Feedback receiver for quantum state discrimination

Highlights from our experiments

Feedback receiver for quantum state discrimination

Coherent states are the best signal carriers for optical communication over a high-loss channel since they remain pure states. On the other hand, their non-orthogonality prohibits us from discriminating different coherent states without error, which limits the amount of information transmitted from sender to receiver.


Conventional measurement strategies detecting classical physical variables of optical fields are often far from optimal performance for coherent state discrimination problems. We experimentally developed a novel feedback receiver combined with photon detection and investigated the performance of our receiver for various discrimination problems of coherent states.


Since the feedback receiver based on photon detection can provide a great advantage for general quantum state discrimination, our receiver technique is expected to be a powerful resource in applications associated with classical coherent communication as well as quantum information protocols.

Readout of optical memories with entangled beams generation

In an experiment reported in the journal Science Advances, INRIM and University of York researchers, collaborating in the QUARTET project, demonstrated that the quantum phenomenon of entanglement between light beams allows reading optical memories more efficiently.  This is an example of emerging quantum technologies, with significant possibilities for rapid application.

Readout of optical memory such as for example CD-rom or BluRays is done by measuring the reflection of a laser on the surface of the memory. The information is encoded in a sequence of cells with two different levels of reflectance, each encoding a bit of information. If the incident optical power on the single cell is too low (for example to reduce consumption or increase the speed) or the two levels are close, the energy fluctuations of the light beam prevent their correct discrimination. On the contrary, the correlations between two entangled beams, of which only one is sent to the memory, allow to cancel the fluctuations, ensuring the information recovery.

The consequences of the study go far beyond applications to optical memories. In fact, the same principle can be used in spectroscopy and in transmission or reflection measurement methods of biological samples, chemical compounds and materials, paving the way for non-invasive ultra-sensitive measurements, as one can greatly reduce the optical power used without reducing the amount of information recovered. Another promising perspective explored by the researchers, is to extend the method to the recognition of complex patterns, formed by many cells, in conjunction with modern 'machine learning' algorithms, with potential implications in the field of bio-imaging.

Microwave quantum illumination and short-range quantum radar

Quantum entanglement is a physical phenomenon where two particles remain interconnected, sharing physical traits regardless of the distance between them. The Fink group and their QUARTET collaborators were able to harness this phenomenon for use in a new type of detection technology known as “microwave quantum illumination”. The prototype, also called a “quantum radar”, is able to detect objects with ultra-low power signals in noisy thermal environments where classical radar systems often fail.

Instead of using conventional microwaves, the team entangled two groups of photons: “signal” and “idler” photons. The signal photons are sent out towards the object of interest, while the idler photos are measured in relative isolation, free from interference and noise. When the signal photons are reflected back, true entanglement is lost, but a small amount of correlation remains, creating a signature or pattern that describes the existence or absence of the target object—irrespective of noise within the environment. While the current experiment relies on post-processed correlations which restrict real-life applications, potential future applications will require the development of analog receivers for detection with high quantum efficiency.

The research was reported in many media outlets.

Johannes Fink and Shabir Barzanjeh at IST Austria