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.

# Category Archives: Photonics and Quantum Optics

## Award for the best poster presentation

## Participation on “1st Physics or Physicists (P4P) Students Conference”

Physics student Mateja Batelić has participated on “1st Physics or Physicists (P4P) Students Conference” that was held in Skopje, North Macedonia from 3rd to 6th October. She gave a presentation about her work in Photonics and Quantum Optics Research Unit of Center of Excellence for Advanced Materials and Sensors titled “Neuronal pulse computing”.

## 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).

## Seminar – Euclid

## 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*_{H }= (99,8 +/- 0,6) %, *V*_{V }= (99,7 +/- 0,4) %, *V*_{D }= (98,5 +/- 0,4) %, *V*_{A }= (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.

## Ostvareno generiranje parova polarizacijski spregnutih fotona

Sa zadovoljstvom objavljujemo da je 9. travnja 2018. istraživači Istraživačke jedinice Fotonika i kvantna optika Znanstvenog centra izvrsnosti za napredne materijale i senzore, na Institutu Ruđer Bošković, dovršili su gradnju eksperimentalnog postava izvora parova spregnutih fotona zasnovanog na procesu spontane parametarske pretvorbe (Engl. *spontaneous parametric downconversion* (SPDC), kolinearni proces tipa II) fotona valne duljine 405 nm u parove infracrvenih fotona u PPKTP kristalu te optičkom postavu u Sagnac-ovoj konfiguraciji. Izvor stabilno generira koincidentne parove polarizacijski spregnutih fotona.

Iako je kvantno sprezanje fotona poznato, ovaj kontraintuitivni efekt i dalje je predmet intenzivnog istraživanja kako na fundamentalnoj tako i na razini mogućih primjena u kvantnoj komunikaciji, kvantnom računanju i kvantnoj metrologiji. Ostvareni rezultat je ključan za buduća istraživanja ove grupe.

## Arbitrarily exhaustive generation of contextual sets

Recently obtained results published in Pavičić, M., Arbitrarily exhaustive hypergraph generation of 4-, 6-, 8-, 16-, and 32-dimensional quantum contextual sets, *Physical Review A*, **95**, 06212–1-25 (2017) will be implemented in a series of experiments in the CEMS Research Unit *Photonics and Quantum Optics*.

Quantum contextuality is a property of quantum systems not to have predetermined values of their observables, in contrast to classical systems. Take an entangled photon pair. Each of the photons is genuinely unpolarized before we let them through polarizers. After polarizers, measurements find the photons in definite polarization states. Can we assume that these polarizations were somehow predetermined when the pair was created? The so-called contextual sets of states of photons prove that we cannot. Such sets are not of just of a foundational theoretical interest. Recently, it turned out that the “contextuality is the source of a quantum computer’s power” (*Nature*; cited in the paper). Therefore, it is important for future applications and implementations to find new classes, instances, and structure of contextual sets as well as to design algorithms and programs for obtaining them. In this paper, arbitrary exhaustive hypergraph-based generation of the most explored contextual sets, Kochen-Specker (KS) ones, is carried out in up to 32 dimensions.

Twelve classes of critical KS sets (the ones that cannot be simplified further) are generated and analyzed, huge number of novel types and instances of them obtained and numerous properties of theirs found. Several thousand times more types and instances of KS sets than previously known are generated. All KS sets in three of the classes and in the upper part of a fourth are novel. The generation was carried out with the help of McKay-Megill-Pavičić (MMP) hypergraph language, algorithms, and programs which generate KS sets (see the feature image for two hypergraphs of 8-dim KS sets; also the figure below) strictly following their definition from the Kochen-Specker theorem, which itself celebrates semicentennial this year. This is in contrast to parity proof based algorithms which prevail in the literature and for which the majority of KS sets and even a whole KS class (as the one shown in the Figure below) are simply invisible.