arXiv subjects

Nikita Leppenen

Publications and source records attributed to Nikita Leppenen.

5 recordsLinked to original sources

arXiv2025

Quantum correlated steady states under competing collective and individual decay

Collective dissipation can generate useful quantum correlations, while ubiquitous individual decay destroys them. We study the interplay between these two competing processes considering a driven system of many spins (``atoms") undergoing both collective and individual dissipation (``radiation"). In steady state and depending on drive, we find that the system exhibits a first-order phase transition and quantum bistability: its quantum state is a mixture of two many-body states associated with the two competing decay processes. Accordingly, one of these states closely resembles a correlated ``coherently radiating spin state" (CRSS) -- the solution of purely collective dissipation -- exhibiting spin-squeezing entanglement. We predict dynamical switching between the two stable states, manifest as many-body quantum jumps in the various observables of spin and radiation. Macroscopically, the switching rate tends to vanish and the system can reside in a correlated CRSS for long times. This reveals how correlated dissipative physics emerges at the presence of decorrelating individual decay, opening a path for unlocking collective dissipation phenomena in realistic quantum platforms and applications. We discuss consequences for experiments in collective radiation.

quant-ph↗

arXiv2025

No oscillating subradiant correlations in a strongly driven quantum emitter array

We theoretically study time-dependent correlations in a strongly driven array of $N$ two-level atoms, coupled to photons in a waveguide. We focus on the spectrum $\{λ\}$ of the Liouvillian superoperator, which determines the correlation decay rates $-\Re λ$ and the frequencies $\Imλ$. Our main finding is the suppression of subradiant oscillating correlations between atomic states by a strong coherent drive of amplitude $Ω$: $|\Re λ|\ge mγ/2$, where $γ$ is the single-atom spontaneous decay rate and $m=|\Im λ/(2Ω)|$ is a nonzero integer for correlations oscillating in time $\propto \exp(\pm 2i m|Ω| t)$. This limit is independent of the number of atoms $N$; it holds both for small arrays and in the macroscopic limit. We demonstrate the suppression of subradiance numerically and provide a rigorous proof based on the analytical decomposition of the Liouvillian using spectral theory of simplicial complexes and posets.

quant-ph↗

arXiv2025

Persistent subradiant correlations in a random driven Dicke model

We study theoretically the driven-dissipative dynamics of an array of two-level emitters, coupled to a single photonic mode, in the presence of disorder in the resonant frequencies. We introduce the notion of subradiant correlations in the dynamics, corresponding to the eigenstates of the Liouvillian with a low decay rate, that can also oscillate in time. While the usual collective subradiant states do not survive the emitter resonant frequency fluctuations, these subradiant correlations are immune to such a type of disorder. These long-living correlations exist in finite-size systems, when their lifetime is parametrically longer than in the so-called Dicke time crystal phase.

quant-ph↗

arXiv2024

Hidden anisotropy controls spin-photon entanglement in a charged quantum dot

Photon entanglement is indispensable for optical quantum technologies. Measurement-based optical quantum computing and all-optical quantum networks rely on multiphoton cluster states consisting of indistinguishable entangled photons. A promising method for creating such cluster states on demand is spin-photon entanglement using the spin of a resident charge carrier in a quantum dot, precessing in a weak external magnetic field. In this work, we show theoretically and experimentally that spin-photon entanglement is strongly affected by the hidden anisotropy of quantum dots, which can arise from mechanical stress, shape anisotropy and even specific crystal structure. In the measurements of time-resolved photoluminescence and cross-polarized second-order photon correlation function in a magnetic field, the anisotropy manifests itself in the spin dynamics and, as a consequence, in the spin-photon concurrence. The measured time-filtered spin-photon Bell state fidelity depends strongly on the excitation polarization and reaches an extremely high value of 94% at maximum. We specify the magnetic field and excitation polarization directions that maximize spin-photon entanglement and thereby enhance the fidelity of multiphoton entangled states.

cond-mat.mes-hall↗

arXiv2024

Birefringent spin-photon interface generates polarization entanglement

A spin-photon interface based on the luminescence of a singly charged quantum dot in a micropillar cavity allows for the creation of photonic entangled states. Current devices suffer from cavity birefringence, which limits the generation of spin-photon entanglement. In this paper, we theoretically study the light absorption and emission by the interface with an anisotropic cavity and derive the maximal excitation and spin-photon entanglement conditions. We show that the concurrence of the spin-photon state equal to one and complete quantum dot population inversion can be reached for a micropillar cavity with any degree of birefringence by tuning the quantum dot resonance strictly between the cavity modes. This sweet spot is also valid for generating a multiphoton cluster state, as we demonstrate by calculating the three-tangle and fidelity with the maximally entangled state.

quant-ph↗