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European Quantum Electronics Conference (EQEC)

Entangled photons from a ps-source - bandwidth considerations

Sebastian Bange, Martin R. Ostermeyer, RalfMenzel

Published Jan 18, 2006• 0 citations
Publisher

Abstract

This study investigates the prospects in the entanglement of the photons in their spectral properties. Furthermore new quantum communication protocols involve the simultaneous entanglement of photons in more than just one parameter. In spontaneous parametric downconversion, it is possible to simultaneously entangle photons in their polarization and energy or wavelength. Schemes to realize this double variable entanglement are also discussed.

DOI
10.1109/eqec.2005.1567271

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This study investigates the prospects in the entanglement of the photons in their spectral properties. Furthermore new quantum communication protocols involve the simultaneous entanglement of photons in more than just one parameter. In spontaneous parametric downconversion, it is possible to simultaneously entangle photons in their polarization and energy or wavelength. Schemes to realize this double variable entanglement are also discussed.
Spectrally correlated photon pairs can be used to improve the performance of long-range fiber-based quantum communication protocols. We present a source based on spontaneous parametric downconversion, which allows one to control spectral correlations within the entangled photon pair without spectral filtering by changing the pump-pulse duration or the characteristics of the coupled spatial modes. The spectral correlations and polarization entanglement are characterized. We find that the generated photon pairs can feature both positive spectral correlations, decorrelation, or negative correlations at the same time as polarization entanglement with a high fidelity of 0.97 (no background subtraction) with the expected Bell state.
We present two types of photon sources designed for secure quantum communication, e.g. for quantum cryptography. Both types are based on the creation of photon pairs by spontaneous parametric downconversion in nonlinear crystals. The first is a heralded single photon source and the second is a source of polarization-entangled photon pairs. For the heralded single photon source the detection of one of the photons of a downconversion pair is used as a trigger to announce the presence of the other: the single photon. The source is characterized by a highly sub-Poisson photon number statistics making it very suitable for use in quantum cryptography protocols using single photonic qubits to create correlated information between a sender and a receiver. The entanglement source instead uses the inherent non-classical correlations between entangled qubits. We also present a hybrid-encoding where the sender uses polarization to encode information while the receiver uses time-bins. Both sources create photons with highly non-degenerate wavelengths of 810 nm and 1550 nm, taking advantage of the efficient detectors at near-infrared and the low transmission loss of optical fibers at telecommunication wavelengths.
Spectrally correlated photon pairs can be used to improve performance of long range fiber based quantum communication protocols. We present a source based on spontaneous parametric down-conversion producing polarization entangled photons without spectral filtering. In addition, the spectral correlation within the photon pair can be controlled by changing the pump pulse duration or coupled spatial modes characteristics. The spectral and polarization correlations were characterized. The generated photon pairs feature both positive spectral correlations, no correlations, or negative correlations and polarization entanglement with the fidelity as high as 0.97 (no background subtraction) with the expected Bell state. © 2014 Optical Society of America OCIS codes: 190.4410,300.6190,270.4180,270.5565 Controlling the spectral correlations of a polariza-tion entangled photon pair produced via spontaneous parametric down-conversion (SPDC) could have impor-tant benefits to applications in optical quantum infor-mation. Photonic quantum gates require pure states, which can be created by heralded sources producing pairs of spectrally decorrelated photons [1–11]. On the other
Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographic protocols, and fundamental tests of quantum theory. Spontaneous parametric down-conversion is the most effective source for polarization entangled photon pairs. Here we show, that a entangled 4-photon state can be directly created by parametric down-conversion. This state exhibit perfect quantum correlations and a high robustness of entanglement against photon loss. We have used this state for four-particle test of local realistic theories. Therefore this state can be used for new types of quantum communication. We also report on possibilities for the experimentally realization of a 3-photon entangled state, the so called W-state, and discuss some of its properties.
The quantum information community is entering a period in which scientists and engineers are transitioning from ‘understanding’ to ‘control’ by harnessing aspects of quantum mechanics to realize capabilities in computing, sensing, and communication that are not possible in the classical world. Although many of these quantum technologies are still in their very early stages, it is clear that our future world will include many quantum devices. The existence of multiple devices will, in turn, necessitate the transfer of quantum information between devices—if isolated devices are useful, it follows that connected devices will be even more useful. Therefore, it is imperative to develop the resources needed for connecting quantum devices at different locations. One such resource is quantum entanglement, a physical phenomenon in which two or more quantum systems cannot be described independently, even when separated by large distances. Entanglement is an important resource for quantum computing and is critical for overcoming loss constraints that limit the transfer of quantum states across large distances. The latter can be accomplished via ‘quantum repeaters,’ which propose to connect multiple shorter entangled photon links to form a longer entangled photon link. Much of our understanding of entanglement comes from experiments involving entangled photon pairs, which are still the best candidates for the exchange of quantum information across any appreciable distance. Since the 1990s, spontaneous parametric downconversion (SPDC) has been established in the academic and research community as a reliable source of entangled photon pairs. During this time, source brightness and stability has improved considerably over the first demonstrations. Yet, entangled photon sources are still built primarily by graduate students, and there is significant variability among sources. Moreover, SPDC entangled photon sources lack flexibility, typically designed to output a single type of entangled state. In this paper we report on the efforts to develop a ‘user-programmable’ entangled photon source capable of producing any type of two-photon entangled state. The goal is a source capable of emitting photon pairs with user-defined polarization states. That is, the user will be able to adjust the probability amplitudes {𝛼,𝛽,𝛾,𝛿} in the general expression for a two-photon polarization state, |𝜓⟩=𝛼|𝐻𝐻⟩+𝛽|𝐻𝑉⟩+𝛾|𝑉𝐻⟩+𝛿|𝑉𝑉⟩, thereby accessing any point in the two-photon polarization entanglement Hilbert space. This requires a mapping between the experimentally controllable parameters and the probability amplitudes listed above. We will describe this mapping for our particular experimental setup and will share results showing the states that our system is able to access.