A trusted node-free eight-user metropolitan quantum communication network, Science Advances, 2 September 2020

Kvantni fizičari CEMS Fotonike s Instituta Ruđer Bošković (IRB) dio su međunarodnog tima znanstvenika koji je otkrio i eksperimentalno realizirao kvantnu komunikacijsku mrežu s više korisnika koju je nemoguće špijunirati. Ovo otkriće veliki  je korak prema potpuno sigurnoj i zaštićenoj mrežnoj komunikaciji. Rezultati ovog značajnog znanstvenog otkrića objavljeni su u prestižnom znanstvenom časopisu Science Advances.

Umjetnički doživljaj kvantne mreže (Anta Bučević, vizualni dizajner).

Participation on Humboldt-Kolleg conferention in Zagreb

PhD student Matej Peranić and physics student Mateja Batelić have participated on Humboldt-Kolleg conferention “Science and educational challenges facing Europe in the next decade” that was held in Zagreb on 10th and 11th October. They presented their work by posters titled Experimental generation of quantum entanglement and testing fundamentals of quantum physics and Improved circuits for a biologically-inspired random pulse computer. Their experimental work was done in Photonics and Quantum Optics Research Unit of Center of Excellence for Advanced Materials and Sensors.

Participation on 7th International Symposium on Optics & its applications (OPTICS-2019) and first prize for best student oral presentation

Member of Photonics and Quantum Optics Research Unit of Center of Excellence for Advanced Materials and Sensors, PhD student Matej Peranić participated on the 7th International Symposium on Optics & its applications (OPTICS-2019) that was held from 20.-24. September in Yerevan, Armenia. He was awarded with the first prize for best student oral presentation with the title The source of polarization entangled pairs of photons and testing Bell’s inequality.

Automated generation of Kochen-Specker sets

Quantum contextuality arguably plays an important role in the field of quantum communication and quantum computation, and in our paper in Scientific Reports (Nature journal; IF 4.122) Mladen Pavičić, Mordecai Waegell, Norman D. Megill and P.K. Aravind, “Automated generation of Kochen-Specker sets,” Scientific Reports,” 9, 6765 (2019) we focus on automated vector-component generation of the most explored and used contextual configurations—the so-called Kochen-Specker (KS) sets. They are represented by hypergraphs whose very structure delimit quantum contextuality from classical noncontextuality. When they can be assigned definite predetermined values, e.g., 0 and 1, as in classical computation, they are noncontextual, and when they cannot be assigned predetermined values, as in quantum computation, they are contextual and possess the KS property and become KS sets.

Since quantum contextuality turns out to be a necessary resource for universal quantum computation it becomes important to generate contextual sets of arbitrary structure and complexity to enable a variety of implementations. Up to now, two approaches have been used for massive generation of non-isomorphic KS sets: exhaustive generation up to a given size and downward generation from big master sets. The former faces low computational limits due to the exponential complexity of hypergraph generation and of finding their coordinatization. On the other hand, the latter masters were obtained together with their coordinatization but from serendipitous or intuitive connections with polytopes or Pauli operators or already known masters in lower dimensions. These masters, which we explored in our previous paper Pavičić, M., Physical Review A, 95, 06212 (2017), therefore provide us with a random choice of KS sets and their coordinatization. But what we need for implementations and applications is a method of finding KS sets for a coordinatization of our choice.

In order to find a solution to this problem we turned it upside-down. Instead of searching for vectors we might assign to chosen masters, we generate masters from basic vector components via automated sweeping through simplest of them, starting, e.g., from {-1,0,1} or {-i,0,i}. Next, we elaborate on features, algorithms, and methods which not only speed up the search for KS sets almost exponentially, but also enable arbitrary exhaustive generation of KS sets and their classes.

In the figure below we can see how much more superior our new method is, with respect to the previous ones, e.g. (a), where a master hypergraph with 60 vertices and 105 edges was obtained via Pauli operators. When we use the same vector components as in (a) we get a huge master hypergraph with 688 vertices and 1305 edges which contais a 432-1177 KS master hypergraph and sixteen 16-8 non-KS hypergraphs as shown in (f). Even when we drop the 5th component (+2), we still get a bigger KS master hypergraph (c) then the original (a).








Quantum entanglement of photons and Bell theorem test at CEMS

Photonics and Quantum Optics Research Unit of Center of Excellence for Advanced Materials and Sensors at the Ruđer Bošković Institute announces realization and measurement of quantum entanglement of photon pairs. The experimental setup is schematically shown in the figure. A 405 nm wavelength purple laser beam is fed into Sagnac interferometer containing a periodically-poled crystal of potassium titanil-phosphate (PPKTP), schematically shown below on the left. The actual setup is shown on the right. Thanks to the nonlinear optical nature of the crystal and the specifically selected orientation of its lattice axes, some of the purple photons undergo a process of spontaneous parametric downconversion and thus split into a pair of infrared photons that are quantum entangled in polarization. Quantum entanglement of photons was evaluated in two ways.


First, we measured correlation of polarization of paired photons. To that end, each photon is sent to one of the polarization-measuring stations, named Alice and Bob. The actual setup is shown below. Alice and Bob are each realized as a polarizer mounted on computer-controlled, motorized mount, followed by an optical-fiber-coupled photon detector, as shown in the photo of the actual setup.

Alice can measure polarization along one of 4 special orientations (horizontal (H), vertical (V), diagonal (D) and anti-diagonal (A)). For each of the Alice’s orientations, Bob rotates his polarization analyzer for a full circle and they evaluate probability of measuring a photon polarization along their respective orientation, as a function of Bob’s analyzer angle. The probability forms a sinusoidal fringe, as shown in the figure below. Visibility greater than 50% for all 4 fringes is not possible if photons in a pair have predetermined polarizations. On the other hand, if photons are entangled, then visibility of all 4 fringes can reach the theoretical maximum of 100%. With our source we have obtained: V= (99,8 +/- 0,6) %, V= (99,7 +/- 0,4) %, V= (98,5 +/- 0,4) %, V= (98,3 +/- 0,4) %, as shown in the figure, which indicates near-maximal entanglement of photons.

We also used the Bell’s theorem and performed measurement of Clauser, Horne, Shimony, Holt (CHSH) parameter S, to test the CHSH form of Bell’s inequality. Classical physics predicts S ≤ 2, while quantum physics allows 2 < S ≤ 2√2  2.828. We have experimentally obtained the value of S = 2,803 +/- 0,007 which is more than 114 standard deviations greater than the maximum value of 2 allowed by classical physics, again indicating the near-maximal entanglement.

These results demonstrate the non-local behaviour of quantum-entangled photon pairs, which is that the measurement of polarization performed on one photon has an immediate impact on the result of measurement of polarization on the other photon.

An Overview of Organized and Planned Conferences

Laboratory for ion beam interactions has long and fruitful experience in organization of international research meetings connected to development and applications of ion beam analysis and ion beam modification techniques as well as related developments of accelerator and detector technology. Since the formation of national center of excellence CEMS, research unit Ion beam physics and technology has organized, or plans organization of many international research meetings.

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Direct photo-capacitive neurostimulation using organic pigments

Localized stimulation of neurons in a safe and effective way is important for both research and therapeutic purposes. The currently available solutions based on micro- and nano-electrodes as well as on ion delivery platforms for neuronal electrical communications led to bioelectronic therapies and opened a new window of research in neuroscience. An important limitation of this approach is the need for wiring the electrode at the site of neurostimulation. The motivation for wireless access to the stimulation site has led to optogenic approaches, necessitating genetically modified  targeted neurons for the expression of light sensitive ion channels. Conventional approaches to addressing the wireless connectivity problem that does not include genetic modification include photoelectric stimulation in which the silicon solar cell of micrometric dimensions is attached to the neuronal excitation electrodes. These solutions are used in clinical applications as artificial retinas that are implanted into blind patients with damaged photoreceptors in the retina.

Electric field distribution arround the electrode for direct photocapacitive stimulation a) and d) made of b) organic pigments of p-type (H2PC) and n-type (PTCDI). c) Energy diagram of the electrode, e) schematic representation of the electrode operation.

A new approach to electrical stimulation of neurons comes from the Organic Electronics Laboratory from the University of Linköping in Sweden. Scientists in the group of prof.dr. Eric Glowacki, in which CEMS member Vedran Đerek participated as a post-doctoral student, presented a new approach to photoelectric stimulation of neurons using thin layers of organic semiconductors – cheap pigments used in cosmetics and the color industry (Advanced Materials, https: //doi.org/10.1002/adma.201707292). These pigments represent a class of new functional materials that are stable under physiological conditions, so they do not need to be protected from water encapsulation influence. The nature of the stimulus is completely capacitive, meaning that active materials – pigments – do not participate in chemical reactions during stimulation, therefore the device is persistent and can not introduce harmful substances into the body. The working principle was demonstrated by associates from prof.dr. Hanein group from Israel on the model of blind chicken retina, where in-vitro neurostimulation of retina neurons was demonstrated.

The Robin Hood Solver software package was used to calculate the three-dimensional distribution of electric potential around the photo-capacitive excitation electrode, which was provided by one of the authors of the package, dr. Predrag Lazić from IRB. The successful advancement of CEMS in the bioelectronic direction with the use of new functional materials can be a motivation for future research in which it would be possible to use the previously developed and explored materials within the framework of CEMS.

Foto: Thor Balkhed, LiU
Illustration source: https://doi.org/10.1002/adma.201707292