Envían con éxito información entre partículas entrelazadas a través de agua de mar. Científicos chinos han enviado con éxito información entre partículas enlazadas a través de agua de mar, siendo la primera vez que este tipo de comunicación cuántica se ha logrado bajo el agua. La información viajó en un tanque de 3,3 metros de largo, pero los investigadores, cuyos resultados han sido publicado por The Optical Society, predicen que deberían ser capaces de usar la misma técnica para enviar comunicaciones estables a 885 metros a través de aguas abiertas, donde la aplicación más obvia sería un submarino que quiera permanecer sumergido pero que se comunique de una manera segura.
Long-distance quantum channels capable of transferring quantum states faithfully for unconditionally secure quantum communication have been so far confirmed to be feasible in both fiber and free-space air. However, it remains unclear whether seawater, which covers more than 70% of the earth, can also be utilized, leaving global quantum communication incomplete. Here we experimentally demonstrate that polarization quantum states including general qubits of single photon and entangled states can survive well after travelling through seawater. We perform experiments with seawater collected over a range of 36 kilometers in the Yellow Sea. For single photons at 405 nm in a blue-green window, we obtain an average process fidelity above 98%. For entangled photons at 810nm, albeit very high loss, we observe the violation of Bell inequality with 33 standard deviations. Our results confirm the feasibility of a seawater quantum channel, representing the first step towards underwater quantum communication.
1. Introduction
Underwater communication is vital for undersea exploitation and modern communication. Conventional ways which employ acoustical technique for underwater communication have their drawbacks [1,2], including high path loss, narrow bandwidth, high bit error rate, among which unconditional security is more demanding due to commercial and secure interest. The question arises of whether quantum communication [3–7] can be achieved by employing seawater as a reliable channel.
Since the seminal work of Bennett and Brassard on transferring quantum states and cryptographic keys through 0.3-meter-long free-space air [8], quantum communication has become an ultimate approach for the goal of unconditional communication security. Massive experimental efforts have been made in channels of fiber [9–11] and free-space air [12–14], pushing the distance up to the order of 100 kilometers [15–19]. Although quantum key distribution and quantum teleportation have been achieved via optical fiber installed underneath Geneva Lake and the River Danube [11] respectively, experimental investigation in free-space seawater has never been done so far.
Fortunately, as existence of transmission window around 800nm in free-space air [14], there is a blue-green optical window at the wavelength regime of 400–500nm in free-space seawater [20], wherein photons experience less loss and therefore can penetrate deeper. Photonic polarization may be a desirable carrier of quantum bits since isotropic seawater induces very limited birefringent effect. In this letter, to verify feasibility of seawater quantum channel, we experimentally explore polarization preservation properties of single photon and quantum entanglement.
Fuente: https://www.osapublishing.org
