arXiv subjects

Özgür Çakır

Publications and source records attributed to Özgür Çakır.

6 recordsLinked to original sources

arXiv2026

Topological Edge States and Collective Radiation in a One-Dimensional Atomic Chain

We investigate the topological and collective radiative properties of a one-dimensional diatomic chain of identical quantum emitters with alternating spacing, coupled to the electromagnetic vacuum. The system realizes an extended, non-Hermitian Su-Schrieffer-Heeger-type model with vacuum-mediated long-range interactions and collective dissipation. We focus on the single-excitation manifold, where the dynamics are described by an effective Hamiltonian. For an infinite chain, the complex band structure reveals subradiant modes associated with wave vectors outside the light line, as well as parameter regimes where real-part band crossings occur. The bulk topology is characterized by a complex Berry phase, which remains quantized in the presence of inversion symmetry and a spectral gap. For finite chains, using exact diagonalization, we identify superradiant, subradiant, and edge states with distinct decay rates and spatial profiles. Edge states emerge in the topologically nontrivial regime when a bulk gap in the real part of the energy spectrum is present, while subradiant states exhibit strongly suppressed decay with system-size dependence. Finally, we analyze the far-field radiation patterns associated with different classes of eigenstates and show that the emission characteristics reflect the decay properties, spatial localization, and the parity of the unit-cell amplitudes.

cond-mat.other↗

arXiv2026

Environment-assisted transport in a strongly correlated boundary-driven Fermi-Hubbard chain

We study steady-state transport in a one-dimensional Fermi-Hubbard chain coupled to particle reservoirs at the boundaries and to local dephasing baths at each site, using the time-evolving block decimation (TEBD) method to solve the Lindblad master equation. In the absence of dephasing, the current exhibits two well-separated maxima as a function of the boundary driving rate, reflecting the distinct charge and spin energy scales of the strongly correlated regime. Upon introducing dephasing, we find two distinct dephasing-induced transport-enhancement regimes, in contrast to the single enhancement previously reported for spinless fermions. Analysis of the non-equilibrium steady state in the Hamiltonian eigenbasis reveals that the two regimes originate from distinct dephasing-induced redistribution processes: the first involves redistribution within the uppermost Hubbard band, while the second involves transitions between Hubbard bands. Our results demonstrate how many-body correlations shape the interplay between coherent driving, dephasing, and quantum Zeno physics in strongly correlated open systems.

cond-mat.str-el↗

arXiv2026

Transition Dipole Rotation Beyond the Condon Approximation in Single hBN Quantum Emitters

The design of polarization-encoded quantum interfaces relies on the assumption that solid-state emitters possess static transition dipoles defined by the host lattice symmetry. Here, we demonstrate that the transition dipole moment of single hexagonal boron nitride quantum emitters is not a static property but rotates as a function of photon energy. Through high-resolution energy-resolved spectroscopy, we reveal a continuous rotation of the emission dipole orientation reaching up to $40^{\circ}$ across the vibronic manifold at room temperature, driven by coupling to the phonon bath. This spectral rotation is effectively suppressed at cryogenic temperatures (6 K), where the acoustic phonon population is negligible, identifying thermally activated lattice vibrations as the primary driver of the reorientation. First-principles calculations on two representative defects spanning weak and strong electron-phonon coupling regimes confirm that phonon-displaced geometries produce a systematic deviation of the transition dipole orientation from the zero-phonon line, with the magnitude scaling with vibronic coupling strength. The experimental observations and calculations demonstrate that single quantum emitters can operate beyond the Condon approximation, with the transition dipole acquiring a dependence on the instantaneous nuclear configuration. Our results identify a fundamental limit for polarization fidelity in solid-state quantum networks and connect solid-state single-emitter physics to a class of effects previously accessible only in ensemble measurements in molecular and biological spectroscopy.

quant-ph↗

arXiv2022

Electromagnetically induced transparency and absorption cross-over with a four-level Rydberg system

Electromagnetically induced transparency (EIT) and absorption (EIA) are quantum coherence phenomena which result due to interference of excitation pathways. Since the first realization of EIT in 1991 and EIA in 1998, mechanisms behind these remarkable effects have been studied extensively, therefore both are relatively easily achievable in controlled environments. Combining EIT and EIA with Rydberg atoms have provided utility in various research fields such as interacting many body systems, quantum optics and quantum information science. We propose an analysis of Rydberg-EIT and Rydberg-EIA effects in cold Cs and Rb atomic ensembles in a four-level ladder type scheme with self-consistent mean field approach. Our results are consistent with experimental realizations and provide a many-body method to simulate EIT-EIA cross-over.

quant-ph↗

arXiv2019

The effect of electron-phonon interactions on the spectral properties of single defects in hexagonal boron nitride

We investigate temperature-dependent spectral properties of a single defect in hexagonal boron nitride (hBN). We observe a sharp zero-phonon line (ZPL) emission accompanied by Stokes and anti-Stokes optical phonon sidebands assisted by the Raman active low-energy ($\approx~6.5$ meV) interlayer shear mode of hBN. Spectral lineshape around the ZPL is measured down to 78 K, at which the linewidth of the ZPL is measured as 172 $μ$eV. By employing a quadratic electron-phonon interaction, the temperature-dependent broadening and the lineshift of the ZPL are found to follow $T+T^5$ and $T+T^3$ temperature dependence, respectively. Furthermore, the temperature-dependent lineshape around the ZPL is modeled with a linear electron-phonon coupling theory, which results in the Debye-Waller factor of the ZPL emission as 0.59.

cond-mat.mes-hall↗

arXiv2009

Proposals of nuclear spin quantum memory in group IV elemental and II-VI semiconductors

New schemes for the nuclear spin quantum memory are proposed based on a system composed of two electrons or one electron coupled to a single nuclear spin in isotopically purified group IV elemental and II-VI compound semiconductors. The qubit consists of the singlet state and one of the triplet states of two electrons or simply of an electron spin. These systems are free from the decoherence due to the nuclear dipole-dipole interaction and are advantageous for the long memory time. In the case of two electrons, the protocol for the quantum state transfer between the electron spin qubit and the nuclear spin qubit is based on the magnetic or electric field tuning of the singlet-triplet state crossing and on the hyperfine coupling supplemented with a well-defined scheme to initialize the nuclear spin. In the case of a single electron qubit, the quantum state transfer is driven by the hyperfine interaction itself without the need of the nuclear spin initialization. Many practical systems are considered, e.g., two electrons loaded on a Si or ZnSe quantum dot, a single electron charged state in a Si quantum dot doped with a P atom, a single electron charged $ ^{28}$Si quantum dot doped with an isotope atom of $ ^{29}$Si, and a localized electron system of Si:P and ZnSe:F in the bulk crystal. General aspects of these systems are investigated and a comparison of merits and demerits is made between the two-electron qubit and the single-electron qubit.

cond-mat.mes-hall↗