Quantum Resonator Research Articles

Parametric coupling between macroscopic quantum resonators

Time-dependent linear coupling between macroscopic quantum resonator modes generates both a parametric amplification also known as a ‘squeezing operation’ and a beam splitter operation, analogous to quantum optical systems. These operations, when applied properly, can robustly generate entanglement and squeezing for the quantum resonator modes. Here, we present such coupling schemes between a nanomechanical resonator and a superconducting electrical resonator using applied microwave voltages as well as between two superconducting lumped-element electrical resonators using a rf SQUID-mediated tunable coupler. By calculating the logarithmic negativity of the partially transposed density matrix, we quantitatively study the entanglement generated at finite temperatures. We also show that characterization of the nanomechanical resonator state after the quantum operations can be achieved by detecting the electrical resonator only. Thus, one of the electrical resonator modes can act as a probe to measure the entanglement of the coupled systems and the degree of squeezing for the other resonator mode.

Modified SQUID Operator Equation for a Single-Qubit Structure Coupled to a Quantum Resonator

Role of self-inductance in superconducting quantum interference device (SQUID) charge qubit is considered. It is found that when an SQUID charge qubit is coupled to a quantum LC resonator, the SQUID voltage operator equation is modified in accompanying with the modification of operator Faraday equation describing the inductance. It is shown that when the extra energy is applied to the junction, the mean phase will be squeezed according to a damping factor.

Simultaneous Cooling of an Artificial Atom and Its Neighboring Quantum System

We propose an approach for cooling both an artificial atom (e.g., a flux qubit) and its neighboring quantum system, the latter modeled by either a quantum two-level system or a quantum resonator. The flux qubit is cooled by manipulating its states, following an inverse process of state population inversion, and then the qubit is switched on to resonantly interact with the neighboring quantum system. By repeating these steps, the two subsystems can be simultaneously cooled. Our results show that this cooling is robust and effective, irrespective of the chosen quantum systems connected to the qubit.

Quantum Dynamics of a Resonator Driven by a Superconducting Single-Electron Transistor: A Solid-State Analogue of the Micromaser

We investigate the behavior of a quantum resonator coupled to a superconducting single-electron transistor (SSET) tuned to the Josephson quasiparticle resonance and show that the dynamics is similar in many ways to that found in a micromaser. Coupling to the SSET can drive the resonator into nonclassical states of self-sustained oscillation via either continuous or discontinuous transitions. Increasing the coupling further leads to a sequence of transitions and regions of multistability.

Spin-dependent electron interferometry

We use interference of electron waves in a quantum well to switch the spin polarization of electrons. The spin quantum resonator consists of a Cu-film of variable thickness sandwiched between vacuum and a magnetic Co-film. Electrons are injected into the resonator from the vacuum side. The Co-film provides a spin dependent reflector. Varying the resonator thickness results in periodic modulation of the spin polarization of the incoming electrons. We use spin interferometry to measure the divergence of the period of interlayer exchange coupling upon approaching a zone boundary. We propose that interferometric spin selection should be observable in spin polarized scanning tunneling spectroscopy.

Transmission coefficient for ballistic transport through quantum resonator

A model for the description of ballistic transport a in 1D-2D-1D system based on the theory of self-adjoint extensions of symmetric operators is suggested. A formula for the transmission coefficient for the system, particularly, in the presense of a magnetic field, is obtained.

Transmission resonances in a semiconductor-superconductor junction quantum interference structure.

Transport properties in a quantum resonator structure of a normal-conductor-superconductor (NS) junction are calculated. Quasiparticles in a cavity region undergo multiple reflections due to an abrupt change in the width of the wire and the NS interface. Quantum interference of the reflections modulates the nominal normal reflection probability at the NS boundary. We show that various NS structures can be regarded as the quantum resonator because of the absence of propagation along the NS interface. When the incident energy coincides with the quasibound state energy levels, the zero-voltage conductance exhibits peaks for small voltages applied to the NS junction. The transmission peaks change to dips of nearly perfect reflection when the applied voltage exceeds a critical value. Two branches of the resonance, which are roughly characterized by electron and hole wavelengths, emerge from the individual dip, and the energy difference between them increases with increasing voltage. The electronlike and holelike resonance dips originating from different quasibound states at zero-voltage cross one after another when the voltage approaches the superconducting gap. We find that both crossing and anticrossing can be produced. It is shown that the individual resonance state in the NS system is associated with two zeros and two poles in the complex energy plane. The behavior of the resonance is explained in terms of splitting and merging of the zero-pole pairs. We examine the Green's function of a one-dimensional NS system in order to find out how the transmission properties are influenced by the scattering from the NS interface.

Effect of an impurity in a quantum resonator.

We present a numerical study of the influence of disorder on quantum-mechanical transmission by considering the presence of an impurity in a T-shaped quantum-wire junction. The device is modeled as an electron waveguide of finite cross section. Transmission and reflection probabilities are computed by use of a waveguide-matching technique. The calculations show that the transmission of the two-dimensional electron gas through the narrow-wide-narrow geometry can be both enhanced and suppressed by the presence of a repulsive impurity potential. Although perfect transparency is not generally achieved with the presence of a single impurity, we find a 100% modulation of conductance as one device parameter is changed, which is robust against the scattering. If the impurity is located in the narrow-wire region, a periodic oscillation appears in transmission due to multiple reflections between the impurity and the stub region. We also examine an attractive scatterer.

A quantum paramagnetic amplifier with coupled resonators for the 10 cm region

Q u AN T U M paramagnetic amplifiers have recently started to be used to increase the sensitivity of radio receivers. There a re serious difficulties in the widespread use of quantum resonator amplifiers in the 10 cm wave region for different applications, because of the narrow bandpass of such amplifiers. It has been suggested T M that coupled resonators should be used to increase the bandpass of quantum resonator amplifiers. Karlov and Prokhorov 2 showed that for two coupled resonators, the first of which is passive (without an active element), the product of bandpass and the amplification factor is given by the formula