266 research outputs found
Soviet-American and Soviet-Chinese Relations under Y.V. Andropov: An Analysis
Get PDFIn this study, the author examines the foreign policy of Y.V. Andropov and assesses the impact of his personal position on the Soviet-US and Sino-Soviet relations during his tenure as the leader of the Soviet Union. The author was guided by the principles of historicism, scientific objectivity, and reliance on sources. It is proved that Y.V. Andropov adopted the foreign policy of the USSR from the position of the “hawks” in the Soviet leadership, which were represented by D.F. Ustinov. According to such a position, foreign policy should be absolutely subordinated to the interests of national security, which can only be guaranteed by the superiority of both nuclear and conventional forces over potential adversaries. Military power should be at the forefront of foreign policy decisions. Therefore, no matter on the issue of Intermediate-Range Nuclear Forces (INF) in Europe, or on the issue of the armed forces on the Sino-Soviet and Sino-Mongolian borders, regardless of the objective changes in the situation, Y.V. Andropov always maintained a tough position. Author proves that the Soviet leader persistently rejected the reasonable proposals of diplomats and was unwilling to make any concessions that contradicted the ideas of military supremacy. This primarily concerned the issue of the INF deployment on the territory of Eastern Europe, which was a key topic of discussion at the Geneva negotiations. As a result, the Soviet Union could not avoid being drawn into the new round of the arms race provoked by the U.S. President Ronald Reagan, one of the results of which was the deployment of the American INF in Western Europe. In the East, he missed the opportunity to improve relations with China, which had begun to reorient itself away from confrontation with the USSR to building a balance in relations with the United States and the Soviet Union and continued a pointless confrontation with it. The foreign policy heritage that Y.V. Andropov left to his successors was an isolated and tense external environment in which there was few freedom of action. In addition, the growth of military expenditures caused by his tough line on foreign affairs aggravated the stagnation and crisis in the social-economic development of the USSR
Theoretical Proposals of Quantum Phase-slip Devices
No full textThis thesis describes a series of theoretical proposals of novel circuits that embed ultrathin superconducting nanowires with coherent quantum phase-slips (QPS). The motivation for our proposals is twofold: firstly, to facilitate unambiguous experimental verification of coherent phase-slips. Secondly, to suggest superconducting devices with new features and functionalities. The first circuit is a driven LC oscillator with the superconducting inductance subject to QPS. We have studied the effect of QPS in two limits: for small and for large phase-slip amplitudes. We expect that weakQPS (whereweakmeans that the QPS amplitude is small compared to the resonance frequency) have ameasurable effect when the oscillator is resonantly driven. The charge induced by the gate affects the quantum interference of QPS with opposite shifts and thus the effect of QPS is periodic in gate voltage. The experimental observation of such dependence would unambiguously identify the quantum coherence of phase-slips. The other crucial effect of QPS is that they produce uncommon non-linearities in the proposed device. The distinctive feature of these non-linearities is the oscillatory dependence on number of photons n with a local period of the order of pn. We analyzed the regime where the QPS correction becomes large by employing a semiclassical approximation as well as by solving the full quantum mechanical equations. At semiclassical level, the QPS-induced non-linearities persist until very high photon number and they result in amultitude ofmetastable states. This was illustrated in the responses of the oscillator: the QPS-induced non-linearities cause deviations fromthe otherwise Lorentzian curve that represents the number of photons N versus detuning. At sufficiently large phase-slip amplitudes this results in an impressive characteristic, a "corkscrew" shape. At any given detuning one finds a multitude of states that differ in the number of photons. About half of these states are stable. At quantum level, we have demonstrated that there is indeed a single quantum state corresponding to the semiclassical metastable states. We have found that these states are robust, their switching time is exponentially long, although they encompass only a few photons. The QPS oscillator is highly tunable: small changes of the driving force, detuning, or charge induced change the number of stable states, thereby enabling easy manipulation of the number of photons. All these features of the phase-slip oscillator make it useful for a wide range of applications, such as ultra-sensitive measurements, quantum manipula- tion and naturally, an unambiguous experimental verification of coherent QPS. In Chapter 3 we propose a series of devices that illustrate the emergence of Coulomb blockade from coherent QPS in thin superconducting wires. The basic ideas is that QPS create an isolation in a wire which is similar to the isolation required by Coulomb blockade. We exploit this idea and suggest setups that are derived from Cooper-pair box and Cooper-pair transistor, so we refer to them as QPS-box and QPS-transistor, respectively. Our main goal was to demonstrate that the devices exhibit sensitivity to a charge induced by a gate electrode, this being the main signature of Coulomb blockade. We analyze the emergence of discrete charging in the limit of strong phaseslips. We investigated six distinct regimes that are realized depending on the relation between the three characteristic energy scales: inductive and charging energy, and QPS amplitude. In both cases the charge sensitivity appears already for small QPS amplitudes as a perturbative correction to the ground-state energy. In both cases, the charge-sensitive part of the perturbative correction is exponentially suppressed in the limit of low impedance. In contrast to the QPS-box, the QPStransistor exhibits both flux and charge sensitivity that makes it potentially useful for measurements. However, if the QPS amplitude becomes of the order of either charging energy (large impedance regime) or inductive energy (small impedance regime), both devices showdiscrete charge states that followthe commonCoulomb blockade pattern of "crossing parabolas". The crossover to Coulomb blockade occurs differently in the limits of large and small impedance. For QPS-transistor, we have analyzed the flux sensitivity (superconducting current) as well as combined flux-charge sensitivity. For a symmetric QPS-transistor we have found a variable separation at specific values of induced charge that leads to double degeneracy of the states at q/e = 1 and half-integer external flux. We have calculated the superconducting current through QPS-transistor to show the non-triviality of the device. Experimental realization of these devices (achievable with the state-or-theart technology), will unambiguously prove the Coulomb blockade as an effect of coherent phase-slip processes. For completeness, we discuss the Josephson-based devices that are dual to QPS-box and QPS-transistor. In Chapter 4 we propose a way to realize quantum synchronization of Josephson and Bloch oscillations in a superconducting device. Essentially, this implies the synchronization of quantumconjugated variables: phase and charge. A circuit comprising of a QPS junction, series resistor and biased by a d.c. voltage exhibits Bloch physics of the charge variable. To ensure well-defined classical oscillations, the resistor needs to be large compared to the resistance quantum. We couple this Bloch subcircuit with a Josephson subcircuit in which the phase variable performs thewell-known Josephson oscillations. Themain effect of the coupling is the transfer of oscillating voltage/current from Josephson/Bloch to Bloch/Josephson part, whereby the voltage/current is multiplied with an amplification coefficient. This amplification coefficient is required to be high for stable synchronization. This is achieved by using an LC-resonator with high quality factorQ. The synchronization takes place in a rather broad interval of frequencies near the LC oscillator’s resonant value. Owing to this synchronization, the transresistance of our device exhibits a typical devil’s staircase curve, very similar to (fractional) Quantum Hall samples. In principle, fluctuations of phase and charge could could destroy the synchronization. Using the quantum description in the framework of Keldysh action formalism, we have investigated the effect of fluctuations on our circuit. The minimum synchronization error rate is shown to be exponential in Q. There is also significant practical interest in the field on metrology. The suggested device can be used to close the famous "metrological triangle" for threemetrological standards: those of resistance, voltage and current—within a single device.Quantum NanoscienceApplied Science
Spin, Vibrations and Radiation in Superconducting Junctions
No full textThis thesis presents the theoretical study of superconducting transport in several devices based on superconducting junctions. The important feature of these devices is that the transport properties of the junction are modified by the interaction with another physical system integrated in the superconducting circuit. The first device discussed is the spin superconducting qubit presented in Chapter 3. Such a unit combines the natural representation of a two level system in terms of electron spin and the advantages of superconducting qubits. We have shown that in spin superconducting qubits the flux and spin degrees of freedom can be easily entangled. Importantly, we have demonstrated feasibility of all electric manipulation of superconducting qubits and more complicated quantum gates made of such qubits. The microscopic analysis of quantum transport through the spin superconducting junction allows us to estimate the spin-dependent part of the Josephson energy. We demonstrate that it can be made sufficiently large, at least for semiconducting devices where spin-orbit interaction is intrinsically strong. The second device discussed in Chapter 4 is a novel qubit design using the spin states of two superconducting quasiparticles trapped in a superconducting junction. Read-out of the qubit is based on spin-blockade that inhibits recombination of quasiparticles in the triplet state. We have detailed the resonant manipulation of singlet-to-triplet and triplet-to-triplet transitions and have described the operation of the qubit. Experimental realization of our proposal would unambiguously demonstrate for the first time the spin properties of superconducting quasiparticles. In the third device studied in Chapter 5 the superconducting transport is modified by the excitation of a mechanical resonator integrated in the superconducting junction. We have demonstrated that the mechanical oscillations can be rectified giving rise to additional d.c. current that can be used for detection. The resonator can be driven by the a.c. voltage applied to the gate electrode as well as a mechanical force that depends on the superconducting phase difference at the junction, termed the Josephson force. We have presented a general and detailed analysis of the coupling between electrical and mechanical degrees of freedom, and have discussed the competing non-linear scales. The analysis has enabled us to derive analytical formulas for the response of the device to mechanical excitations in a wide interval of excitation strengths and for various biasing schemes. In Chapter 6 we have studied the full counting statistics of the radiation emitted by a Josephson junction circuit in the regime of parametric resonance. This is important in view of recent experiments that enable the detection of full power dissipated. We present the interpretation of the statistics in terms of bursts of multiple pairs of photons. This interpretation has been supported by investigating the time-dependent and frequency-resolved correlations.Quantum NanoscienceApplied Science
Nuclear Spins in Quantum Dots
No full textThe main theme of this thesis is the hyperfine interaction between the many lattice nuclear spins and electron spins localized in GaAs quantum dots. This interaction is an intrinsic property of the material. Despite the fact that this interaction is rather weak, it can, as shown in this thesis, strongly influence the dynamics of electron spins in quantum dots. In chapter 1 some basic features of quantum dots are described and the most important sources for the mixing of spin components in GaAs, i.e. the hyperfine and spin-orbit interaction, are introduced and discussed. The hyperfine mediated transition rate from a triplet state to the ground state singlet is considered in chapter 2. The transition involves changing both the orbital and spin degree of freedom so the phonon scattering alone cannot facilitate the transition. Also, the hyperfine interaction alone can not cause the transition because the nuclear spin system cannot absorb the energy released by the singlet-to-triplet transition. Thus, both a source of inelastic scattering and the hyperfine interaction are required for the transition. The resulting transition involves virtual excited states. Assuming a small exchange splitting a simple expression for the transition rate is obtained that involves only the first excited singlet state. The subject of chapter 3 is the hyperfine mediated transitions between Zeeman split doublet components of the ground state orbital of a single-electron quantum dot. The spin-flip mechanism is the same as for the singlet-triplet case, i.e. the transition goes via higher orbital virtual states and involves both the hyperfine interaction and phonon scattering. A closer look is taken at the relevant electron-phonon coupling mechanism: The piezoelectric phonons. In addition, a semiclassical picture of the nuclear system is formulated. The great number of nuclei in the quantum dot makes it possible to consider them as an effective nuclear magnetic field acting on the electron spin. The transition amplitude between the doublet components due to the hyperfine interaction remains finite even if the external magnetic field goes to zero. This is in contrast to spin-orbit interaction where the transition amplitude vanishes at zero field. Thus, at sufficiently low magnetic field the hyperfine related spin-flip rate will dominate the spin-orbit one. The rates obtained in chapters 2 and 3 are usually much smaller than non-spin-flip transition rates in quantum dots. Transport through a GaAs double quantum dot in the socalled spin-blockade regime is the subject of chapter 4. The current is blocked due to the absence of transitions between singlet and triplet states within the quantum dots. Mo tivated by a recent experiment, we consider the influence of the hyperfine in teraction on transport in the spin-blockade regime. A small transport current will flow if the singlet and triplet states are mixed. In our model the mixing is induced by the different effective nuclear magnetic fields acting the electron spins in the two dots. Not only does the nuclear system affect the electron spins and lift the spin-blockade, there is also a back-action on the nuclear system that is determined by the average electron spin in the two dots. The nuclear system precesses around the average electron spin, leading to a time dependent transport current whose characteristics are in qualitative agreement with the experimental observations. In the last chapter the dynamics induced by the hyperfine coupling of the electron and nuclear spins in a quantum dot are studied. An effective spin Hamiltonian is considered where the spatial dependence of the electron wave- function results in an inhomogenoues hyperfine coupling of the electron spin to different nuclear spins. Generally, it is not possible to solve this Hamiltonian except in the special case of homogeneous coupling. To obtain an approximate solution, we split the nuclear system into Nb subsystems where all nuclei within a given subsystem have equal coupling to the electron spin. An important fea ture of the original Hamiltonian is the separation of the timescales, i.e. the electron spin dynamics are much faster than that of the nuclear spins. This allows us to use the adiabatic approximation when calculating the average elec tron spin which each nuclear spin sees. In this way the electron spin is removed from the problem leaving 3Nb coupled differential equations. These are solved numerically and the results used to calculate certain electron spin correlation functions. Contrary to what one may guess, the dynamics are not chaotic and the correlation functions show no decay in time, only complicated oscillations. This may be be attributed to the fact that the system has many integrals of motion and that it is close to exactly solvable. This behavior persists even for Nb » 1, which is the limit in which our approximation becomes more accurate.Applied Science
Nuclear Spin Effects in Nanostructures
No full textIn this thesis we theoretically investigate effects of the interaction between electron spins and nuclear spins in different nanoscopic devices, quantum dots and spin valves. A quantum dot is a tiny potential well in which one can trap single electrons. One of the proposed applications of the quantum dot is to use the spin of the trapped electrons as qubits, the computational units in a quantum computer. The main obstacle for this application is the fact that the electron spin in the dot is coupled via the hyperfine interaction to roughly one million randomly fluctuating nuclear spins (those in the host material of the quantum dot). These fluctuations manifest themselves as a small but unpredictable magnetic field, causing the spin state of the electron to be not stable enough to be useful for quantum computation. The hyperfine interaction however works both ways: Several recent experiments have showed clear evidence that the nuclear spins, in turn, are also affected by the electron spin. So, it might be possible to suppress the fluctuations of the nuclear field by a clever manipulation of the electron spin in the dot. If so, this would bring the realization of the quantum dot spin qubit one big step closer. In this thesis we investigate the coupled electron-nuclear spin dynamics in several realistic experimental situations. We consider both single and double quantum dots, and concentrate on the combination of electronic transport (current) and electron spin resonance (a magnetic microwave field). We find that in these situations the fluctuations of the nuclear field indeed can be strongly suppressed, and we support this with experimental results. Further, we investigate the effect of strong spin-orbit coupling on the transport properties of a double quantum dot, and we also consider hyperfine effects in a metallic spin valve.Kavli Institute of Nanoscience DelftApplied Science
The Half-Josephson Laser: Essentials and Applications
No full textIn this dissertation, we study a novel class of laser devices, referred to as half-Josephson lasers (HJL). Here, lasing at half the Josephson frequency is induced by a parametric instability, resulting from an oscillating supercurrent flowing through a Josephson junction that contains quantum emitters. The device combines the electronic coherence of superconductivity with the optical coherence of lasers, resulting in a phase lock between the optical phase and the superconducting phase difference across the junction. Apart from the introductory first chapter, this dissertation consists of two parts. In the first part, the essentials of HJL devices are investigated. First, we study a model containing a Josephson junction with a single quantum emitter. Spontaneous switchings in the quantum emitter lead to decoherence of the laser. After this, a general HJL model is derived for the case of many quantum emitters driving the laser mode. Here, small fluctuations in the laser lead to a broad background in the laser spectrum. Large fluctuations, occurring on exponentially long timescales, cause decoherence. In the second part of the dissertation, two possible applications of the HJL are investigated, both exploiting the phase lock. First, a well-known optical feedback technique is found to lead to significant stabilization of fluctuations in the bias voltage of the HJL, which cause drift of the optical and superconducting phases. After this, a HJL device is proposed based on the Superconducting Quantum Interference Device. In this `light-superconducting interference device’ (LSID), magnetic fields can be used to manipulate the laser light. The LSID is also shown to facilitate multimode lasing.Quantum nanoscience (theoretical physics)Applied Science
Topological properties of superconducting nanostructures
No full textOne of the pillars of the scientific method is the fact that... Oh wait, it’s a different one. One of the pillars of the technological development is the fact that if the existing design does not achieve the goal or cannot be applied in new conditions, one could propose a totally different design that may achieve the goal. The only constraints in this way being the laws of physics. This is the main message of the lecture by Richard Feynman on tiny machines. The role of different designs can also be noted on a purely theoretical level. There, changing the well-known model can have far reaching consequences on its properties and possible applications.One of the main goals in the focus of modern quantum technology is realization of a quantum computer. The appeal of this device is in the difference from the classical analogous computer, being reasonable proposals for error correction. Another aspect is that one may use topological quantum states that are robust by themselves against certain noises. There is a lot of effort in trying different approaches and designs to experimentally realize and detect these states. Two main approaches are to either realize topological compounds or combine topologically trivial compounds to effectively realize non-trivial topological properties. There have been advances in both topological and non-topological quantum computation. One of the most famous examples being the achieved quantum supremacy (or, after censorship, quantum advantage). Despite that, the technology is still far away from being used at home. Also, during the process of development of technology other things may come about on the way. Anyhow, regardless of the outcome, the way itself is always more important than the resulting point. In this thesis we discuss certain theoretical findings discovered on the way.QN/Nazarov Grou
Statistics of continuous weak linear measurement
No full textOne of the pillars of the scientific method is the fact that all scientific predictions and explanations of phenomena in the universe are testable. Testing in the context of physics involves the action of measuring. Thus, the measurement process plays an important role in physics.While classically we all understand the idea of measurement in a very straightforward fashion, in quantum mechanics the concept of measurement departs fromour everyday experience in physics. In fact, although the quantum measurement obeys rather simple rules, its interpretation has been a subject of discussion since the beginning of the 20th century.Some of the physics involved in the process of a quantum measurement have no classical analogues, challenging in this way our intuition: the famous paradox of a cat in a box is a clear example of this.Casimir PhD Series, Delft-Leiden 2018-50QN/Nazarov Grou
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