Long-distance distribution of genuine energy-time entanglement

Abstract

Any practical realization of entanglement-based quantum communication must be intrinsically secure and able to span long distances avoiding the need of a straight line between the communicating parties. The violation of Bell’s inequality offers a method for the certification of quantum links without knowing the inner workings of the devices. Energy-time entanglement quantum communication satisfies all these requirements. However, currently there is a fundamental obstacle with the standard configuration adopted: an intrinsic geometrical loophole that can be exploited to break the security of the communication, in addition to other loopholes. Here we show the first experimental Bell violation with energy-time entanglement distributed over 1 km of optical fibres that is free of this geometrical loophole. This is achieved by adopting a new experimental design, and by using an actively stabilized fibre-based long interferometer. Our results represent an important step towards long-distance secure quantum communication in optical fibres.

Postselection-Loophole-Free Bell Test Over an Installed Optical Fiber Network

“…Unconditional security is possible provided there is an observation of a loopholefree Bell inequality violation. During the last years there have been important advances towards this accomplishment [15][16][17][18][19][20][21][22][23]. In this work we present another step towards fiber-based loophole-free Bell tests, which are specially relevant for device-independent quantum communication due to the existing optical telecommunication networks.…”

“…We have also designed coincidence electronics based on a field programmable gate array (FPGA) platform, in order to implement 1 ns coincidence windows, reducing accidental counts, compared to Ref. [20] where 4 ns windows were employed. We obtained on the order of 300 000 counts=s at either of Alice's detectors, 9000 counts=s at Bob's, and a total of 40 mean coincidence counts=s.…”

“…These pulses are detected by Bob through a dedicated p-i-n photodiode, and fed to the coincidence electronics. Bob's interferometer is phase locked, and then by performing coincidence measurements, the delay line is properly adjusted in the indistinguishability position [20].…”

“…To deal with such phase fluctuations, we extended a technique previously demonstrated in Refs. [20,36]. At Bob's station, a piezoelectric fiber stretcher (FS) composed of 40 m of singlemode optical fiber is placed in the S B arm, which is used by an electronic control system to actively compensate the environmental phase fluctuations in the interferometer.…”

“…However, if loopholes are present in the experimental implementation, security can no longer be guaranteed [14]. Although considerable progress has taken place in the last few years, and Bell inequality violations have been observed free of some loopholes [15][16][17][18][19][20][21][22][23], the goal of a violation free of all loopholes is still missing.…”