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Journal of Physics B Atomic Molecular and Optical Physics

Performing high-quality multi-photon experiments with parametric down-conversion

Thomas D. Jennewein, Rupert Ursin, Markus Aspelmeyer, Anton Zeilinger

Published May 15, 2009• 12 citations
Publisher

Abstract

We demonstrate entanglement swapping of polarization entangled photons using an interferometric Bell-state measurement operating at the theoretical limit of 50% efficiency capable of identifying two of the four Bell states. Our experiment represents the highest quality of entanglement swapping so far, with the correlations of the final entangled states showing an overlap fidelity with the ideal Bell states of F = 0.892, and a violation Bell's inequality for both swapped entangled states. In order to achieve this high-quality operation we have optimized various experimental error sources of our setup including the quality of optical elements, as well as the constraints due to the coherence and the group-velocity mismatch of the pump and down-conversion photons. Our results are relevant for experiments that involve interferences of independent photons and pulsed multi-photon states, such as in quantum computing, quantum metrology and quantum communication with quantum repeaters.

DOI
10.1088/0953-4075/42/11/114008

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We demonstrate entanglement swapping of polarization entangled photons using an interferometric Bell-state measurement operating at the theoretical limit of 50% efficiency capable of identifying two of the four Bell states. Our experiment represents the highest quality of entanglement swapping so far, with the correlations of the final entangled states showing an overlap fidelity with the ideal Bell states of F = 0.892, and a violation Bell's inequality for both swapped entangled states. In order to achieve this high-quality operation we have optimized various experimental error sources of our setup including the quality of optical elements, as well as the constraints due to the coherence and the group-velocity mismatch of the pump and down-conversion photons. Our results are relevant for experiments that involve interferences of independent photons and pulsed multi-photon states, such as in quantum computing, quantum metrology and quantum communication with quantum repeaters.
We demonstrate an experiment on entanglement swapping using an optimal Bell-state measurement capable of identifying two of the four Bell-states for polarization entangled photons, which is the optimum with linear optical elements. The two final photons belong to separately created paris. They are entangled after their original partner photons have been subjected to the Bell state measurement, whose outcome determines the type of entanglement of the final photon pair. The resulting violation of Bell's inequality in both cases confirms the success of the teleportation protocol.
We have demonstrated entanglement swapping with 1550 nm polarization-entangled photon pairs created by spontaneous parametric down conversion in two spatially separated type-II periodically poled lithium niobate bulk crystals. In a partial Bell-state measurement, entanglement was observed between two photons from separate sources. We employed sinusoidally gated InGaAs/InP avalanche photodiodes operated at a repetition frequency of 1.28 GHz, which increased the four-fold coincidence count rate without degrading the fidelity of entanglement swapping. The obtained fidelity exceeds 90%, which indicates that the entangled states obtained in this experiment are high enough for immediate further use in quantum communication protocols such as quantum teleportation or quantum cryptography.
Multi-particle entangled states are becoming more and more interesting for different applications in quantum information, such as quantum multi-particle teleportation, entangled based cryptography and quantum communication schemes based on multi-particle states, but also for fundamental reasons such as testing local realism by means of the Greenberger-Horne-Zeilinger states (GHZ) arguments . The advantage of multiparticle entanglement versus the most common two particle one lies in the fact that the greater the number of particles involved in the entangled state, the more clearly the quantum effects are exhibited. Our final goal is to exploit the multi-photon entangled states for quantum metrology. By the interaction of those states with nanostructures (mostly nanoholes, but in a future also nanospheres), we expect to reach sub-wavelength sensitivity phase measurements beating the Standard Quantum Limit as well as to detect plasmonic effects. In this thesis we present our work consisting on designing and building an experimental setup for the generation of states of 4-photon entangled in the transverse linear momentum. In the first part we explain some basics in the form of a technical introduction. We then adapt a general theoretical model to our particular case, explaining the physics of the process of generation of entangled states of photons. We proceed explaining the details of our setup, as well as the procedure of measurement. We then present the results of our tests in a "standard" 2-photon regime to show the correct functioning of the setup itself. Some more sophisticated measurements using structured light in the form of a first-order Hermite-Gaussian mode has been performed to generate entanglement in higher spatial modes are also presented. We conclude with theoretical and experimental details regarding the 4-photon experiments feasible with our setup. Our source is based on Spontaneous Parametric Down-Conversion (SPDC). In the last two decades it has been extensively studied for the generation of two-photon entangled states, since Kwiat et al. published in 1995 the new scheme based on polarization entangled photons. Most of the studies regarding entangled states involving more than two photons are indeed based on quantum correlations in the polarization or in the path of the down converted beams. The great advantage of entangling a spatial degree of freedom (as we do) is related to the dimensionality of the Hilbert space in which the state is defined. In the particular case of linear momentum, which is a continuous variable, we are dealing with infinite dimensions. Since in each dimension of the state one quanta of information can be encoded, the advantage of having multidimension entanglement for the field of quantum communication and computing is clear. The specs of our setup include a periodically-poled KTP crystals offering high conversion efficiency and cut for type-0 SPDC. The photons generated are well known to be inherently entangled in different degrees of freedom such as polarization, energy-time, linear momentum, and orbital angular momentum. Since each single SPDC event generates one photon pair, in order to reach four photon we entangle two consecutive SPDC events per each state. Furthermore, it is possible to manipulate (to enhance) the dimensionality of the entanglement by properly tailoring the pump spatial profile. The key point for the generation of four-photon entangled states is that, under some conditions, two consecutive pairs generated via SPDC cannot be described as two independent pairs but necessarily as a four-photon state showing genuine entanglement. This can happen when using a pulsed laser to generate the entangled photons; in particular, in the regime in which the coherence time of the down-converted photons τc is (much) longer than the temporal length of the pumping pulse Δt. In this condition the emission of a second pair is stimulated by the presence of the first one. That effect can be ultimately seen as a constructive multi-particle interference effect, the deep origin of which has to be sought in the boson statistic of photons.
We present a theoretical and experimental study of preparing maximally entangled two-photon polarization states, or Bell states, using femtosecond-pulse-pumped spontaneous parametric down-conversion (SPDC). First, we show how the inherent distinguishability in femtosecond-pulse-pumped type-II SPDC can be removed by using an interferometric technique without spectral and amplitude postselection. We then analyze the recently introduced Bell-state preparation scheme using type-I SPDC. Theoretically, both methods offer the same results, however, type-I SPDC provides experimentally superior methods of preparing Bell states in femtosecond-pulse-pumped SPDC. Such a pulsed source of highly entangled photon pairs is useful in quantum communications, quantum cryptography, quantum teleportation, etc.
Invariant entangled states remain unchanged under simultaneous identical unitary transformations of all their subsystems. We experimentally generate and characterize such invariant two-, four-, and six-photon polarization entangled states. This is done only with a suitable filtering procedure of multiple emissions of entangled photon pairs from a single source without any interferometric overlaps. We get the desired states utilizing bosonic emission enhancement due to indistinguishability. The setup is very stable and gives high interference contrasts. Thus, the process is a very likely candidate for various photonic demonstrations of quantum information protocols.