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Cristiano Ciuti

Publications and source records attributed to Cristiano Ciuti.

At least 91 records · Page 5Linked to original sources
arXiv2012

Ultrastrong coupling of the cyclotron transition of a two-dimensional electron gas to a THz metamaterial

Artificial cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems, due to the possibility of manipulating the cavity quantum electrodynamic ground state with controllable physical properties. We report here experiments showing ultrastrong light-matter coupling in a terahertz metamaterial where the cyclotron transition of a high mobility two-dimensional electron gas is coupled to the photonic modes of an array of electronic split-ring resonators. We observe a normalized coupling ratio $\fracΩ{ω_c}=0.58$ between the vacuum Rabi frequency $Ω$ and the cyclotron frequency $ω_c$. Our system appears to be scalable in frequency and could be brought to the microwave spectral range with the potential of strongly controlling the magnetotransport properties of a high-mobility 2DEG.

cond-mat.mes-hall↗
arXiv2012

Quantum theory of intersubband polarons

We present a microscopic quantum theory of intersubband polarons, quasiparticles originated from the coupling between intersubband transitions and longitudinal optical phonons. To this aim we develop a second quantized theory taking into account both the Fröhlich interaction between phonons and intersubband transitions and the Coulomb interaction between the intersubband transitions themselves. Our results show that the coupling between the phonons and the intersubband transitions is extremely intense, thanks both to the collective nature of the intersubband excitations and to the natural tight confinement of optical phonons. Not only the coupling is strong enough to spectroscopically resolve the resonant splitting between the modes (strong coupling regime), but it can become comparable to the bare frequency of the excitations (ultrastrong coupling regime). We thus predict the possibility to exploit intersubband polarons both for applied optoelectronic research, where a precise control of the phonon resonances is needed, and also to observe fundamental quantum vacuum physics, typical of the ultrastrong coupling regime.

cond-mat.mtrl-sci↗
arXiv2011

Cavity QED of the graphene cyclotron transition

We investigate theoretically the cavity quantum electrodynamics of the cyclotron transition for Dirac fermions in graphene. We show that the ultrastrong coupling regime characterized by a vacuum Rabi frequency comparable or even larger than the transition frequency can be obtained for high enough filling factors of the graphene Landau levels. Important qualitative differences occur with respect to the corresponding physics of massive electrons in a semiconductor quantum well. In particular, an instability for the ground state analogous to the one occuring in the Dicke-model is predicted for increasing value of the electron density.

cond-mat.mes-hall↗
arXiv2011

Double symmetry breaking and 2D quantum phase diagram in spin-boson systems

The quantum ground state properties of two independent chains of spins (two-levels systems) interacting with the same bosonic field are theoretically investigated. Each chain is coupled to a different quadrature of the field, leading to two independent symmetry breakings for increasing values of the two spin-boson interaction constants $Ω_C$ and $Ω_I$. A phase diagram is provided in the plane ($Ω_C$,$Ω_I$) with 4 different phases that can be characterized by the complex bosonic coherence of the ground states and can be manipulated via non-abelian Berry effects. In particular, when $Ω_C$ and $Ω_I$ are both larger than two critical values, the fundamental subspace has a four-fold degeneracy. Possible implementations in superconducting or atomic systems are discussed.

quant-ph↗
arXiv2011

Counter-polarized single-photon generation from the auxiliary cavity of a weakly nonlinear photonic molecule

We propose a scheme for the resonant generation of counter-polarized single photons in double asymmetric cavities with a small Kerr optical nonlinearity (as that created by a semiconductor quantum well) compared to the mode broadening. Due to the interplay between spatial intercavity tunneling and polarization coupling, by weakly exciting with circularly polarized light one of the cavities, we predict strong antibunching of counter-polarized light emission from the non-pumped auxiliary cavity. This scheme due to quantum interference is robust against surface scattering of pumping light, which can be suppressed both by spatial and polarization filters.

quant-ph↗
arXiv2011

Protected quantum computation with multiple resonators in ultrastrong coupling circuit QED

We investigate theoretically the dynamical behavior of a qubit obtained with the two ground eigenstates of an ultrastrong coupling circuit-QED system consisting of a finite number of Josephson fluxonium atoms inductively coupled to a transmission line resonator. We show an universal set of quantum gates by using multiple transmission line resonators (each resonator represents a single qubit). We discuss the intrinsic 'anisotropic' nature of noise sources for fluxonium artificial atoms. Through a master equation treatment with colored noise and manylevel dynamics, we prove that, for a general class of anisotropic noise sources, the coherence time of the qubit and the fidelity of the quantum operations can be dramatically improved in an optimal regime of ultrastrong coupling, where the ground state is an entangled photonic 'cat' state.

quant-ph↗
arXiv2010

Classical and Quantum Theory of Photothermal Cavity Cooling of a Mechanical Oscillator

Photothermal effects allow very efficient optomechanical coupling between mechanical degrees of freedom and photons. In the context of cavity cooling of a mechanical oscillator, the question of if the quantum ground state of the oscillator can be reached using photothermal back-action has been debated and remains an open question. Here we address this problem by complementary classical and quantum calculations. Both lead us to conclude that: first, the ground-state can indeed be reached using photothermal cavity cooling, second, it can be reached in a regime where the cavity detuning is small allowing a large amount of photons to enter the cavity.

quant-ph↗
arXiv2010

On the origin of strong photon antibunching in weakly nonlinear photonic molecules

In a recent work [T. C. H. Liew and V. Savona, Phys. Rev. Lett. {\bf104}, 183601 (2010)] it was numerically shown that in a photonic 'molecule' consisting of two coupled cavities, near-resonant coherent excitation could give rise to strong photon antibunching with a surprisingly weak nonlinearity. Here, we show that a subtle quantum interference effect is responsible for the predicted efficient photon blockade effect. We analytically determine the optimal on-site nonlinearity and frequency detuning between the pump field and the cavity mode. We also highlight the limitations of the proposal and its potential applications in demonstration of strongly correlated photonic systems in arrays of weakly nonlinear cavities.

quant-ph↗
arXiv2010

Hydrodynamic nucleation of vortices and solitons in a resonantly excited polariton superfluid

We present a theoretical study of the hydrodynamic properties of a quantum gas of exciton-polaritons in a semiconductor microcavity under a resonant laser excitation. The effect of a spatially extended defect on the superfluid flow is investigated as a function of the flow speed. The processes that are responsible for the nucleation of vortices and solitons in the wake of the defect are characterized, as well as the regimes where the superfluid flow remains unperturbed. Specific features due to the non-equilibrium nature of the polariton fluid are put in evidence.

cond-mat.other↗
arXiv2010

Is there a no-go theorem for superradiant quantum phase transitions in cavity and circuit QED ?

In cavity quantum electrodynamics (QED), the interaction between an atomic transition and the cavity field is measured by the vacuum Rabi frequency $Ω_0$. The analogous term "circuit QED" has been introduced for Josephson junctions, because superconducting circuits behave as artificial atoms coupled to the bosonic field of a resonator. In the regime with $Ω_0$ comparable to the two-level transition frequency, "superradiant" quantum phase transitions for the cavity vacuum have been predicted, e.g. within the Dicke model. Here, we prove that if the time-independent light-matter Hamiltonian is considered, a superradiant quantum critical point is forbidden for electric dipole atomic transitions due to the oscillator strength sum rule. In circuit QED, the capacitive coupling is analogous to the electric dipole one: yet, such no-go property can be circumvented by Cooper pair boxes capacitively coupled to a resonator, due to their peculiar Hilbert space topology and a violation of the corresponding sum rule.

cond-mat.supr-con↗
arXiv2010

Ultrastrong coupling between a cavity resonator and the cyclotron transition of a 2D electron gas in the case of integer filling factor

We investigate theoretically the coupling between a cavity resonator and the cyclotron transition of a two dimensional electron gas under an applied perpendicular magnetic field. We derive and diagonalize an effective quantum Hamiltonian describing the magnetopolariton excitations of the two dimensional electron gas for the case of integer filling factors. The limits of validity of the present approach are critically discussed. The dimensionless vacuum Rabi frequency $Ω_0/ω_0$ (i.e., normalized to the cyclotron frequency $ω_0$) is shown to scale as $\sqrt{α\: n_{QW} ν}$, where $α$ is the fine structure constant, $n_{QW}$ is the number of quantum wells and $ν$ is the filling factor in each well. We show that with realistic parameters of a high-mobility semiconductor two dimensional electron gas, the dimensionless coupling $Ω_0/ω_0$ can be much larger than 1 in the case of $ν\gg 1$, the latter condition being typically realized for cyclotron transitions in the microwave range. Implications of such ultrastrong coupling regime are discussed.

cond-mat.other↗
arXiv2010

Quantum squeezing generation versus photon localization in a disordered microcavity

We investigate theoretically the nonlinear dynamics induced by an intense pump field in a disordered planar microcavity. Through a self-consistent theory, we show how the generation of quantum optical noise squeezing is affected by the breaking of the in-plane translational invariance and the occurrence of photon localization. We find that the generation of single-mode Kerr squeezing for the ideal planar case can be prevented by disorder as a result of multimode nonlinear coupling, even when the other modes are in the vacuum state. However, the excess noise is a non-monotonous function of the disorder amplitude. In the strong localization limit, we show that the system becomes protected with respect to this fundamental coupling mechanism and that the ideal quadrature squeezing generation can be obtained.

cond-mat.other↗
arXiv2010

Vacuum degeneracy of a circuit-QED system in the ultrastrong coupling regime

We investigate theoretically the quantum vacuum properties of a chain of $N$ superconducting Josephson atoms inductively coupled to a transmission line resonator. We derive the quantum field Hamiltonian for such circuit-QED system, showing that, due to the type and strength of the interaction, a quantum phase transition can occur with a twice degenerate quantum vacuum above a critical coupling. In the finite-size case, the degeneracy is lifted, with an energy splitting decreasing exponentially with increasing values of $g^2 N^2$, where $g$ is the dimensionless vacuum Rabi coupling per artificial atom. We determine analytically the ultrastrong coupling asymptotic expression of the two degenerate vacua for an arbitrary number of artificial atoms and of resonator modes. In the ultrastrong coupling regime the degeneracy is protected with respect to random fluctuations of the transition energies of the Josephson elements.

quant-ph↗
arXiv2009

Extracavity quantum vacuum radiation from a single qubit

We present a theory of the quantum vacuum radiation that is generated by a fast modulation of the vacuum Rabi frequency of a single two-level system strongly coupled to a single cavity mode. The dissipative dynamics of the Jaynes-Cummings model in the presence of anti-rotating wave terms is described by a generalized master equation including non-Markovian terms. Peculiar spectral properties and significant extracavity quantum vacuum radiation output are predicted for state-of-the-art circuit cavity quantum electrodynamics systems with superconducting qubits.

quant-ph↗
arXiv2009

Light-matter excitations in the ultra-strong coupling regime

In a microcavity, light-matter coupling is quantified by the vacuum Rabi frequency $Ω_R$. When $Ω_R$ is larger than radiative and non-radiative loss rates, the system eigenstates (polaritons) are linear superposition of photonic and electronic excitations, a condition actively investigated in diverse physical implementations. Recently, a quantum electrodynamic regime (ultra-strong coupling) was predicted when $Ω_R$ becomes comparable to the transition frequency. Here we report unambiguous signatures of this regime in a quantum-well intersubband microcavity. Measuring the cavity-polariton dispersion in a room-temperature linear optical experiment, we directly observe the anti-resonant light-matter coupling and the photon-energy renormalization of the vacuum field.

quant-ph↗
arXiv2009

Quantum theory of electron tunneling into intersubband cavity polariton states

Through a non-perturbative quantum theory, we investigate how the quasi-electron excitations of a two-dimensional electron gas are modified by strong coupling to the vacuum field of a microcavity. We show that the electronic dressed states originate from a Fano-like coupling between the bare electron states and the continuum of intersubband cavity polariton excitations. In particular, we calculate the electron spectral function modified by light-matter interactions and its impact on the electronic injection of intersubband cavity polaritons. The domain of validity of the present theoretical results is critically discussed. We show that resonant electron tunneling from a narrow-band injector can selectively excite superradiant states and produce efficient intersubband polariton electroluminescence.

quant-ph↗
arXiv2008

Fermionized photons in an array of driven dissipative nonlinear cavities

We theoretically investigate the optical response of a one-dimensional array of strongly nonlinear optical microcavities. When the optical nonlinearity is much larger than both losses and inter-cavity tunnel coupling, the non-equilibrium steady state of the system is reminiscent of a strongly correlated Tonks-Girardeau gas of impenetrable bosons. Signatures of strong correlations are identified in the absorption spectrum of the system, as well as in the intensity correlations of the emitted light. Possible experimental implementations in state-of-the-art solid-state devices are discussed.

cond-mat.other↗