Papers by Ali Ümit Cemal Hardal
Physical Review A, 2013
We propose a quantum-electrodynamics scheme for implementing the discrete-time, coined quantum wa... more We propose a quantum-electrodynamics scheme for implementing the discrete-time, coined quantum walk with the walker corresponding to the phase degree of freedom for a quasi-magnon field realized in an ensemble of nitrogen-vacancy centres in diamond. The coin is realized as a superconducting flux qubit. Our scheme improves on an existing proposal for implementing quantum walks in cavity quantum electrodynamics by removing the cumbersome requirement of varying drive-pulse durations according to mean quasiparticle number. Our improvement is relevant to all indirect-coinflip cavity quantum-electrodynamics realizations of quantum walks. Our numerical analysis shows that this scheme can realize a discrete quantum walk under realistic conditions.
Quantum physics revolutionized classical disciplines of mechanics, statistical physics, and elect... more Quantum physics revolutionized classical disciplines of mechanics, statistical physics, and electrodynamics. One branch of scientific knowledge however seems untouched: thermodynamics. Major motivation behind thermodynamics is to develop efficient heat engines. Technology has a trend to miniaturize engines, reaching to quantum regimes. Development of quantum heat engines (QHEs) requires emerging field of quantum thermodynamics. Studies of QHEs debate whether quantum coherence can be used as a resource. We explore an alternative where it can function as an effective catalyst. We propose a QHE which consists of a photon gas inside an optical cavity as the working fluid and quantum coherent atomic clusters as the fuel. Utilizing the superradiance, where a cluster can radiate quadratically faster than a single atom, we show that the work output becomes proportional to the square of the number of the atoms. In addition to practical value of cranking up QHE, our result is a fundamental difference of a quantum fuel from its classical counterpart.
Physical Review A, 2015
ABSTRACT We propose a multilevel quantum heat engine with a working medium described by a general... more ABSTRACT We propose a multilevel quantum heat engine with a working medium described by a generalized Rabi model which consists of a two-level system coupled to a single-mode bosonic field. The model is constructed to be a continuum limit of a quantum biological description of light-harvesting complexes so that it can amplify quantum coherence by a mechanism which is a quantum analog of classical Huygens clocks. The engine operates in a quantum Otto cycle where the working medium is coupled to classical heat baths in the isochoric processes of the four-stroke cycle, while either the coupling strength or the resonance frequency is changed in the adiabatic stages. We found that such an engine can produce work with an efficiency close to the Carnot bound when it operates at low temperatures and in the ultrastrong-coupling regime. The interplay of the effects of quantum coherence and quantum correlations on the engine performance is discussed in terms of second-order coherence, quantum mutual information, and the logarithmic negativity of entanglement. We point out that the proposed quantum Otto engine can be implemented experimentally with modern circuit quantum electrodynamic systems where flux qubits can be coupled ultrastrongly to superconducting transmission-line resonators.
Physica Scripta, 2014
ABSTRACT We investigate Einstein–Podolsky–Rosen (EPR)-type spatial entanglement between two coupl... more ABSTRACT We investigate Einstein–Podolsky–Rosen (EPR)-type spatial entanglement between two coupled, driven, dissipative and nonlinear optical cavities. We identify the required parameter regimes of polariton-exchange and nonlinearity coefficients as having robust EPR-type entanglement at the steady state. In addition, we examine the influence of weak and strong drives on these parameter regimes.
Journal of the Optical Society of America B, 2014
We investigate the Hermitian and the non-Hermitian dynamics of the mode entanglement in two ident... more We investigate the Hermitian and the non-Hermitian dynamics of the mode entanglement in two identical optical cavities coupled by a chiral mirror. By employing the non-Hermitian quantum evolution, we calculate the logarithmic negativity measure of entanglement for initially Fock, coherent and squeezed states, separately. We verify the non-conservation of mean spin for the initially coherent and squeezed states when the coupling is non-reciprocal and report the associated spin noise for each case. We examine the effects of non-conserved symmetries on the mode correlations and determine the degree of non-reciprocal coupling to establish robust quantum entanglement.
Journal of the Optical Society of America B, 2014
ABSTRACT We investigate spin squeezing, quantum entanglement and second order coherence in two co... more ABSTRACT We investigate spin squeezing, quantum entanglement and second order coherence in two coupled, driven, dissipative, nonlinear cavities. We compare these quantum statistical properties for the cavities coupled with either single or two-photon exchange. Solving the quantum optical master equation of the system numerically in the steady state, we calculate the zero-time delay second-order correlation function for the coherence, genuine two-mode entanglement parameter, and an optimal spin squeezing inequality associated with particle entanglement. We identify regimes of distinct quantum statistical character depending on the relative strength of photon-exchange and nonlinearity. Moreover, we examine the effects of weak and strong drives on these quantum statistical regimes.
Physical Review E, 2014
We propose a four-level quantum heat engine in an Otto cycle with a working substance of two spin... more We propose a four-level quantum heat engine in an Otto cycle with a working substance of two spins subject to an external magnetic field and coupled to each other by a one-axis twisting spin squeezing nonlinear interaction. We calculate the positive work and the efficiency of the engine for different parameter regimes. In particular, we investigate the effects of quantum correlations at the end of the two isochoric processes of the Otto cycle, as measured by the entanglement of formation and quantum discord, on the work extraction and efficiency. The regimes where the quantum correlations could enhance the efficiency and work extraction are characterized.
Journal of the Optical Society of America B, 2012
ABSTRACT We examine transfer of particle entanglement and spin squeezing between atomic and photo... more ABSTRACT We examine transfer of particle entanglement and spin squeezing between atomic and photonic subsystems in optical cavities coupled by two-photon exchange. Each cavity contains a single atom, interacting with cavity photons with a two-photon cascade transition. Particle entanglement is characterized by evaluating optimal spin squeezing inequalities, for the cases of initially separable and entangled two-photon states. It is found that particle entanglement is first generated among the photons in separate cavities and then transferred to the atoms. The underlying mechanism is recognized as an inter-cavity two-axis twisting spin squeezing interaction, induced by two-photon exchange, and its optimal combination with the intra-cavity atom-photon coupling. Relative effect of non-local two-photon exchange and local atom-photon interactions of cavity photons on the spin squeezing and entanglement transfer is pointed out.
Physical Review A, 2013
We propose a quantum-electrodynamics scheme for implementing the discrete-time, coined quantum wa... more We propose a quantum-electrodynamics scheme for implementing the discrete-time, coined quantum walk with the walker corresponding to the phase degree of freedom for a quasi-magnon field realized in an ensemble of nitrogen-vacancy centres in diamond. The coin is realized as a superconducting flux qubit. Our scheme improves on an existing proposal for implementing quantum walks in cavity quantum electrodynamics by removing the cumbersome requirement of varying drive-pulse durations according to mean quasiparticle number. Our improvement is relevant to all indirect-coinflip cavity quantum-electrodynamics realizations of quantum walks. Our numerical analysis shows that this scheme can realize a discrete quantum walk under realistic conditions.
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Papers by Ali Ümit Cemal Hardal