Faithful Entanglement Swapping Based on Sum-Frequency Generation

Abstract

We show that an entanglement swapping operation performed with spontaneous parametric down-conversion can be made faithful without postselection using sum-frequency generation. This invites us to revisit the sum-frequency process and from a proof-of-principle experiment, we demonstrate that it provides a realistic solution for nonlinear optics at the single-photon level. This opens the way to attractive alternatives to six-photon protocols based on linear optics used, e.g., for the heralded creation of maximally entangled pairs or for device-independent quantum key distribution.

Interaction of independent single photons based on integrated nonlinear optics

“…The telecom photons generated by the SPDC source are combined with the coherent state pulses from the DFG source using a dense wavelength division multiplexer (DWDM). We verified the single-photon nature of the SPDC source by measuring the conditional second-order correlation function of the telecom photon after the DWDM to be g (2) (0) ¼ 0.03.…”

“…Realizing such an interaction is not only fundamentally fascinating but holds great potential for emerging technologies. It has recently been shown that even weak optical nonlinearities between single photons can be used to perform important quantum communication tasks more efficiently than methods based on linear optics 2 , which have fundamental limitations 3 . Nonlinear optical effects at single-photon levels in atomic media have been studied 4,5 and demonstrated [6][7][8][9] , but these are neither flexible nor compatible with quantum communication, as they impose restrictions on photons' wavelengths and bandwidths.…”

“…1 shows how the SFG of two photons g 2 and g 3 from independent spontaneous parametric down conversion (SPDC) sources can herald the presence of two distant photons g 1 and g 4 , as proposed in ref. 2. If the photon pairs are initially entangled using time-bin encoding, the detection of the photon g 5 on a basis that erases the 'which path' information about its creation time projects the pair g 1 g 4 in an entangled state.…”

Challenging preconceptions about Bell tests with photon pairs

“…1 to be used for device-independent quantum key distribution based on the CHSH inequality, as the secret key fraction goes to zero for S ∼ 2.43 under the assumption of collective attacks [7]. Note that a higher violation can be obtained through amplification [8] or non-linear filtering [9] at the price of increasing complexity. An alternative solution is based on conditioning, i.e.…”

Passive Decoy-State Quantum Key Distribution with Coherent Light

“…Our method employs sum-frequency generation (SFG) [43][44][45][46] together with linear optical components and classical photodetectors. SFG has already exhibited its usefulness in quantum information [47][48][49][50][51][52] and device-independent QKD [53] at the single-photon level. Here we use it in the conventional non-linear optics paradigm with strong coherent light.…”