Strong Spectral Entanglement in Spontaneous Parametric Down-Conversion

Photon pairs with a high degree of spectral entanglement have a very large capacity for carrying information. We describe methods for generating this type of entanglement and discuss applications. ! 2009 Optical Society of America OCIS codes: (270.5585) Quantum information and processing; (270.5565) Quantum communications. Spontaneous parametric down-conversion (SPDC) has proven to be a reliable source of photon pairs for a variety of quantum information applications. In most cases, the SPDC sources are configured for either polarization or timebin entanglement. However, entanglement may also exist in auxiliary degrees of freedom, such as the photons’ energies or momenta. As with polarization or time-bin entanglement, these entanglements can also carry information, often with unwanted consequences. For example, asymmetric spectral information has been shown to reduce the visibility in two-photon Hong-Ou-Mandel interference [1]; and for a broad class of multi-photon experiments, results are best in the absence of spectral entanglement [2]. In these cases, the auxiliary entanglement carries information that distinguishes between interference terms, thus diminishing the interference effect. For this reason, the elimination of auxiliary entanglements has been the subject of several recent works [3]. However, the effects of spectral and/or spatial entanglement are not always harmful and can, in fact, serve as an important resource in quantum information applications. Here, we describe an SPDC source that has been engineered for a high degree of spectral entanglement and we discuss two applications that exploit this type of entanglement. The general expression for a two-photon state entangled in a continuous degree of freedom (like energy or momentum) is ! " = dxd # x f x, # x ( ) x # x $$ , (1)