We theoretically analyze two setups of low energy single-photon counters based on Josephson junct... more We theoretically analyze two setups of low energy single-photon counters based on Josephson junctions (JJ). For this, we propose two simple and general models, based on the macroscopic quantum tunneling formalism (MQT). The first setup is similar to the photon counter based on the "cold electron bolometer" (CEB), where the JJ replaces the CEB in the center of the superconducting antenna. In the second setup, the JJ is capacitively coupled to the antenna. We derive the Hamiltonians for the two setups and we write the Schrödinger equations taking into account both, the antenna and the JJ. The quantum particles of the MQT models move in twodimensional potential landscapes, which are parabolic along one direction and may have the form of a washboard potential along another direction. Such a potential landscape has a series of local minima, separated by saddle points. If the particle is prepared in the initial state in the metastable "ground state" of a local minimum, then the photon absorption causes it to jump into an excited state. If the excitation energy is bigger than the potential barrier seen by the quantum particle (the difference between the "ground state" and the saddle point), the photon is detected. The models are simple and allow us to do mostly analytical calculations. We show that the two setups are equivalent form the MQT point of view, since one Hamiltonian can be transformed into the other by changes of variables. For typical values of the JJ and antenna parameters, the setups may work as counters of photons of wavelengths up to 1 cm, at least. Dark count rates due to the phase particle tunneling directly from the ground state into the running state have also been evaluated.
The influence of a noisy environment on coherent transport in Andreev states through a point cont... more The influence of a noisy environment on coherent transport in Andreev states through a point contact between two superconductors is considered. The amount of dephasing is estimated for a microwave-activated quantum interferometer. Possibilities of experimentally investigating the coupling between a superconducting quantum point contact and its electromagnetic environment are discussed.
The dc current induced by a coherent surface acoustic wave (SAW) of wave vector q in a ballistic ... more The dc current induced by a coherent surface acoustic wave (SAW) of wave vector q in a ballistic channel of length L is calculated. The current contains two contributions, even and odd in q. The even current exists only in a asymmetric channel, when the electron reflection coefficients r1 and r2 at both channel ends are different. The direction of the even current does not depend on the direction of the SAW propagation, but is reversed upon interchanging r1 and r2. The direction of the odd current is correlated with the direction of the SAW propagation, but is insensitive to the interchange of r1 and r2. It is shown that both contributions to the current are non zero only when the electron reflection coefficients at the channel ends are energy dependent. The current exhibits geometric oscillations as function of qL. These oscillations are the hallmark of the coherence of the SAW and are completely washed out when the current is induced by a flux of non-coherent phonons. The results are compared with those obtained previously by different methods and under different assumptions.
The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in g... more The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in glasses is investigated within the soft-potential model. The same bilinear coupling constant is assumed for all three different kinds of soft modes. The model reproduces the results of the tunneling model at low temperatures and frequencies. In addition, it explains the fast rise of the relaxational absorption above 1 K and the plateau in the thermal conductivity around 5 K. The universal features of the sound absorption in glasses are described with good accuracy up to 20 K.
The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in g... more The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in glasses is investigated within the soft-potential model. The same bilinear coupling constant is assumed for all three different kinds of soft modes. The model reproduces the results of the tunneling model at low temperatures and frequencies. In addition, it explains the fast rise of the relaxational absorption above 1 K and the plateau in the thermal conductivity around 5 K. The universal features of the sound absorption in glasses are described with good accuracy up to 20 K.
Physica E: Low-dimensional Systems and Nanostructures, 2005
Dense (n = 4 × 10 11 cm −2) arrays of Ge quantum dots in Si host were studied using attenuation o... more Dense (n = 4 × 10 11 cm −2) arrays of Ge quantum dots in Si host were studied using attenuation of surface acoustic waves (SAWs) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, ∆Γ = Γ(ω, H) − Γ(ω, 0), and change of velocity of SAW, ∆V /V = (V (H) − V (0))/V (0), were measured in the temperature interval T = 1.5-4.2 K as a function of magnetic field H up to 6 T for the waves in the frequency range f = 30-300 MHz. Basing on the dependences of ∆Γ on H, T and ω, as well as on its sign, we believe that the AC conduction mechanism is a combination of diffusion at the mobility edge with hopping between localized states at the Fermi level. The measured magnetic field dependence of the SAW attenuation is discussed basing on existing theoretical concepts.
The acoustelectric current for composite fermions in a two-dimensional electron gas (2DEG) close ... more The acoustelectric current for composite fermions in a two-dimensional electron gas (2DEG) close to the half-filled Landau level is calculated in the random phase approximation. The Boltzmann equation is used to find the nonequilibrium distribution of composite fermions to second order in the acoustic field. It is shown that the oscillating Chern-Simons field created by the induced density fluctuations in the 2DEG is important for the acoustoelectric current. This leads to a violation of the Weinreich relation between the acoustoelectric current and acoustic intensity. The deviations from the Weinreich relation can be detected by measuring the angle between the longitudinal and the Hall components of the acoustoelectric current. This departure from the Weinreich relation gives additional information on the properties of the compostite fermion fluid.
ABSTRACT The stability of pinned vortex systems is constantly challenged in superconductors. In t... more ABSTRACT The stability of pinned vortex systems is constantly challenged in superconductors. In this work, magneto-optical imaging was used to reveal a new type of intermittent flux behavior in films of YBa2Cu3Ox. Films were grown on tilted NdGaO3 substrates, where the terrace structure creates a high density of planar defects. The flux penetration along the terrace steps consists of numerous 1-dimensional avalanches, some starting at the film edge, some fully internal. In spite the vivid dynamics the flux front advances in accordance with the critical state model. Analysing more than 10000 avalanche events, we find a power-law size distribution and finite-size-scaling with the depth of the flux front as crossover length. The intermittent behaviour shows no threshold value in the applied field. These new characteristics largely contrast those of the thermo-magnetic avalanches observed in many superconducting films, and suggest that a different mechanism is responsible for the 1-dimensional avalanches.
A study is presented concerning the influence of flicker noise in the junction transparency on co... more A study is presented concerning the influence of flicker noise in the junction transparency on coherent transport in Andreev states. The amount of dephasing is estimated for a microwave-activated quantum interferometer. Possibilities of experimentally investigating the coupling between a superconducting quantum point contact and its electromagnetic environment are discussed.
The efficiency of the future devices for quantum information processing will be limited mostly by... more The efficiency of the future devices for quantum information processing will be limited mostly by the finite decoherence rates of the qubits. Recently, substantial progress was achieved in enhancing the time within which a solid-state qubit demonstrates coherent dynamics. This progress is based mostly on a successful isolation of the qubits from external decoherence sources. Under these conditions, the material-inherent sources of noise start to play a crucial role. In most cases, the noise that the quantum device demonstrates has a 1/f spectrum. This suggests that the environment that destroys the phase coherence of the qubit can be thought of as a system of two-state fluctuators, which experience random hops between their states. In this short review, the current state of the theory of the decoherence due to the qubit interaction with the fluctuators is discussed. The effect of such an environment on two different protocols of the qubit manipulations, free induction and echo signa...
ABSTRACT We present study of stimulated THz emission from uniaxially strained Ge at low electric ... more ABSTRACT We present study of stimulated THz emission from uniaxially strained Ge at low electric field. The cw regime was reached at electric field below 10V/cm. The carrier population inversion is a result of capture and emission of free holes at strain-induced resonant acceptor states.
A theory of low-frequency shot noise in a resonant tunneling double-barrier device has been worke... more A theory of low-frequency shot noise in a resonant tunneling double-barrier device has been worked out. The calculations have been carried out within the coherent tunneling model; only the electron-phonon interaction inside the quantum well is taken into account. The average current I dc and the noise spectrum S() are expressed in terms of intrawell two-, three-, and four-electron Green's functions. The expression is valid, in principle, for arbitrary temperatures and for any type of intrawell scattering. We use it to analyze excess noise in phonon-assisted resonant tunneling through a double-barrier device at zero temperature and to the lowest order in the electron-phonon interaction. Our results show that the suppression of excess noise due to a correlation in electron transport is expected for both elastic and inelastic tunneling. In particular, we note that the contribution of the elastic processes to the ratio S()/eI dc is very sensitive to asymmetry of the tunneling barrier heights. Such a sensitivity is reduced for phonon-assisted processes. ͓S0163-1829͑97͒04604-3͔
Low-temperature conductance mechanism in low-density Si/SiGe heterostructures in high magnetic fi... more Low-temperature conductance mechanism in low-density Si/SiGe heterostructures in high magnetic fields (ultra-quantum limit) is studied using AC and DC transport measurements. Evidence of the Wigner crystal in this regime is given.
We theoretically analyze two setups of low energy single-photon counters based on Josephson junct... more We theoretically analyze two setups of low energy single-photon counters based on Josephson junctions (JJ). For this, we propose two simple and general models, based on the macroscopic quantum tunneling formalism (MQT). The first setup is similar to the photon counter based on the "cold electron bolometer" (CEB), where the JJ replaces the CEB in the center of the superconducting antenna. In the second setup, the JJ is capacitively coupled to the antenna. We derive the Hamiltonians for the two setups and we write the Schrödinger equations taking into account both, the antenna and the JJ. The quantum particles of the MQT models move in twodimensional potential landscapes, which are parabolic along one direction and may have the form of a washboard potential along another direction. Such a potential landscape has a series of local minima, separated by saddle points. If the particle is prepared in the initial state in the metastable "ground state" of a local minimum, then the photon absorption causes it to jump into an excited state. If the excitation energy is bigger than the potential barrier seen by the quantum particle (the difference between the "ground state" and the saddle point), the photon is detected. The models are simple and allow us to do mostly analytical calculations. We show that the two setups are equivalent form the MQT point of view, since one Hamiltonian can be transformed into the other by changes of variables. For typical values of the JJ and antenna parameters, the setups may work as counters of photons of wavelengths up to 1 cm, at least. Dark count rates due to the phase particle tunneling directly from the ground state into the running state have also been evaluated.
The influence of a noisy environment on coherent transport in Andreev states through a point cont... more The influence of a noisy environment on coherent transport in Andreev states through a point contact between two superconductors is considered. The amount of dephasing is estimated for a microwave-activated quantum interferometer. Possibilities of experimentally investigating the coupling between a superconducting quantum point contact and its electromagnetic environment are discussed.
The dc current induced by a coherent surface acoustic wave (SAW) of wave vector q in a ballistic ... more The dc current induced by a coherent surface acoustic wave (SAW) of wave vector q in a ballistic channel of length L is calculated. The current contains two contributions, even and odd in q. The even current exists only in a asymmetric channel, when the electron reflection coefficients r1 and r2 at both channel ends are different. The direction of the even current does not depend on the direction of the SAW propagation, but is reversed upon interchanging r1 and r2. The direction of the odd current is correlated with the direction of the SAW propagation, but is insensitive to the interchange of r1 and r2. It is shown that both contributions to the current are non zero only when the electron reflection coefficients at the channel ends are energy dependent. The current exhibits geometric oscillations as function of qL. These oscillations are the hallmark of the coherence of the SAW and are completely washed out when the current is induced by a flux of non-coherent phonons. The results are compared with those obtained previously by different methods and under different assumptions.
The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in g... more The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in glasses is investigated within the soft-potential model. The same bilinear coupling constant is assumed for all three different kinds of soft modes. The model reproduces the results of the tunneling model at low temperatures and frequencies. In addition, it explains the fast rise of the relaxational absorption above 1 K and the plateau in the thermal conductivity around 5 K. The universal features of the sound absorption in glasses are described with good accuracy up to 20 K.
The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in g... more The interaction of sound waves with tunneling, relaxational, and resonant vibrational states in glasses is investigated within the soft-potential model. The same bilinear coupling constant is assumed for all three different kinds of soft modes. The model reproduces the results of the tunneling model at low temperatures and frequencies. In addition, it explains the fast rise of the relaxational absorption above 1 K and the plateau in the thermal conductivity around 5 K. The universal features of the sound absorption in glasses are described with good accuracy up to 20 K.
Physica E: Low-dimensional Systems and Nanostructures, 2005
Dense (n = 4 × 10 11 cm −2) arrays of Ge quantum dots in Si host were studied using attenuation o... more Dense (n = 4 × 10 11 cm −2) arrays of Ge quantum dots in Si host were studied using attenuation of surface acoustic waves (SAWs) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, ∆Γ = Γ(ω, H) − Γ(ω, 0), and change of velocity of SAW, ∆V /V = (V (H) − V (0))/V (0), were measured in the temperature interval T = 1.5-4.2 K as a function of magnetic field H up to 6 T for the waves in the frequency range f = 30-300 MHz. Basing on the dependences of ∆Γ on H, T and ω, as well as on its sign, we believe that the AC conduction mechanism is a combination of diffusion at the mobility edge with hopping between localized states at the Fermi level. The measured magnetic field dependence of the SAW attenuation is discussed basing on existing theoretical concepts.
The acoustelectric current for composite fermions in a two-dimensional electron gas (2DEG) close ... more The acoustelectric current for composite fermions in a two-dimensional electron gas (2DEG) close to the half-filled Landau level is calculated in the random phase approximation. The Boltzmann equation is used to find the nonequilibrium distribution of composite fermions to second order in the acoustic field. It is shown that the oscillating Chern-Simons field created by the induced density fluctuations in the 2DEG is important for the acoustoelectric current. This leads to a violation of the Weinreich relation between the acoustoelectric current and acoustic intensity. The deviations from the Weinreich relation can be detected by measuring the angle between the longitudinal and the Hall components of the acoustoelectric current. This departure from the Weinreich relation gives additional information on the properties of the compostite fermion fluid.
ABSTRACT The stability of pinned vortex systems is constantly challenged in superconductors. In t... more ABSTRACT The stability of pinned vortex systems is constantly challenged in superconductors. In this work, magneto-optical imaging was used to reveal a new type of intermittent flux behavior in films of YBa2Cu3Ox. Films were grown on tilted NdGaO3 substrates, where the terrace structure creates a high density of planar defects. The flux penetration along the terrace steps consists of numerous 1-dimensional avalanches, some starting at the film edge, some fully internal. In spite the vivid dynamics the flux front advances in accordance with the critical state model. Analysing more than 10000 avalanche events, we find a power-law size distribution and finite-size-scaling with the depth of the flux front as crossover length. The intermittent behaviour shows no threshold value in the applied field. These new characteristics largely contrast those of the thermo-magnetic avalanches observed in many superconducting films, and suggest that a different mechanism is responsible for the 1-dimensional avalanches.
A study is presented concerning the influence of flicker noise in the junction transparency on co... more A study is presented concerning the influence of flicker noise in the junction transparency on coherent transport in Andreev states. The amount of dephasing is estimated for a microwave-activated quantum interferometer. Possibilities of experimentally investigating the coupling between a superconducting quantum point contact and its electromagnetic environment are discussed.
The efficiency of the future devices for quantum information processing will be limited mostly by... more The efficiency of the future devices for quantum information processing will be limited mostly by the finite decoherence rates of the qubits. Recently, substantial progress was achieved in enhancing the time within which a solid-state qubit demonstrates coherent dynamics. This progress is based mostly on a successful isolation of the qubits from external decoherence sources. Under these conditions, the material-inherent sources of noise start to play a crucial role. In most cases, the noise that the quantum device demonstrates has a 1/f spectrum. This suggests that the environment that destroys the phase coherence of the qubit can be thought of as a system of two-state fluctuators, which experience random hops between their states. In this short review, the current state of the theory of the decoherence due to the qubit interaction with the fluctuators is discussed. The effect of such an environment on two different protocols of the qubit manipulations, free induction and echo signa...
ABSTRACT We present study of stimulated THz emission from uniaxially strained Ge at low electric ... more ABSTRACT We present study of stimulated THz emission from uniaxially strained Ge at low electric field. The cw regime was reached at electric field below 10V/cm. The carrier population inversion is a result of capture and emission of free holes at strain-induced resonant acceptor states.
A theory of low-frequency shot noise in a resonant tunneling double-barrier device has been worke... more A theory of low-frequency shot noise in a resonant tunneling double-barrier device has been worked out. The calculations have been carried out within the coherent tunneling model; only the electron-phonon interaction inside the quantum well is taken into account. The average current I dc and the noise spectrum S() are expressed in terms of intrawell two-, three-, and four-electron Green's functions. The expression is valid, in principle, for arbitrary temperatures and for any type of intrawell scattering. We use it to analyze excess noise in phonon-assisted resonant tunneling through a double-barrier device at zero temperature and to the lowest order in the electron-phonon interaction. Our results show that the suppression of excess noise due to a correlation in electron transport is expected for both elastic and inelastic tunneling. In particular, we note that the contribution of the elastic processes to the ratio S()/eI dc is very sensitive to asymmetry of the tunneling barrier heights. Such a sensitivity is reduced for phonon-assisted processes. ͓S0163-1829͑97͒04604-3͔
Low-temperature conductance mechanism in low-density Si/SiGe heterostructures in high magnetic fi... more Low-temperature conductance mechanism in low-density Si/SiGe heterostructures in high magnetic fields (ultra-quantum limit) is studied using AC and DC transport measurements. Evidence of the Wigner crystal in this regime is given.
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Papers by Yuri Galperin