# List of publications

1. P. Sinkovicz, T. Kiss, and J. K. Asbóth
Generalized Kac’s Lemma for Recurrence Time in Iterated Open Quantum Systems
Phys. Rev. A, 93. 050101(R) (2016)
We consider recurrence to the initial state after repeated actions of a quantum channel. After each iteration a projective measurement is applied to check recurrence. The corresponding return time is known to be an integer for the special case of unital channels, including unitary channels. We prove that for a more general class of quantum channels the expected return time can be given as the inverse of the weight of the initial state in the steady state. This statement is a generalization of the Kac lemma for classical Markov chains.

2. P. Sinkovicz, G. Szirmai, and K. Penc
Order by disorder in a four flavor Mott-insulator on the fcc latticePhys. Rev. B, 93. 7. 075137 (2016)The classical ground states of the SU(4) Heisenberg model on the face-centered-cubic lattice constitute a highly degenerate manifold. We explicitly construct all the classical ground states of the model. To describe quantum fluctuations above these classical states, we apply linear flavor-wave theory. At zero temperature, the bosonic flavor waves select the simplest of these SU(4) symmetry-breaking states, the four-sublattice-ordered state defined by the cubic unit cell of the fcc lattice. Due to geometrical constraints, flavor waves interact along specific planes only, thus rendering the system effectively two dimensional and forbidding ordering at finite temperatures. We argue that longer-range interactions generated by quantum fluctuations can shift the transition to finite temperatures.

3. P. Sinkovicz, Z. Kurucz, T. Kiss, and J. K. Asbóth
Quantized recurrence time in iterated open quantum dynamics
Phys. Rev. A 91. 4., 042108 (2015)
The expected return time to the original state is a key concept characterizing systems obeying both classical or quantum dynamics. We consider iterated open quantum dynamical systems in finite-dimensional Hilbert spaces, a broad class of systems that includes classical Markov chains and unitary discrete-time quantum walks on networks. Starting from a pure state, the time evolution is induced by repeated applications of a superoperator (quantum channel) in each time step followed by a measurement to detect whether the system has returned to the original state. We prove that if the superoperator is unital in the part of the Hilbert space explored by the system, then the expectation value of the return time is an integer, equal to the dimension of this relevant Hilbert space. We illustrate our results on partially coherent quantum walks on finite graphs. Our work shows that the expected return time is a quantitative measure of the size of the part of the Hilbert space available to the system when the dynamics is started from a certain state.

4. P. Sinkovicz, A. Zamora, E. Szirmai, M. Lewenstein, and G. Szirmai
Spin liquid phases of alkaline-earth-metal atoms at finite temperature
Phys. Rev. A 88 4., 043619 (2013)
We study spin liquid phases of spin-5/2 alkaline-earth-metal atoms on a honeycomb lattice at finite temperatures. Our analysis is based on a Gutzwiller projection variational approach recast to a path-integral formalism. In the framework of a saddle-point approximation we determine spin liquid phases with lowest free energy and study their temperature dependence. We identify a critical temperature, where all the spin liquid phases melt and the system goes to the paramagnetic phase. We also study the stability of the saddle-point solutions and show that a time-reversal symmetry breaking state, a so-called chiral spin liquid phase, is realized even at finite temperatures. We also determine the spin structure factor, which, in principle, is an experimentally measurable quantity and is the basic tool to map the spectrum of elementary excitations of the system.

5. Z. Szoboszlai, Gy. Nagy, Zs. Kertész, A. Angyal, E. Furu, Zs. Török, K. Ratter, P. Sinkovicz, and A.Z. Kiss
Characterization of atmospheric aerosols in different indoor environments
ATOMKI Annual Report, ISSN 0231-3596 (no.26) 43, 41., 43102323 (2011)
In this study indoor aerosol concentration levels in three different microenvironments of Institute of Nuclear Research (ATOMKI) were investigated. We determined coarse and fine mass concentrations and elemental composition during two summer and two winter campaigns at each location (laboratory, workshop, and library). We also defined the possible emission sources of the indoor aerosols and sturied the effects of the windows replacements of the buildings to the levels of the aerosol concentrations. We demonstrated that the ion beam analysis combined with electron microscopy is an excellent tool for studying indoor air pollution.

6. G. Mandula, Z. Kis, P. Sinkovicz, and L. Kovács
Homogeneous linewidth of the I_11/2--I_15/2 optical transition of erbium in LiNbO_3:Er^3+
Journal-ref: IOP Conference Series: Materials Science and Engineering; Volume 15, Page, 012062 (2010)
We work out a simple, pulsed pump-probe measurement scheme to measure the homogeneous linewidth of an atomic transition in an inhomogeneously broadened spectral line in a solid state environment. We apply the theory to the I_11/2 – I_15/2 optical transition of erbium in LiNbO3:Er3+ crystal. Beside obtaining the homogeneous linewidth, we have estimated the population relaxation time as well.