Abstract
Propagation losses in transmission media limit the transmission distance of optical signals. In the case where the signal is made up of quantum optical states, conventional deterministic optical amplification schemes cannot be used to increase the transmission distance as the copying of an arbitrary and unknown quantum state is forbidden. One strategy that can offset propagation loss is the use of probabilistic, or non-deterministic, amplification schemes - an example of which is the state comparison amplifier. Here we report a state comparison amplifier implemented in a compact, fiber-coupled femtosecond laser-written waveguide chip as opposed to the large, bulk-optical components of previous designs. This pathfinder on-chip implementation of the quantum amplifier has resulted in several performance improvements: the polarization integrity of the written waveguides has resulted in improved visibility of the amplifier interferometers; the potential of substantially-reduced losses throughout the amplifier configuration; and a more compact and environmentally-stable amplifier which is scalable to more complex networks.
Highlights
The no-cloning theorem forbids the copying of an unknown quantum state [1] and gives rise to a minimum bound on the amount of noise that an ideal deterministic amplifier must necessarily add to a signal [2]. It places a fundamental limit on the amount of information that an eavesdropper can extract from a signal composed of non-orthogonal states which is the foundation on which the security of quantum communications protocols is built [3]
This paper reports the first step towards fully on-chip state comparison amplification, which has been achieved by implementing the fundamental core beamsplitters of the amplifier in compact on-chip integrated directional couplers fabricated using femtosecond laser writing (FLW)
The state comparison amplifier (SCAMP) is a probabilistic amplifier operating on a known set of coherent state signals of arbitrary dimension
Summary
The no-cloning theorem forbids the copying of an unknown quantum state [1] and gives rise to a minimum bound on the amount of noise that an ideal deterministic amplifier must necessarily add to a signal [2]. The advanced infrastructure that has already been developed for the electronics industry makes it attractive to use silicon or III-V compound semiconductor systems These techniques produce highly compact devices but they suffer from high propagation losses [28], and, while the technology for coupling electrical signals on- and off-chip is advanced, the same cannot be said for coupling of light which remains a non-trivial task due to the large size of a single-mode fiber core in comparison to the resultant waveguides [30]. These drawbacks are improved (at some cost) by using glass waveguides such as etched silica-on-silicon or writing waveguides directly into glass using femtosecond laser writing (FLW)
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