swapping. We report the generation of polarization-entangled photon pairs using grating structures of nonlinear materials such as Ti -diffused Lithium Niobate. This approach is useful for quantum integrated optics because the size of this element is less than 325 micron. Polarization-entangled photon pairs are created using the nonlinear optical process of type-II spontaneous parametric down-conversion. The pump laser at 800 nm was doubled to a wavelength of 400 nm, which was used to produce SPDC in the grating structure of Lithium Niobate. The entangled photon pairs were generated at wavelengths 692 nm and 950 nm. The used grating structure consists of two sections. Grating period and length of the first section are 162 nm and 129.6 micron, respectively. Grating period of the second section is 193 nm and its length is equal to 193 micron.

Evaluation of polarization entanglement generated by pulsed spontaneous parametric down-conversion with multi-pairs using four single-photon detectors for quantum state tomography

Quantum state tomography (QST) is widely used to evaluate entanglement generated by spontaneous parametric down-conversion. Two-fold coincidence counts for 16 different configurations have been measured using single-photon detectors to estimate the fidelity (i.e., the probability overlap between ideal and real states) from the reconstructed density matrix. However, multi-pair emission events degrade the fidelity. Here, we numerically analyze pulse-pumped polarization-entangled photon pairs for single-photon detectors that can or cannot count the number of incident photons to estimate the fidelity given by QST. Threshold single-photon detectors are desirable for QST from a practical point of view. Our analysis suggests that using four threshold single-photon detectors for two-fold coincidence measurements offers almost the same fidelity as that given by four photon-number-resolving single-photon detectors. The fidelity estimated by QST becomes poor and loss independent when two threshold single-photon detectors are used.

Polarization-based entanglement swapping at the telecommunication wavelength using spontaneous parametric down-conversion photon-pair sources

We experimentally demonstrated polarization-based entanglement swapping using spatially separated periodically poled lithium niobate waveguides that emit polarization-entangled photon pairs by spontaneous parametric down-conversion (SPDC). Entanglement swapping using such SPDC sources was theoretically analyzed by accounting for losses along photon propagation paths, accidental fourfold coincidence counts (FCC's), and the dead time in single-photon detection. In a partial Bell-state measurement, entanglement was observed between two photons that were initially uncorrelated and that did not directly interact with each other. The obtained fidelity was $52.5\ifmmode\pm\else\textpm\fi{}0.8%$. Accidental FCC's were measured to clarify the effect of simultaneous generation of three pairs in SPDC on the fidelity. The fidelity increased to $80.5\ifmmode\pm\else\textpm\fi{}2.9%$ when accidental coincidence counts were subtracted.

Front Cover: Positioning of single nanostructures — single quantum devices (Phys. Status Solidi B 4/2012)

In a series of 16 articles, reports of the DFG Research Unit “Positioning of single nanostructures - single quantum devices” (FOR 730) are presented in this issue. Some of the topics are illustrated on the cover: The control of quantum-dot emission by integration of semiconductor nanomembranes onto piezoelectric actuators is demonstrated by Rastelli et al., pp. 687–696 (top left figure). This is a promising method for the generation of polarization-entangled photon pairs (Jöns et al., pp. 697–701, center figure). Site-controlled SiGe islands on patterned Si(001) are developed by Zhang et al., pp. 752–763 (top right figure). Zallo et al., pp. 702–709, discuss the realization of laterally ordered InAs quantum dot molecules for quantum gate applications (bottom left figure). First steps towards the electrical detection of plasmons in all-silicon pin-diodes are presented by Fischer et al., pp. 773–777 (bottom center). A self-assembled quantum dot can be coupled in a controlled way to a plasmonic nanoantenna, as demonstrated by Pfeiffer et al., pp. 678–686 (bottom right figure).

Quantum state tomography measurements on strain‐tuned InxGa1−xAs/GaAs quantum dots

AbstractWe present quantum state tomography (QST) measurements on strain‐tuned self‐assembled InxGa1−xAs/GaAs quantum dots. By applying uniaxial stress we tune the fine structure splitting (FSS) of the exciton transition and investigate the change in the two‐photon polarization state (TPPS) of the quantum dot. We tune the FSS to a minimum value of 4.7 µeV. The density matrix of the TPPS indicates an onset of the anti‐crossing of the exciton transition. Our experiments show that strain‐tuning is a promising method for the generation of triggered polarization entangled photon pairs.

Entangled photon generation in two-period quasi-phase-matched parametric down-conversion

We proposed and demonstrated a simple but deterministic scheme for generating polarization-entangled photon pairs at telecommunication wavelengths with type-II quasi-phase-matched spontaneous parametric down-conversion (QPM-SPDC) having two poling periods. We fabricated a LiNbO3 crystal having two poling periods so as to generate entangled photons at two wavelengths, i.e., 1506 nm and 1594 nm. We characterized the two-photon polarization state with state tomography and confirmed that the state was highly entangled.

Experimental implementation of bit commitment in the noisy-storage model

Fundamental primitives such as bit commitment and oblivious transfer serve as building blocks for many other two-party protocols. Hence, the secure implementation of such primitives is important in modern cryptography. Here we present a bit commitment protocol that is secure as long as the attacker's quantum memory device is imperfect. The latter assumption is known as the noisy-storage model. We experimentally executed this protocol by performing measurements on polarization-entangled photon pairs. Our work includes a full security analysis, accounting for all experimental error rates and finite size effects. This demonstrates the feasibility of two-party protocols in this model using real-world quantum devices. Finally, we provide a general analysis of our bit commitment protocol for a range of experimental parameters.

Mode Matching of Type II SPDC Polarization Entangled Photon Pairs by Using a Digital Camera

Mode Matching of Type II SPDC Polarization Entangled Photon Pairs by Using a Digital Camera

Compact source of narrow-band counterpropagating polarization-entangled photon pairs using a single dual-periodically-poled crystal

We propose a scheme for the generation of counterpropagating polarization-entangled photon pairs from a dual-periodically-poled crystal. Compared with the usual forward-wave-type source, this source, in the backward-wave way, has a much narrower bandwidth. With a 2-cm-long bulk crystal, the bandwidths of the example sources are estimated to be $3.6$ GHz, and the spectral brightnesses are more than 100 pairs$/$(s GHz mW). Two concurrent quasi-phase-matched spontaneous parametric down-conversion processes in a single crystal enable our source to be compact and stable. This scheme does not rely on any state projection and applies to both degenerate and nondegenerate cases, facilitating applications of the entangled photons.

Evaluation of polarization entanglement generated by spontaneous parametric downconversion using photon number counting

Spontaneous parametric downconversion (SPDC) is widely used to generate entangled photon pairs; however, multi-pair emissions degrade the quality of the entanglement. We numerically evaluate polarization-entangled photon pairs created by SPDC. The effects of multi-pair emission events on the visibility of two-photon interference and on the fidelity (the probability overlap for ideal and real states) are analyzed using single-photon detectors that can count the number of incoming photons and discard multiphoton events. Compared with conventional threshold single-photon detectors, photon-number resolving single-photon detectors have higher fidelity for the same or lower visibility.

Analytical solution of the quantum-state tomography of the biexciton cascade in semiconductor quantum dots: Pure dephasing does not affect entanglement

A fundamental approach to generate polarization-entangled photon pairs from solid-state devices is the biexciton cascade in semiconductor quantum dots. For efficient photon pair generation in the weak coupling regime, we present an analytical solution for the two-photon matrix elements (quantum-state tomography), including dephasing and relaxation mechanisms. In particular, we show that a pure dephasing mechanism does not affect the degree of entanglement for quantum dots with degenerated exciton levels.