State Comparison Amplification

Lead Research Organisation: University of Strathclyde
Department Name: Physics


Part of the work in the EPSRC quantum communications hub is to analyse amplifiers and repeaters for quantum information. These are essential to offset the substantial loss in typical quantum communication systems.

One such amplification mechanism is the state comparison amplifier, first proposed by Jeffers in 2013 and implemented in 2015. This amplifies specific sets of quantum coherent states with high success probability and fidelity. Such states of light are easy to make and can be used in quantum communications systems.

The device works by comparing the input state to be amplified with a guess state using a beam splitter and a detector. When the detector fires the input state and the guess state are different and the output is discarded. Otherwise the two states add their amplitudes and the output is accepted. A low success probability photon subtraction is then performed on the output to filter the amplified state of imperfections.

One improvement that might be made to such a device is the inclusion of feed-forward to correct incorrect amplifications. When the first detector fires we know that the guess state was incorrect so we can correct the output perfectly by feeding this knowledge forward to a second amplification stage that simply adds enough of the right state. This improves both the fidelity of the output and the overall success probability of the device. Moreover, a photon subtraction filtration is no longer required.

This project will investigate some of the physics behind the device, leading to improvements that could help with real world implementations.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509802/1 30/09/2016 30/03/2022
1810881 Studentship EP/N509802/1 30/09/2016 29/06/2020 Gioan Tatsi
EP/N509760/1 30/09/2016 29/09/2021
1810881 Studentship EP/N509760/1 30/09/2016 29/06/2020 Gioan Tatsi
Description The primary goal of this project has been to further develop and improve the comparison state amplifier, originally proposed by Jeffers in 2013 and first implemented in 2015; a light amplifying device that works by comparing an input signal to a guess state and discarding the signal when the two do not match, or constructively adding their amplitude otherwise, hence amplifying the input signal. The original proposal works for coherent states of light; states which are easy to produce and of great importance to quantum communication tasks.

We have since then investigated the potential of the device to amplify Schrödinger cat states of light. Cat states are superposition states of some degree of freedom of light, e.g polarisation- a cat state can be though of as a superposition of horizontally and vertically polarised photons. In the original thought experiment Schrödinger in his attempt to explain the superposition principle of quantum physics envisioned a cat trapped in a closed box. The cat could be dead or alive, but to an external observer who has not opened the box the cat is dead and alive at the same time. These are states of light which are very fragile but at the same time great resources in quantum computation tasks. For these tasks cat states of high amplitude and high fidelity are required. The first key finding of this project has been that the state comparison amplifier can be used to amplify input cat states with high probability of success and high fidelity using a guess state, a beamsplitter and a detector.

Among other investigations we noticed that the state comparison amplifier could be used to implement an all photonic Maxwell's demon. Maxwell envisioned a demon standing at the interface between two chambers of gas and sorting the gas molecules depending on their speed so that the all the fast molecules will be in one reservoir and the slow ones on the other, thus decreasing the overall entropy (the degree of disorder in the system) of the system and apparently violating the second law of thermodynamics,which states that the entropy of a system must never decrease. Maxwell failed to account for the memory of the demon that needed to be wiped and which would require an increase in the entropy, thus saving the second law. The idea of the demon is of fundamental interest despite the fact that there is no "free" lunch. A second key finding has been that the state comparison amplifier with input light from a thermal source (meaning that the only information we know is the average energy of the input state) can be used to implement an all photonic Maxwell's demon. The role of the demon in this case is played by the detector, if it records a photon we have fast (high energy) photons while if it does not we have slow (low energy) photons on the other output port of the beamsplitter.
Exploitation Route The first step would be to experimentally verify both of these findings and we are already under talks with a group in Edinburgh. While the all photonic Maxwell's demon is mostly of fundamental interest to the scientific community, the state comparison amplifier has been already implemented on a chip and one could potentially see it in the future as a quantum repeater device in quantum communication networks.
Sectors Digital/Communication/Information Technologies (including Software)