**NOTE: This is archived profile page. From September 2017 I am not at the JQC anymore. You can contact me on nikolaREMOVEsibalicATphysicsDOTorg**

After attending Mathematical High School in Belgrade, Serbia, I have studied Theoretical and Experimental Physics at the University of Belgrade, Faculty of Physics. I joined Durham's AtMol group as a postgraduate student in October 2013. To find out more about the research I am involved in, please continue reading below.

**Research interest**

Highly excited atoms, in so-called Rydberg states, have strong atom-atom interactions, and long lifetimes. This makes them flexible platform for both quantum engineering on few-excitation level (light storage, single photon non-linearities, quantum logic gates) and exploration of fundamental physics that opens in the many-body regime with many excitations. Given the inherently non-equilibrium and non-linear nature of this system, understanding the phase diagrams, transitions and ordering in them represents one of the major challenges of the modern physics. All of this has a practical spin-offs too, due to the fact that atoms are sensitive probes of EM fields, with constant and indentical properties everywhere in the Universe.

**1) Fundamental physics**

Rydberg atoms driven with the laser fields, dissipating through spontaneous emission and forced decay channels, with interactions tunable with static and microwave/terahertz EM fields present ideal platform for exploration of non-equilibrium dynamics in well-controlable way. We recently explored numerically non-equilibrium phase diagrams of these systems, and found that phase diagrams depend qualitatively on the cross-over range between short-range (van der Waals) and long-range (resonant dipole-dipole) interactions. We explored transition between the frozen and hot systems, and found that qualitative changes in dynamics can occur even at $\mu K$ temperatures. Finally, we develop new efficient theoretical framework for highly disordered (hot) systems.

Obtained non-equilibrium phase transition diagrams can be explored in current experiments, while the new theoretical framework can help us in describing complex multi-level, multi-component situations, since it offers efficient, consistent and expendable framework.

Reference: *Phys. Rev. A* **94**, 011401(R) (2016)

**2.1) Tools for working with highly excited states - concepts and ideas**

Storing of light field excitation as a collective excitation of atomic clouds is important for quantum memories and gates. The storage time so far is limited by atomic motion that smears out information about the relative phases of the stored excitation within the cloud, preventing succesful retrival of light in well defined spatial output channel. This is especially problem for ledder type excitation to Rydberg states, due to big mismatch of the wavelenghs of light used for excitation.

In recent work, we proposed new approach for saving information in the form of uniform phase spin-waves, that would be insensitive to motion. We showed that by performing the strong resonant driving of the two excited states, one can get engineered dressed state, that can be used as a proxy in stadard three-level storage and retrival protocols. Adiabatic mapping of excitations, used in EIT based three level storage protocols, can be directly applied in unchanged form in the new scheme. However, generalised scheme uses actually three lasers and four states, allowing one to achieve Doppler free excitation of the uniform phase spin-waves, thus overcoming limitations of storage time due to atomic motion. Additionaly, off-resonant excitation with three lasers, aranged in plane, allows one to selectivily (de)excite atoms in very small volume (diameter >10 $\mu$m) within bigger cold atomic clouds or vapour cells, which can allow new types of experiments.

Detailed theoretical discussion of possiblities and limitations is followed with proof-of-principle experiment.

Reference: *Phys. Rev. A*** 94**, 033840 (2016)

We have embraced motion of the collective atomic excitation, to store single photon in superposition of two states: one nearly stationary, while the other moves away. Observed (collective) quantum beats due to emission of single photon from both of this states demonstrated coherent nature of the storage.

See news article for more details.

Reference: *Phys. Rev. Lett.* **118**, 253601 (2017)

Very short laser pulses, or spontaneous decays, can prepare system in superposition of states with well defined relative phase. If this states happen to decay radiatively, we can see signature of this coherent superposition as oscilation of the fluorescence intensity. This so-called quantum beats happen due to interference of multiple decay paths whose relative phases evolve in time due to differences in energies, imposed external drivings and relative atomic motion (if they are are collective, not single-atom based). We have explored this effect to follow dynamics between two excited states driven by coherent laser field, and observed collapses and rivivals of beating.

Reference: *Phys. Rev A 90*, 033424 (2014)

Different excitation schemes can offer not only technological simplifications, but also new possibilities. We have recently explored four photon Rydberg excitation scheme, and observed absorptions and transparencies due to complex multi-level interference effects (EIT and EIA). This was possible even with very low laser powers ($\mu$W and nW)

Reference: *Optics Letters 40*, 5570 (2015)

**2.2) Tools for working with highly excited states - software**

Have you ever wished to be able to quickly perform calculations in Rydberg atomic physics? From quick estimates during meetings, to easy building up of calculations from primitives and data at hand, suitably wrapped in useful tools? From asymptotic C6 coefficients, to Stark maps and spaghetti diagrams, in easily form that facilitates their exploration and use? Something easy to expand for your own research purposes? We have developed **ARC - Alkali Rydberg Calculator**, a hierarchical Python library of calculation methods and data, precisely with this in mind. Check our code, detailed .html documentation and iPython notebook with examples on GitHub (and contribute expansions!).

Reference: *Computer Physics Communications* **220**, 319 (2017)

**Online Atom calculator**: atomcalc.jqc.org.uk

**3) Applications**

Atomic vapours provide a medium with properties that are fixed in time, and easily reproduced. This makes them an excellent resource for metrology. In particular, wide range of the transitions between the highly excited atoms corresponds to the terahertz range (0.3-3 THz) in electromagnetic spectrum. Traditionally, sources and detectors in this range have been scarce, since these frequencies are too high for usual semiconductor technology on one hand, and too low for optical technology, on the other.

Recently we managed to achieve atomic excitation (and subsequent observable fluorescence) proportionally to the THz intensity, allowing mapping of THz field into optical domain. This is achieved by driving off-resonant optical-terahertz Raman transition to the excited states. Excitable atomic medium is prepared with laser ladder driving, which selects atomic velocities, and prevents smearing of fluorescence pattern, allowing sub-wavelength resolution in 2D. Using usual resonant Autler-Townes splitting measurements, and calculated values of dipole matrix elements (that in principle can be fixed to the absolute standards), measurements are calibrated, providing 3D sub-wavelength THz field sampling, and simultaneous measurement along 1D. Importantly, since lifetimes of the excited atomic states are of the order of 10$~\mu$s, the refresh rate of the excitable medium is quite fast, allowing real-time imaging of dynamic terahertz fields. This is demonstrated imaging fields at 25fps with consumer camera.

Reference: *Nature Photonics* **11**, 40 (2017)

**The most recent results...**

For most recent work, please see Publications tab, where you can find arXiv preprints too.

**My PhD thesis**

Here are the links for chapters of my thesis "Rydberg atom ensembles under dephasing and dissipation: from single- to many-body dynamics" (Durham University, 2017)

Introduction, Overview of dephasing and dissipation mechanishms and their impact on dynamics. Short history of Rydberg physics.

Rydberg atomic states: energy level structure and dynamics. Check here for more details about ARC project, atomic structure, Rydberg interactions and THz imaging.

Spin-wave motion. Check here for schemes that provide light storage that is insensitive to motion (uniform-phase spin-waves) in ladder storage schemes, and for quantum beat fenomena, both signle-atom qunatum beats, and collective quantum beats.

Driven-dissipative systems with power-law interactions. Check here for more details about non-equilibrium transitions of driven-dissipative systems, occurance of bistability, and impact of spin/atom motion on the non-equilibrium phase diagrams.

Appendix A3 contains derivation of Ensemble Averaged Mean Field, that gives analytical solution (equations A.15 and 4.10) for driven-dissipative dynamics of interacting spins that becomes exact solution in the llimit of strong mixing of spins due to fast motion.

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