Category Archives: Fotonika i kvantna optika

Quantum contextual sets have been recognized as resources for universal quantum computation, quantum steering and quantum communication. Therefore, in our paper in “Quantum” (Impact Factor 6.4) Mladen Pavičić, “Quantum Contextuality,” Quantum 7, 953 (2023); DOI 10.22331 q-2023-03-17-953 we focus on engineering the sets that support those resources and on determining their structures and properties. Such engineering and subsequent implementation rely on discrimination between statistics of measurement data of quantum states and those of their classical counterparts. Their discriminators are hypergraphs which determines how states supporting a computation or communication are arranged.

It turns out that contextual quantum non-binary hypergraphs, in contrast to classical binary ones, are essential for designing quantum computation and communication and that their structure and implementation rely on such non-binary vs. binary differentiation. We are able to generate arbitrarily many contextual sets from simplest possible vector components and then make use of their structure by implementing the hypergraphs with the help of YES-NO measurements so as to collect data from each gate/edge and then postselect them. At the same time this procedure shows us that we have to carry out measurements on complete set of states before we postselect them. As an example the Klyachko pentagon cannot lie in a plane, as shown in the figure; only its postselected states do.

Klyachko’s pentagon

Other considered discriminators are six hypergraph inequalities. They follow from two kinds of statistics of data. One kind of statistics, often applied in the literature, turn out to be inappropriate and consequently two kinds of inequalities turn out not to be noncontextuality inequalities. Results are obtained by making use of universal automated algorithms which generate hypergraphs with both odd and even numbers of hyperedges in any odd and even dimensional space – in this paper, from the smallest contextual set with just three hyperedges and three vertices to arbitrarily many contextual sets in up to 8-dimensional spaces. Higher dimensions are computationally demanding although feasible.

5. 8. 2021. Uspostavljena je šifrirana audio-video komunikacija između Italije, Slovenije i Hrvatske uz pomoć kvantne tehnologije.

Prva javna demonstracija kvantne komunikacije između tri zemlje: Italije, Slovenije i Hrvatske, održana je danas u sklopu sastanka G20 u Trstu. Taj je prijenos ostvaren između Trsta, Ljubljane i Rijeke. Zahvaljujući znanstvenicima i stručnjacima predvođenima Institutom Ruđer Bošković (IRB) u sklopu konzorcija Hrvatske kvantne komunikacijske infrastrukture (CroQCI), Hrvatska se sudjelovanjem u ovom poduhvatu, iako nije članica G20, našla uz rame najbogatijih i najutjecajnijih zemalja.

dr. sc. Mario Stipčević i dr. sc. Martin Lončarić

Demonstraciju su organizirali prof. Angelo Bassi s Odjela za fiziku Sveučilišta u Trstu i grupa za kvantne komunikacije Nacionalnog instituta za optiku (CNR-INO) pod vodstvom dr. sc. Alessandra Zavatte.

U Sloveniji su demonstraciju predvodili prof. Rainer Kaltenbaek i prof. Anton Ramšak s Fakulteta za matematiku i fiziku Sveučilišta u Ljubljani, uz tehničku podršku Telekoma Slovenije, dok su u Hrvatskoj ovaj poduhvat predvodili dr. sc. Mario Stipčević i dr. sc. Martin Lončarić s Instituta Ruđer Bošković uz podršku kolega s Fakulteta prometnih znanosti u Zagrebu te tvrtke OIV – Digitalni signali i mreže.

“Hrvatska strana, pored pune kvantne komunikacije na 100,5 kilometara dugoj relaciji Trst-Rijeka, ovdje je po prvi puta u svijetu javno demonstrirala takozvanu “kvantno pojačanu kriptografiju”, putem koje je Rijeka povezana sa Zagrebom. Uređaj koji je to omogućio plod je suradnje Pomorskog centra za elektroniku (PCE) iz Splita i Instituta Ruđer Bošković iz Zagreba. To je međukorak koji nudi sigurnost manju od kvantno kriptografske (koja je apsolutna), ali i značajno veću od uobičajenih kripto rješenja, a po bitno nižoj cijeni od kvantne kriptografije”, objasnio je dr. sc. Mario Stipčević, voditelj Laboratorija za fotoniku i kvantnu optiku na IRB-u te jedan od koordinatora demonstracije u Hrvatskoj.

”Uspješnom realizacijom ovog poduhvata naši su znanstvenici i stručnjaci probili led i utrli put realizaciji kvantne infrastrukture u Republici Hrvatskoj. Ovime je dokazana spremnost Hrvatske za sudjelovanje u izgradnji sveeuropske kvantne komunikacijske mreže,” izjavio je dr. Stipčević.

Kvantna komunikacija zadovoljava potrebu za sigurnom komunikacijom, što je prioritet svih vlada diljem svijeta. Ova tehnologija postiže apsolutnu sigurnost zahvaljujući kvantnoj enkripciji koja funkcionira putem razmjene fotona, a koja omogućuje trenutnu detekciju pokušaja hakerskog upada.

“Važnost današnje demonstracije dodatno je naglašena i u kontekstu buduće Europske kvantne komunikacijske infrastrukture (EuroQCI) koju promiče svih 27 država članica i Europska komisija uz potporu Europske svemirske agencije. Hrvatska je od početaka uključena u oblikovanje EuroQCI”, objašnjava dr. sc. Martin Lončarić s IRB-a, koji je uz dr. sc. Stipčevića koordinator ovih aktivnosti u Hrvatskoj.

“Danas smo položili kamen temeljac nove europske kvantne infrastrukture”, objasnio je predsjednik europske Quantum Community Network (QCN) Tommaso Calarco. “Kruna istraživanja provedenog tijekom prve faze programa ‘Quantum Flagship’ nudi svim europskim građanima infrastrukturu za potpunu zaštitu privatnosti njihovih podataka uz sigurnost bez presedana, koja nam po prirodi i pripada”, zaključio je.

Demonstracija je započela službenim pozdravima predstavnika institucija, nakon čega je je uslijedila prava glazbena poslastica koju su priredila tri glazbena kvarteta. Naime, glazbenici su koristeći kvantnu kriptografiju uspostavljenu za demonstraciju ove komunikacije izveli posebno pripremljena glazbena djela u svakoj od triju država.

Glazbeni nastup ostvaren je zahvaljujući suradnji tršćanskog Konzervatorija “Giuseppe Tartini” zajedno s Muzičkim akademijama u Ljubljani i Zagrebu.

Kvartet saksofonista Muzičke akademije u Ljubljani (Miha Rogina sopran, saksofon, Nika Deželak alt saksofon, Agata Živoder tenor saksofon, Domen Koren bariton) izveo je “Tango virtuoso” Thierryja Escaicha. Gudački kvartet Muzičke akademije u Zagrebu (Matej Žerovnik i Luka Kojundžić violina, Filip Kojundžić viola, Lucija Mušac violončelo) izveli su “Scherzo” Frana Lhotke, dok je kvartet saksofonista Tršćanskog konzervatorija “Giuseppe Tartini” (sopran-saksofon Elia Sorchiotti, alt-saksofon Emma Marcolin, tenor saksofon Matilda Travain, baritonski saksofon Marin Komadina), izveo “Suite Hellenique” Pedra Iturraldea.

Potporu realizaciji dali su Ministarstvo vanjskih i europskih poslova, Ministarstvo znanosti i obrazovanja, Ministarstvo mora, prometa i infrastrukture, Ministarstvo obrane i Agencija Alan te projekti Hrvatske zaklade za znanost ug. br. IPS-2020-01-2616 i MZO ug. br. KK.01.1.1.01.0001.

Dodatne informacije možete pronaći ovdje.

Video URL: https://youtu.be/LLc_YP7FngI

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).

Doktorand Matej Peranić iz istraživačke jedinice Fotonika i kvantna optika Znanstvenog centra izvrsnosti za napredne materijale i senzore dobio je priznanje za najbolje postersko priopćenje na 4. Simpoziju studenata doktorskih studija PMF-a koje se održalo u Zagrebu 28. veljače 2020. godine.

Na 7. međunarodnoj konferenciji Optika i njene primjene (OPTICS-2019, http://www.ift.uni.wroc.pl/~optics2019/) koja se održala od 20. do 24. rujna u Jerevanu, Armenija sudjelovao je član istraživačke jedinice Fotonika i kvantna optika Znanstvenog centra izvrsnosti za napredne materijale i senzore, doktorand Matej Peranić. Za svoju prezentaciju pod nazivom “The source of polarization entangled pairs of photons and testing Bell’s inequality” dobio je nagradu za najbolje studentsko usmeno izlaganje.

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).

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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.