Elisabetta Paladino | Università di Catania (original) (raw)

Papers by Elisabetta Paladino

Research paper thumbnail of Decoherence in Josephson qubits

Eprint Arxiv Cond Mat 0312550, Dec 20, 2003

A high degree of quantum coherence is a crucial requirement for the implementation of quantum log... more A high degree of quantum coherence is a crucial requirement for the implementation of quantum logic devices. Solid state nanodevices seem particularly promising from the point of view of integrability and flexibility in the design. However decoherence is a serious limitation, due to the presence of many types low energy excitations in the ``internal'' environment and of ``external'' sources due to the control circuitery. Here we study both kind of dephasing in a special implementation, the charge Josephson qubit, however many of our results are applicable to a large class of solid state qubits. This is the case of 1/f noise for which we introduce and study a model of an environment of bistable fluctuatiors. External sources of noise are analized in terms of a suitable harmonic oscillator environment and the explicit mapping on the spin boson model is presented. We perform a detailed investigation of various computation procedures (single shot measurements, repeated measurements) and discuss the problem of the information needed to characterize the effect of the environment. For a fluctuator environment with 1/f spectrum memory effects turn out to be important. Although in general information beyond the power spectrum is needed, in many situations this results in the knowledge of only one more microscopic parameter of the environment. This allows to determine which degrees of freedom of the environment are effective sources of decoherence in each different physical situation considered.

Research paper thumbnail of Protected Computational Subspaces of Coupled Superconducting Qubits

Int J Quantum Inf, 2008

ABSTRACT We discuss the effect of low-frequency noise on interacting superconducting qubits in a ... more ABSTRACT We discuss the effect of low-frequency noise on interacting superconducting qubits in a fixed coupling scheme. By properly choosing operating conditions, within the adiabatic framework the systems develops two decoupled subspaces. The subspace where a SWAP operation takes place turns out to be resilient to low frequency fluctuations. The possibility to encode a single qubit in a protected two-physical-qubit system subspace is briefly discussed.

Research paper thumbnail of Characterization of Adiabatic Noise in Charge-Based Coherent Nanodevices

Controllable Quantum States - Mesoscopic Superconductivity and Spintronics (MS+S2006) - Proceedings of the International Symposium, 2008

ABSTRACT Low-frequency noise, often with 1/f spectrum, has been recognized as the main mechanism ... more ABSTRACT Low-frequency noise, often with 1/f spectrum, has been recognized as the main mechanism of decoherence in present-day solid state coherent nanodevices. The responsible degrees of freedom are almost static during the coherent time evolution of the device leading to effects analogous to inhomogeneous broadening in NMR. Here we present a characterization of the effects of adiabatic noise exploiting the tunability of nanodevices.

Research paper thumbnail of Experimental on-demand recovery of entanglement by local operations within non-Markovian dynamics

Scientific reports, 2015

In many applications entanglement must be distributed through noisy communication channels that u... more In many applications entanglement must be distributed through noisy communication channels that unavoidably degrade it. Entanglement cannot be generated by local operations and classical communication (LOCC), implying that once it has been distributed it is not possible to recreate it by LOCC. Recovery of entanglement by purely local control is however not forbidden in the presence of non-Markovian dynamics, and here we demonstrate in two all-optical experiments that such entanglement restoration can even be achieved on-demand. First, we implement an open-loop control scheme based on a purely local operation, without acquiring any information on the environment; then, we use a closed-loop scheme in which the environment is measured, the outcome controling the local operations on the system. The restored entanglement is a manifestation of "hidden" quantum correlations resumed by the local control. Relying on local control, both schemes improve the efficiency of entanglement...

Research paper thumbnail of Coherence correlations in the dissipative two-state system

Physical Review E, 1998

We study the dynamical equilibrium correlation function of the polarondressed tunneling operator ... more We study the dynamical equilibrium correlation function of the polarondressed tunneling operator in the dissipative two-state system. Unlike the position operator, this coherence operator acts in the full system-plus-reservoir space. We calculate the relevant modified influence functional and present the exact formal expression for the coherence correlations in the form of a series in the number of tunneling events. For an Ohmic spectral density with the particular damping strength K = 1 2 , the series is summed in analytic form for all times and for arbitrary values of temperature and bias. Using a diagrammatic approach, we find the long-time dynamics in the regime K < 1. In general, the coherence correlations decay algebraically as t −2K at T = 0. This implies that the linear static susceptibility diverges for K ≤ 1 2 as T → 0, whereas it stays finite for K > 1 2 in this limit. The qualitative differences with respect to the asymptotic behavior of the position correlations are explained.

Research paper thumbnail of Detection of Finite-Frequency Current Moments with a Dissipative Resonant Circuit

Physical Review Letters, 2007

We consider the measurement of higher current moments with a dissipative resonant circuit, which ... more We consider the measurement of higher current moments with a dissipative resonant circuit, which is coupled inductively to a mesoscopic device in the coherent regime. Information about the higher current moments is coded in the histograms of the charge on the capacitor plates of the resonant circuit. Dissipation is included via the Caldeira-Leggett model, and it is essential to include it in order for the charge fluctuations (or the measured noise) to remain finite. We identify which combination of current correlators enter the measurement of the third moment. The latter remains stable for zero damping. Results are illustrated briefly for a quantum point contact.

Research paper thumbnail of Hidden entanglement in the presence of random telegraph dephasing noise

Physica Scripta T153, 014014, 2013

Research paper thumbnail of Purcell effect in a circuit-QED architecture implementation of a universal two-qubit gate

Physica Scripta, 2012

ABSTRACT We study spontaneous emission processes of two capacitively coupled superconducting qubi... more ABSTRACT We study spontaneous emission processes of two capacitively coupled superconducting qubits embedded in transmission-line resonators in a circuit-quantum electrodynamics architecture. We find that under proper conditions of the entanglement generating operation, a Purcell protection from spontaneous emission occurs. We discuss the dependence of this effect on the coupling energy between the two qubits and comment on the possibility of finding a trade-off between optimization of the gate with respect to low-frequency noise and relaxation effects.

Research paper thumbnail of Focus on Quantum Dissipation in Unconventional Environments

New Journal of Physics, 2008

Quantum dissipation has been the object of study within the physics and chemistry communities for... more Quantum dissipation has been the object of study within the physics and chemistry communities for many years. Despite this, the field is in constant evolution, largely due to the fact that novel systems where the understanding of dissipation and dephasing processes is of crucial importance have become experimentally accessible in recent years. Among the ongoing research themes, we mention the

Research paper thumbnail of Effects of low-frequency noise cross-correlations in coupled superconducting qubits

New Journal of Physics, 2008

We study the effects of correlated low frequency noise sources acting on a two qubit gate in a fi... more We study the effects of correlated low frequency noise sources acting on a two qubit gate in a fixed coupling scheme. A phenomenological model for the spatial and cross-talk correlations is introduced. The decoherence inside the SWAP subspace is analyzed by combining analytic results based on the adiabatic approximation and numerical simulations. Results critically depend on amplitude of the low frequency noise with respect to the qubits coupling strength. Correlations between noise sources induce qualitative different behaviors depending on the values of the above parameters. The possibility to reduce dephasing due to correlated low frequency noise by a recalibration protocol is discussed.

Research paper thumbnail of An operator approach to the construction of generating functions for products of associated Laguerre polynomials

Journal of Physics A: Mathematical and General, 1996

ABSTRACT We introduce an iteratively-defined operator sequence allowing the construction of new u... more ABSTRACT We introduce an iteratively-defined operator sequence allowing the construction of new useful mathematical identities, involving the product of associated Laguerre polynomials. Possible physical applications as well as some methodological aspects of our approach are pointed out.

Research paper thumbnail of Spin-boson dynamics: A unified approach from weak to strong coupling

Europhysics Letters (EPL), 2007

We present a novel approximation scheme to describe the influence of a harmonic bath on the dynam... more We present a novel approximation scheme to describe the influence of a harmonic bath on the dynamics of a two-level particle over almost the whole regime of temperatures and coupling to the environment, for a wide class of bath spectral densities. Starting from the exact path-integral solution for the two-level system density matrix, effective intra-blip correlations are fully included, while inter-blip and blip-sojourn interactions are considered up to first order. In the proper regimes, an excellent agreement with conventional perturbative approaches and ab-initio path-integral results is found.

Research paper thumbnail of Coupled Josephson qubits: Characterization of low-frequency charge noise

The European Physical Journal Special Topics, 2008

. The realization of coupled qubit setups is a fundamental step towards implementation of univer... more . The realization of coupled qubit setups is a fundamental step towards implementation of universal quantum computing architectures. Solid state nano- devices, despite being very promising from the point of view of scalability and integration, strongly suffer from various noise sources. Particular detrimental role is played by low-frequency noise components. Here we identify stability conditions against low-frequency charge noise of two

Research paper thumbnail of Dissipation, decoherence and preparation effects in the spin-boson system

The European Physical Journal B, 1999

: The dynamics of the reduced density matrix of the driven dissipative two-state system is stud... more : The dynamics of the reduced density matrix of the driven dissipative two-state system is studied for a general diagonal/off-diagonal initial state. We derive exact formal series expressions for the populations and coherences and show that they can be cast into the form of coupled nonconvolutive exact master equations and integral relations. We show that neither the asymptotic distributions, nor

Research paper thumbnail of Dynamics of the spin-boson model with a structured environment

Chemical Physics, 2004

We investigate the dynamics of the spin-boson model when the spectral density of the boson bath s... more We investigate the dynamics of the spin-boson model when the spectral density of the boson bath shows a resonance at a characteristic frequency Ω but behaves Ohmically at small frequencies. The time evolution of an initial state is determined by making use of the mapping onto a system composed of a quantum mechanical twostate system (TSS) which is coupled to a harmonic oscillator (HO) with frequency Ω. The HO itself is coupled to an Ohmic environment. The dynamics is calculated by employing the numerically exact quasiadiabatic path-integral propagator technique. We find significant new properties compared to the Ohmic spin-boson model. By reducing the TSS-HO system in the dressed states picture to a three-level system for the special case at resonance, we calculate the dephasing rates for the TSS analytically. Finally, we apply our model to experimentally realized superconducting flux qubits coupled to an underdamped dc-SQUID detector.

Research paper thumbnail of Electron transfer in the nonadiabatic regime: Crossover from quantum-mechanical to classical behaviour

Chemical Physics, 1999

We study nonadiabatic electron transfer within the biased spin-boson model. We calculate the inco... more We study nonadiabatic electron transfer within the biased spin-boson model. We calculate the incoherent transfer rate in analytic form at all temperatures for a power law form of the spectral density of the solvent coupling. In the Ohmic case, we present the exact low temperature corrections to the zero temperature rate for arbitrarily large bias energies between the two redox sites. Both for Ohmic and non-Ohmic coupling, we give the rate in the entire regime extending from zero temperature, where the rate depends significantly on the detailed spectral behaviour, via the crossover region, up to the classical regime. For low temperatures, the rate shows characteristic quantum features, in particular the shift of the rate maximum to a bias value below the reorganization energy, and the asymmetry of the rate around the maximum. We study in detail the gradual extinction of the quantum features as temperature is increased.

Research paper thumbnail of Detection of finite-frequency photoassisted shot noise with a resonant circuit

Physical Review B, 2010

Photoassisted transport through a mesoscopic conductor occurs when an oscillatory (ac) voltage is... more Photoassisted transport through a mesoscopic conductor occurs when an oscillatory (ac) voltage is superposed to the constant (dc) bias which is imposed on this conductor. Of particular interest is the photoassisted shot noise, which has been investigated theoretically and experimentally for several types of samples. For dc-biased conductors, a detection scheme for finite-frequency noise using a dissipative resonant circuit, which

Research paper thumbnail of Spin-boson dynamics beyond conventional perturbation theories

Physical Review B, 2007

A novel approximation scheme is proposed to describe the dynamics of the spin-boson problem. Bein... more A novel approximation scheme is proposed to describe the dynamics of the spin-boson problem. Being nonperturbative in the coupling strength nor in the tunneling frequency, it gives reliable results over a wide regime of temperatures and coupling strength to the thermal environment for a large class of bath spectral densities. We use a path-integral approach and start from the exact solution for the two-level system population difference in the form of a generalized master equation (GME). Then, we approximate inter-blip and blip-sojourns interactions up to linear order, while retaining all intra-blip correlations to find the kernels entering the GME in analytical form. Our approximation scheme, which we call Weakly-Interacting Blip Approximation (WIBA), fully agrees with conventional perturbative approximations in the tunneling matrix element (Non-Interacting Blip Approximation) or in the system-bath coupling strength.

Research paper thumbnail of Optimal operating conditions of an entangling two-transmon gate

New Journal of Physics, 2012

We identify optimal operating conditions of an entangling two-qubit gate realized by a capacitive... more We identify optimal operating conditions of an entangling two-qubit gate realized by a capacitive coupling of two transmons in a circuit-QED architecture. Sensitivity of the optimized gate to 1/f flux and critical current noise is suppressed to leading order. The procedure only requires a preliminary estimate of the 1/f noise amplitudes. No additional control or bias line beyond those used for the manipulation of individual qubits are needed. The proposed optimization is effective also in the presence of relaxation processes and of spontaneous emission through the resonator (Purcell effect).

Research paper thumbnail of Hidden entanglement, system-environment information flow and non-Markovianity

International Journal of Quantum Information, 2014

It is known that entanglement dynamics of two noninteracting qubits, locally subjected to classic... more It is known that entanglement dynamics of two noninteracting qubits, locally subjected to classical environments, may exhibit revivals. A simple explanation of this phenomenon may be provided by using the concept of hidden entanglement, which signals the presence of entanglement that may be recovered without the help of nonlocal operations. Here we discuss the link between hidden entanglement and the (non-Markovian) flow of classical information between the system and the environment.

Research paper thumbnail of Decoherence in Josephson qubits

Eprint Arxiv Cond Mat 0312550, Dec 20, 2003

A high degree of quantum coherence is a crucial requirement for the implementation of quantum log... more A high degree of quantum coherence is a crucial requirement for the implementation of quantum logic devices. Solid state nanodevices seem particularly promising from the point of view of integrability and flexibility in the design. However decoherence is a serious limitation, due to the presence of many types low energy excitations in the ``internal'' environment and of ``external'' sources due to the control circuitery. Here we study both kind of dephasing in a special implementation, the charge Josephson qubit, however many of our results are applicable to a large class of solid state qubits. This is the case of 1/f noise for which we introduce and study a model of an environment of bistable fluctuatiors. External sources of noise are analized in terms of a suitable harmonic oscillator environment and the explicit mapping on the spin boson model is presented. We perform a detailed investigation of various computation procedures (single shot measurements, repeated measurements) and discuss the problem of the information needed to characterize the effect of the environment. For a fluctuator environment with 1/f spectrum memory effects turn out to be important. Although in general information beyond the power spectrum is needed, in many situations this results in the knowledge of only one more microscopic parameter of the environment. This allows to determine which degrees of freedom of the environment are effective sources of decoherence in each different physical situation considered.

Research paper thumbnail of Protected Computational Subspaces of Coupled Superconducting Qubits

Int J Quantum Inf, 2008

ABSTRACT We discuss the effect of low-frequency noise on interacting superconducting qubits in a ... more ABSTRACT We discuss the effect of low-frequency noise on interacting superconducting qubits in a fixed coupling scheme. By properly choosing operating conditions, within the adiabatic framework the systems develops two decoupled subspaces. The subspace where a SWAP operation takes place turns out to be resilient to low frequency fluctuations. The possibility to encode a single qubit in a protected two-physical-qubit system subspace is briefly discussed.

Research paper thumbnail of Characterization of Adiabatic Noise in Charge-Based Coherent Nanodevices

Controllable Quantum States - Mesoscopic Superconductivity and Spintronics (MS+S2006) - Proceedings of the International Symposium, 2008

ABSTRACT Low-frequency noise, often with 1/f spectrum, has been recognized as the main mechanism ... more ABSTRACT Low-frequency noise, often with 1/f spectrum, has been recognized as the main mechanism of decoherence in present-day solid state coherent nanodevices. The responsible degrees of freedom are almost static during the coherent time evolution of the device leading to effects analogous to inhomogeneous broadening in NMR. Here we present a characterization of the effects of adiabatic noise exploiting the tunability of nanodevices.

Research paper thumbnail of Experimental on-demand recovery of entanglement by local operations within non-Markovian dynamics

Scientific reports, 2015

In many applications entanglement must be distributed through noisy communication channels that u... more In many applications entanglement must be distributed through noisy communication channels that unavoidably degrade it. Entanglement cannot be generated by local operations and classical communication (LOCC), implying that once it has been distributed it is not possible to recreate it by LOCC. Recovery of entanglement by purely local control is however not forbidden in the presence of non-Markovian dynamics, and here we demonstrate in two all-optical experiments that such entanglement restoration can even be achieved on-demand. First, we implement an open-loop control scheme based on a purely local operation, without acquiring any information on the environment; then, we use a closed-loop scheme in which the environment is measured, the outcome controling the local operations on the system. The restored entanglement is a manifestation of "hidden" quantum correlations resumed by the local control. Relying on local control, both schemes improve the efficiency of entanglement...

Research paper thumbnail of Coherence correlations in the dissipative two-state system

Physical Review E, 1998

We study the dynamical equilibrium correlation function of the polarondressed tunneling operator ... more We study the dynamical equilibrium correlation function of the polarondressed tunneling operator in the dissipative two-state system. Unlike the position operator, this coherence operator acts in the full system-plus-reservoir space. We calculate the relevant modified influence functional and present the exact formal expression for the coherence correlations in the form of a series in the number of tunneling events. For an Ohmic spectral density with the particular damping strength K = 1 2 , the series is summed in analytic form for all times and for arbitrary values of temperature and bias. Using a diagrammatic approach, we find the long-time dynamics in the regime K < 1. In general, the coherence correlations decay algebraically as t −2K at T = 0. This implies that the linear static susceptibility diverges for K ≤ 1 2 as T → 0, whereas it stays finite for K > 1 2 in this limit. The qualitative differences with respect to the asymptotic behavior of the position correlations are explained.

Research paper thumbnail of Detection of Finite-Frequency Current Moments with a Dissipative Resonant Circuit

Physical Review Letters, 2007

We consider the measurement of higher current moments with a dissipative resonant circuit, which ... more We consider the measurement of higher current moments with a dissipative resonant circuit, which is coupled inductively to a mesoscopic device in the coherent regime. Information about the higher current moments is coded in the histograms of the charge on the capacitor plates of the resonant circuit. Dissipation is included via the Caldeira-Leggett model, and it is essential to include it in order for the charge fluctuations (or the measured noise) to remain finite. We identify which combination of current correlators enter the measurement of the third moment. The latter remains stable for zero damping. Results are illustrated briefly for a quantum point contact.

Research paper thumbnail of Hidden entanglement in the presence of random telegraph dephasing noise

Physica Scripta T153, 014014, 2013

Research paper thumbnail of Purcell effect in a circuit-QED architecture implementation of a universal two-qubit gate

Physica Scripta, 2012

ABSTRACT We study spontaneous emission processes of two capacitively coupled superconducting qubi... more ABSTRACT We study spontaneous emission processes of two capacitively coupled superconducting qubits embedded in transmission-line resonators in a circuit-quantum electrodynamics architecture. We find that under proper conditions of the entanglement generating operation, a Purcell protection from spontaneous emission occurs. We discuss the dependence of this effect on the coupling energy between the two qubits and comment on the possibility of finding a trade-off between optimization of the gate with respect to low-frequency noise and relaxation effects.

Research paper thumbnail of Focus on Quantum Dissipation in Unconventional Environments

New Journal of Physics, 2008

Quantum dissipation has been the object of study within the physics and chemistry communities for... more Quantum dissipation has been the object of study within the physics and chemistry communities for many years. Despite this, the field is in constant evolution, largely due to the fact that novel systems where the understanding of dissipation and dephasing processes is of crucial importance have become experimentally accessible in recent years. Among the ongoing research themes, we mention the

Research paper thumbnail of Effects of low-frequency noise cross-correlations in coupled superconducting qubits

New Journal of Physics, 2008

We study the effects of correlated low frequency noise sources acting on a two qubit gate in a fi... more We study the effects of correlated low frequency noise sources acting on a two qubit gate in a fixed coupling scheme. A phenomenological model for the spatial and cross-talk correlations is introduced. The decoherence inside the SWAP subspace is analyzed by combining analytic results based on the adiabatic approximation and numerical simulations. Results critically depend on amplitude of the low frequency noise with respect to the qubits coupling strength. Correlations between noise sources induce qualitative different behaviors depending on the values of the above parameters. The possibility to reduce dephasing due to correlated low frequency noise by a recalibration protocol is discussed.

Research paper thumbnail of An operator approach to the construction of generating functions for products of associated Laguerre polynomials

Journal of Physics A: Mathematical and General, 1996

ABSTRACT We introduce an iteratively-defined operator sequence allowing the construction of new u... more ABSTRACT We introduce an iteratively-defined operator sequence allowing the construction of new useful mathematical identities, involving the product of associated Laguerre polynomials. Possible physical applications as well as some methodological aspects of our approach are pointed out.

Research paper thumbnail of Spin-boson dynamics: A unified approach from weak to strong coupling

Europhysics Letters (EPL), 2007

We present a novel approximation scheme to describe the influence of a harmonic bath on the dynam... more We present a novel approximation scheme to describe the influence of a harmonic bath on the dynamics of a two-level particle over almost the whole regime of temperatures and coupling to the environment, for a wide class of bath spectral densities. Starting from the exact path-integral solution for the two-level system density matrix, effective intra-blip correlations are fully included, while inter-blip and blip-sojourn interactions are considered up to first order. In the proper regimes, an excellent agreement with conventional perturbative approaches and ab-initio path-integral results is found.

Research paper thumbnail of Coupled Josephson qubits: Characterization of low-frequency charge noise

The European Physical Journal Special Topics, 2008

. The realization of coupled qubit setups is a fundamental step towards implementation of univer... more . The realization of coupled qubit setups is a fundamental step towards implementation of universal quantum computing architectures. Solid state nano- devices, despite being very promising from the point of view of scalability and integration, strongly suffer from various noise sources. Particular detrimental role is played by low-frequency noise components. Here we identify stability conditions against low-frequency charge noise of two

Research paper thumbnail of Dissipation, decoherence and preparation effects in the spin-boson system

The European Physical Journal B, 1999

: The dynamics of the reduced density matrix of the driven dissipative two-state system is stud... more : The dynamics of the reduced density matrix of the driven dissipative two-state system is studied for a general diagonal/off-diagonal initial state. We derive exact formal series expressions for the populations and coherences and show that they can be cast into the form of coupled nonconvolutive exact master equations and integral relations. We show that neither the asymptotic distributions, nor

Research paper thumbnail of Dynamics of the spin-boson model with a structured environment

Chemical Physics, 2004

We investigate the dynamics of the spin-boson model when the spectral density of the boson bath s... more We investigate the dynamics of the spin-boson model when the spectral density of the boson bath shows a resonance at a characteristic frequency Ω but behaves Ohmically at small frequencies. The time evolution of an initial state is determined by making use of the mapping onto a system composed of a quantum mechanical twostate system (TSS) which is coupled to a harmonic oscillator (HO) with frequency Ω. The HO itself is coupled to an Ohmic environment. The dynamics is calculated by employing the numerically exact quasiadiabatic path-integral propagator technique. We find significant new properties compared to the Ohmic spin-boson model. By reducing the TSS-HO system in the dressed states picture to a three-level system for the special case at resonance, we calculate the dephasing rates for the TSS analytically. Finally, we apply our model to experimentally realized superconducting flux qubits coupled to an underdamped dc-SQUID detector.

Research paper thumbnail of Electron transfer in the nonadiabatic regime: Crossover from quantum-mechanical to classical behaviour

Chemical Physics, 1999

We study nonadiabatic electron transfer within the biased spin-boson model. We calculate the inco... more We study nonadiabatic electron transfer within the biased spin-boson model. We calculate the incoherent transfer rate in analytic form at all temperatures for a power law form of the spectral density of the solvent coupling. In the Ohmic case, we present the exact low temperature corrections to the zero temperature rate for arbitrarily large bias energies between the two redox sites. Both for Ohmic and non-Ohmic coupling, we give the rate in the entire regime extending from zero temperature, where the rate depends significantly on the detailed spectral behaviour, via the crossover region, up to the classical regime. For low temperatures, the rate shows characteristic quantum features, in particular the shift of the rate maximum to a bias value below the reorganization energy, and the asymmetry of the rate around the maximum. We study in detail the gradual extinction of the quantum features as temperature is increased.

Research paper thumbnail of Detection of finite-frequency photoassisted shot noise with a resonant circuit

Physical Review B, 2010

Photoassisted transport through a mesoscopic conductor occurs when an oscillatory (ac) voltage is... more Photoassisted transport through a mesoscopic conductor occurs when an oscillatory (ac) voltage is superposed to the constant (dc) bias which is imposed on this conductor. Of particular interest is the photoassisted shot noise, which has been investigated theoretically and experimentally for several types of samples. For dc-biased conductors, a detection scheme for finite-frequency noise using a dissipative resonant circuit, which

Research paper thumbnail of Spin-boson dynamics beyond conventional perturbation theories

Physical Review B, 2007

A novel approximation scheme is proposed to describe the dynamics of the spin-boson problem. Bein... more A novel approximation scheme is proposed to describe the dynamics of the spin-boson problem. Being nonperturbative in the coupling strength nor in the tunneling frequency, it gives reliable results over a wide regime of temperatures and coupling strength to the thermal environment for a large class of bath spectral densities. We use a path-integral approach and start from the exact solution for the two-level system population difference in the form of a generalized master equation (GME). Then, we approximate inter-blip and blip-sojourns interactions up to linear order, while retaining all intra-blip correlations to find the kernels entering the GME in analytical form. Our approximation scheme, which we call Weakly-Interacting Blip Approximation (WIBA), fully agrees with conventional perturbative approximations in the tunneling matrix element (Non-Interacting Blip Approximation) or in the system-bath coupling strength.

Research paper thumbnail of Optimal operating conditions of an entangling two-transmon gate

New Journal of Physics, 2012

We identify optimal operating conditions of an entangling two-qubit gate realized by a capacitive... more We identify optimal operating conditions of an entangling two-qubit gate realized by a capacitive coupling of two transmons in a circuit-QED architecture. Sensitivity of the optimized gate to 1/f flux and critical current noise is suppressed to leading order. The procedure only requires a preliminary estimate of the 1/f noise amplitudes. No additional control or bias line beyond those used for the manipulation of individual qubits are needed. The proposed optimization is effective also in the presence of relaxation processes and of spontaneous emission through the resonator (Purcell effect).

Research paper thumbnail of Hidden entanglement, system-environment information flow and non-Markovianity

International Journal of Quantum Information, 2014

It is known that entanglement dynamics of two noninteracting qubits, locally subjected to classic... more It is known that entanglement dynamics of two noninteracting qubits, locally subjected to classical environments, may exhibit revivals. A simple explanation of this phenomenon may be provided by using the concept of hidden entanglement, which signals the presence of entanglement that may be recovered without the help of nonlocal operations. Here we discuss the link between hidden entanglement and the (non-Markovian) flow of classical information between the system and the environment.