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Quantum Electronics and Laser Science Conference

Bell state preparation using pulsed non-degenerate two-photon entanglement

Yoon-Ho Kim, S. P. Kulik, Yanhua H. Shih

Published Nov 13, 2002• 0 citations
Publisher

Abstract

Summary form only given. In femtosecond pulse pumped single photon downconversion (SPDC), polarization Bell states have been realized only after post-selections of the two-photon amplitudes are made: especially, spectral post-selection by using narrow-band filters. Instead of the spectral post-selection, one can use a thin nonlinear crystal. However, both methods reduce the available flux of the entangled photon pair significantly. In this paper, we report a Bell state preparation experiment in which we effectively eliminate any post-selection in femtosecond pulse pumped SPDC for the first time.

DOI
10.1109/qels.2001.961895

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Summary form only given. In femtosecond pulse pumped single photon downconversion (SPDC), polarization Bell states have been realized only after post-selections of the two-photon amplitudes are made: especially, spectral post-selection by using narrow-band filters. Instead of the spectral post-selection, one can use a thin nonlinear crystal. However, both methods reduce the available flux of the entangled photon pair significantly. In this paper, we report a Bell state preparation experiment in which we effectively eliminate any post-selection in femtosecond pulse pumped SPDC for the first time.
We report a Bell-state preparation experiment. High-purity Bell states are prepared by using femtosecond pulse pumped nondegenerate collinear spontaneous parametric down-conversion. The use of a femtosecond pump pulse does not result in a reduction of quantum interference visibility in our scheme in which the postselection of amplitudes and other traditional mechanisms, such as using thin nonlinear crystals or narrow-band spectral filters are not used. Another distinct feature of this scheme is that the pump, the signal, and the idler wavelengths are all distinguishable, which is very useful for quantum communications.
Summary form only given. By far the most efficient source of two-particle entangled state is spontaneous parametric downconversion (SPDC). Especially, cw laser pumped type-II SPDC has been widely treated as a good source of entangled photon pairs, since one can easily prepare a polarization Bell state. However, femtosecond pulse pumped type-II SPDC shows poor quantum interference visibility when a thick crystal is used. To restore the visibility, one has to use narrow-band spectral filters at the expense of available photon flux. We demonstrate a scheme for high-visibility two-photon interference with thick (3.4 mm.) type-II BaB/sub 2/O/sub 4/ (BBO) crystals in femtosecond pulse pumped SPDC. Polarization Bell states can be prepared using this method without spectral and amplitude post-selection.
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.
Polarization-entangled photon pairs can be efficiently prepared into pure Bell states with a high fidelity via type-II spontaneous parametric down-conversion (SPDC) of narrow-band pump light. However, the use of femtosecond pump pulses to generate multiphoton states with precise timing often requires spectral filtering to maintain a high quality of polarization entanglement. This typically reduces the efficiency of photon pair collection. We experimentally map the polarization correlations of photon pairs from such a source over a range of down-converted wavelengths with a high spectral resolution and find strong polarization correlations everywhere. A spectrally dependent imbalance between contributions from the two possible decay paths of SPDC is identified as the reason for a reduction in entanglement quality observed with femtosecond pump pulses. Our spectral measurements allow us to predict the polarization correlations for arbitrary filter profiles when the frequency degree of freedom of the photon pairs is ignored.
The quantum state of the photon pair generated from type-II spontaneous parametric downconversion pumped by an ultrafast laser pulse exhibits strong decoherence in its polarization entanglement, an effect which can be attributed to the clock effect of the pump pulse or, equivalently, to distinguishing spectral information in the two-photon state. Here, we propose novel temporal and spectral engineering techniques to eliminate these detrimental decoherence effects. The temporal engineering of the two-photon wavefunction results in a universal Bell-state synthesizer that is independent of the choice of pump source, crystal parameters, wavelengths of the interacting photons and the bandwidth of the spectral filter. In the spectral engineering technique, the distinguishing spectral features of the two-photon state are eliminated through modifications to the two-photon source. In addition, spectral engineering also provides a means for the generation of polarization-entangled states with novel spectral characteristics: the frequency-correlated state and the frequency-uncorrelated state.
The generation of entangled states of three or more photons through the nonlinear process of spontaneous parametric down-conversion requires the use of a femtosecond pump to provide an indistinguishability of photon pairs generated from different nonlinear crystals. We demonstrate experimentally that the quantum interference between two photons generated in each of the nonlinear crystals will degrade significantly as the duration of the femtosecond pump pulse becomes shorter than the coherence time of the signal and idler photons. Spectral postselection using narrow-band filters compensates this effect at the expense of the photon flux available for detection.