Superconducting Spin Qubits

We propose and theoretically investigate spin superconducting qubits. Spin superconducting qubit consists of a single spin confined in a Josephson junction. We show that owing to spin-orbit interaction, superconducting difference across the junction can polarize this spin. We demonstrate that this enables single qubit operations and more complicated quantum gates, where spins of different qubits interact via a mutual inductance of superconducting loop where the junctions are embedded. Recent experimental realizations of Josephson junctions made of semiconductor quantum dots in contact with superconducting leads have shown that the number of electrons in the quantum dot can be tuned by a gate voltage. Spin superconducting qubit is realized when the number of electrons is odd. We discuss the qubit properties at phenomenological level. We present a microscopic theory that enables us to make accurate estimations of the qubit parameters by evaluating the spin-dependent Josephson energy in the framework of fourth-order perturbation theory.Comment: 11 pages, 8 figure

The Effect of Mechanical Resonance on Josephson Dynamics

We study theoretically dynamics in a Josephson junction coupled to a mechanical resonator looking at the signatures of the resonance in d.c. electrical response of the junction. Such a system can be realized experimentally as a suspended ultra-clean carbon nanotube brought in contact with two superconducting leads. A nearby gate electrode can be used to tune the junction parameters and to excite mechanical motion. We augment theoretical estimations with the values of setup parameters measured in the samples fabricated. We show that charging effects in the junction give rise to a mechanical force that depends on the superconducting phase difference. The force can excite the resonant mode provided the superconducting current in the junction has oscillating components with a frequency matching the resonant frequency of the mechanical resonator. We develop a model that encompasses the coupling of electrical and mechanical dynamics. We compute the mechanical response (the effect of mechanical motion) in the regime of phase bias and d.c. voltage bias. We thoroughly investigate the regime of combined a.c. and d.c. bias where Shapiro steps are developed and reveal several distinct regimes characteristic for this effect. Our results can be immediately applied in the context of experimental detection of the mechanical motion in realistic superconducting nano-mechanical devices.Comment: 18 pages, 11 figure

Quartet currents in a biased three-terminal diffusive Josephson junction

Biasing a three-terminal Josephson junction (TTJ) with symmetrical voltages $0,V,-V$ leads to new kinds of DC currents, namely quartet Josephson currents and phase-dependent multiple Andreev reflection (MAR) currents. We study these currents in a system where a normal diffusive metallic node $N$ is connected to three terminals $S_{0,1,2}$ by barriers of arbitrary transparency. We use the quantum circuit theory to calculate the current in each terminal, including decoherence. In addition to the stationary combination $\varphi_Q=\varphi_1+\varphi_2-2\varphi_0$ of the terminal phases $\varphi_i$, the bias voltage $V$ appears as a new and unusual control variable for a DC Josephson current. A general feature is the sign changes of the current-phase characteristics, manifesting in minima of the quartet ``critical current". Those sign changes can be triggered by the voltage, by the junction transparency or by decoherence. We study the possible separation of quartet currents from MAR currents in different regimes of parameters, including an "funnel" regime with very asymmetric couplings to $S_{0,1,2}$. In the regime of low transparency and asymetric couplings, we provide an analytic perturbative expression for the currents which shows an excellent agreement with the full numerical results

Quantum Synchronization in Presence of Shot Noise

Synchronization is a widespread phenomenon encountered in many natural and engineered systems with nonlinear classical dynamics. How synchronization concepts and mechanisms transfer to the quantum realm and whether features are universal or platform specific are timely questions of fundamental interest. Here, we present a new approach to model incoherently driven dissipative quantum systems susceptible to synchronization within the framework of Josephson photonics devices, where a dc-biased Josephson junction creates (non-classical) light in a microwave cavity. The combined quantum compound constitutes a self-sustained oscillator with a neutrally stable phase. Linking current noise to the full counting statistics of photon emission allows us to capture phase diffusion, but moreover permits phase locking to an ac-signal and mutual synchronization of two such devices. Thereby one can observe phase stabilization leading to a sharp emission spectrum as well as unique photon emission statistics revealing shot noise induced phase slips. Two-time perturbation theory is used to obtain a reduced description of the oscillators phase dynamics in form of a Fokker-Planck equation in generalization of classical synchronization theories.Comment: 14 pages, 10 figure

Concept of Suicide: Neurophysiological/Genetic Theories and Possible Oxytocin Relevance

The suicidal behavior is regarded as the act by which a person seeks to take his life, being aware of the consequences of his action. In our review, besides describing the main introductory aspects for the concept of suicide, we focus our attention on the main neurophysiological and genetical mechanisms relevant for this extremely difficult to manage and controversial behavior. Moreover, considering the latest interests in the current literature on the relevance of central oxytocin to various superior cognitive behaviors, we will also make a short description on how important effects of oxytocin could be in the context of suicidal behavior.Суїцидальна поведінка – це дії, в результаті яких особа намагається позбавити себе життя, усвідомлюючи наслідки таких дій. У даному огляді, окрім опису основних загальних аспектів концепції суїциду, ми концентрували увагу на основних нейрофізіологічних та генетичних аспектах, котрі мають відношення до цього вкрай важко контрольованого та повного протиріч типу поведінки. Окрім того, враховуючи велику цікавість, яку викликає в сучасній літературі задіяність центральної окситоцинової системи в контроль когнітивної поведінки вищих типів, ми надали короткий опис того, наскільки ефекти окситоцину можуть бути важливими в контексті суїцидальної поведінки

Spatial navigation deficits — overlooked cognitive marker for preclinical Alzheimer disease?

Detection of incipient Alzheimer disease (AD) pathophysiology is critical to identify preclinical individuals and target potentially disease-modifying therapies towards them. Current neuroimaging and biomarker research is strongly focused in this direction, with the aim of establishing AD fingerprints to identify individuals at high risk of developing this disease. By contrast, cognitive fingerprints for incipient AD are virtually non-existent as diagnostics and outcomes measures are still focused on episodic memory deficits as the gold standard for AD, despite their low sensitivity and specificity for identifying at-risk individuals. This Review highlights a novel feature of cognitive evaluation for incipient AD by focusing on spatial navigation and orientation deficits, which are increasingly shown to be present in at-risk individuals. Importantly, the navigation system in the brain overlaps substantially with the regions affected by AD in both animal models and humans. Notably, spatial navigation has fewer verbal, cultural and educational biases than current cognitive tests and could enable a more uniform, global approach towards cognitive fingerprints of AD and better cognitive treatment outcome measures in future multicentre trials. The current Review appraises the available evidence for spatial navigation and/or orientation deficits in preclinical, prodromal and confirmed AD and identifies research gaps and future research priorities

Spin, Vibrations and Radiation in Superconducting Junctions

This 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

Spin blockade qubit in a superconducting junction

We interpret a recent pioneering experiment (Zgirski M

Tunable pseudogaps due to nonlocal coherent transport in voltage-biased three-terminal Josephson junctions

We investigate the proximity effect in junctions between N=3 superconductors under commensurate voltage bias. The bias is chosen to highlight the role of transport processes that exchange multiple Cooper pairs coherently between more than two superconductors. Such nonlocal processes can be studied in the dc response, where local transport processes do not contribute. We focus on the proximity-induced normal density of states that we investigate in a wide parameter space. We reveal the presence of deep and highly tunable pseudogaps and other rich structures. These are due to a static proximity effect that is absent for N=2 and is sensitive to an emergent superconducting phase associated to nonlocal coherent transport. In comparison with results for N=2, we find similarities in the signature peaks of multiple Andreev reflections. We discuss the effect of electron-hole decoherence and of various types of junction asymmetries. Our predictions can be investigated experimentally using tunneling spectroscopy.Peer reviewe