Quantum-optical signals, like conventional optical signals, experience loss as they propagate through a communication channel (e.g. an optical‑fibre). As this loss becomes significant, some form of optical amplification is required so the signal can still be distinguished by a receiver.
In the early days of conventional optical communications, a simple measure and resend measurement method was used to amplify the optical signal. Later, all‑optical amplifiers were adopted as the preferred approach, allowing for a greater signal bandwidth. This is one of the main reasons for the growth of the internet at the start of the millennium. So, if all‑optical amplifiers for classical signals exist, why is not possible to apply them to quantum signals as well? Unfortunately, the deterministic nature of the amplification (meaning that we always amplify a signal when we want to) adds optical noise to the signal from various mechanisms. This would swamp the quantum properties of our quantum signal. Therefore other methods of optical amplification which can be noiseless need to be explored.
The idea of probabilistic optical amplifiers was proposed to overcome the addition of optical noise. These are based on post‑selection conditions (generally correlations between photon detector events), so we cannot know if a quantum signal will be successfully amplified before the conditions have actually been met.
Our research focuses on the experimental realisations of devices based on a state comparison measurement with phase encoded coherent states. We target our resrach towards improved gain devices with increased success probability through enhanced selection and conditioning of the quantum states.
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State Comparison Amplifier
R.J. Donaldson, R.J. Collins, E. Eleftheriadou, S.M. Barnett, J. Jeffers, and G.S. Buller, "Experimental Implementation of a Quantum Optical State Comparison Amplifier", Physical Review Letters 114(12), Article Number 120505 (2015)