Experimental Demonstration of Counterfactual Quantum Communication

Quantum effects, besides offering substantial superiority in many tasks over classical methods, are also expected to provide interesting ways to establish secret keys between remote parties. A striking scheme called “counterfactual quantum cryptography” proposed by Noh [Phys. Rev. Lett. 103, 230501 (2009).] allows one to maintain secure key distributions, in which particles carrying secret information are seemingly not being transmitted through quantum channels. We have experimentally demonstrated, for the first time, a faithful implementation for such a scheme with an on-table realization operating at telecom wavelengths. To verify its feasibility for extension over a long distance, we have furthermore reported an illustration on a 1 km fiber. In both cases, high visibilities of more than 98% are achieved through active stabilization of interferometers. Our demonstration is crucial as a direct verification of such a remarkable application, and this procedure can become a key communication module for revealing fundamental physics through counterfactuals.

Figure 1

Schematic setup of fiber-based counterfactual quantum cryptography system. A signal photonic pulse is sent from Alice’s side before it passes through an optical circulator and then is split by a BS to travel along paths $a$ and $b$. Then, the pulse goes through a Michelson-type interferometer. Sifted keys will be established by selecting only the events for which $D1$ alone detects a photon with a correct final polarization state. LD: laser diode; PBS: polarization beam splitter; ATT: fiber attenuator; CIRC: optical circulator; PC: polarization controller; BS: beam splitter; FS: fiber stretcher; DWDM: dense wavelength division multiplexer; FF: fiber filter; OD: optical delay line; OL: optical loop; SW: fiber switch; FM: Faraday rotator mirror; $D0$, $D1$, $D2$, $D3$: detectors; $D1H$ and $D1V$ are short notions of detector $D1$ for detecting horizontal and vertical polarization, the same for $D3H$ and $D3V$.