Abstract
Generation of time-bin entangled photon pairs requires the use of the Franson interferometer which consists of two spatially separated unbalanced Mach-Zehnder interferometers through which the signal and idler photons from spontaneous parametric down-conversion (SPDC) are made to transmit individually. There have been two SPDC pumping regimes where the scheme works: the narrowband regime and the double-pulse regime. In the narrowband regime, the SPDC process is pumped by a narrowband cw laser with the coherence length much longer than the path length difference of the Franson interferometer. In the double-pulse regime, the longitudinal separation between the pulse pair is made equal to the path length difference of the Franson interferometer. In this paper, we propose another regime by which the generation of time-bin entanglement is possible and demonstrate the scheme experimentally. In our scheme, differently from the previous approaches, the SPDC process is pumped by a cw multi-mode (i.e., short coherence length) laser and makes use of the coherence revival property of such a laser. The high-visibility two-photon Franson interference demonstrates clearly that high-quality time-bin entanglement source can be developed using inexpensive cw multi-mode diode lasers for various quantum communication applications.
Highlights
The coherent superposition of physically exclusive single-or many-particle path amplitudes is exemplified best with the minimalistic paradigm of a single particle prepared in the state∣1: 0〉 ≡ (∣1, 0〉 + ∣0, 1〉) 2, a coherent superposition of the upper and lower arms of a Mach–Zehnder interferometer
We introduce a versatile formalism for many-boson interferometry based on double-sided Feynman diagrams, which we apply to a protocol for differential decoherence diagnosis: twin-Fock states
—possibly strong—coherent states, for which the signal intensity replaces the event probability in equation (1), there is no possibility for a qualitative differentiation of decoherence mechanisms either, nor for N 00N -states, as we show below
Summary
The coherent superposition of physically exclusive single-or many-particle path amplitudes is exemplified best with the minimalistic paradigm of a single particle prepared in the state∣1: 0〉 ≡ (∣1, 0〉 + ∣0, 1〉) 2 , a coherent superposition of the upper and lower arms of a Mach–Zehnder interferometer. The interferometer in figure illustrates three prominent mechanisms for decoherence: dephasing, mixing, and path distinguishability. The three decoherence effects are, not differentiated in practice by the single-particle interferometric signal. The parsimonious description inherent to (2) is certainly sufficient to predict the combined impact of dephasing, distinguishability and mixing on an interferometer [7, 8], and an impressive level of understanding of decoherence in nature has been achieved through studies that essentially monitor only the visibility, as demonstrated for large molecules [8]. The formalism naturally allows us to treat mixed states and to thereby incorporate decoherence processes such as mixing, distinguishability and dephasing. A four-particle double-Fock state∣2, 2〉 allows a clear diagnosis of mixing against distinguishability, while the entangled doubleFock superposition∣2: 1〉 ≡ (∣1, 2〉 + ∣2, 1〉) 2 quantifies dephasing. States with larger particle numbers promise an even more detailed revelation of the processes that deteriorate interference
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