Seminars

February 16 Tuesday 4 pm, 2021

Amnon Aharony, Tel Aviv University, Israel

Breaking Time-Reversal Symmetry and Spin Selection in chiral molecules

Location: PCS video conference (further details: pcs@ibs.re.kr)

February 9 Tuesday 4 pm, 2021

Leonid Gorelik, Chalmers University of Technology, Sweden

Nanoelectromechanical phenomena induced by Josephson force

Location: PCS video conference (further details: pcs@ibs.re.kr)

January 12 Tuesday 4 pm, 2021

Ora Entin-Wohlman, Tel Aviv University, Israel

Magnetization generated by microwave-induced Rashba interaction

Location: PCS video conference (further details: pcs@ibs.re.kr)

ZOOM meetings

Participants: Junho Suh, Chulki Kim, Dong Hun Lee, and Hee Chul Park

Date: December 10, 2021

Title: Nanomechanical devices as hybrid and interface devices for quantum computing

Contents: We have a plan to have a discussion meeting for an establishing research topic related to quantum computing. When we want to make quantum systems to calculate high performance tasks, we need to integrate all of the devices such as various qubits, interpretational devices, and human interface. If we don't have knowledge for each qubit then we can not use the high performance toy. Since all computations have their own pros and cons, we propose nanomechanical devices which have high compatibility with different qubits. The nano-electromechanical systems are robust against the temperature change and small perturbation of the environment. For quantum computing, we need both pros and cons of quantum devices which are robustness and sensitivity. The mechanical degree of freedom is one of the critical windows to open the controllability using geometrical deformations.

Participants: Robert Shekhter, Leonid Gorelik, Sergei Kulinich, Olha Baroba, and Hee Chul Park

Date: December 7, 2021

Title: Nanomechanics driven by superconducting proximity effect

Contents: We have a plan to have a discussion meeting for an establishing research topic related to the superconducting shuttle with Olha Baroba. The Josephson energy will introduce the mechanical force on the CNT by tunnel coupling in the mechanical SINIS junction. We expected that the nano-electromechanical system shows instability driven by the Josephson energy. This work is wrapped up and submitted in the New Journal of Physics. Our main goal is to extend this work in the quantum regimes and we want to estimate that this system can show quantum cooling.

Participants:  Olha Barova, Anton Parafilo, Robert Shekhter, Leonid Gorelik, Sergey Kulinich, and Hee Chul Park

Date: August 30, 2021

Title: Electronic properties of superconducting nano-electromechanical shuttle

Contents: We have a plan to have a discussion meeting on the superconducting shuttle. Olha Bahrova will give us a presentation and Prof. Robert Shekhter, Prof. Leonid Gorelik, and Prof. Sergey Kulinich will consult for her recent results. She calculates the Josephson current and normal current through a CNT quantum dot as a mechanical vibrator. 

Participants:  Robert Shekhter, Leonid Gorelik, Olha Baroba, and Hee Chul Park

Date: July 22, 2021

Title: Josephson energy driven nano-electromechanical system using CNT and superconducting leads

Contents: We have a plan to have a discussion meeting for an establishing research topic related to the superconducting shuttle with a new student Olha Baroba. She is involved in this topic for formulating the nontrivial transport on the carbon nanotube coupled with superconducting leads. The Josephson energy will introduce the mechanical force on the CNT by tunnel coupling in the mechanical SINIS junction. We expect that the nano-electromechanical system shows instability driven by the Josephson energy.

Participants:  Robert Shekhter, Leonid Gorelik, and Hee Chul Park

Date: July 7, 2021

Title: Electronic properties of superconducting nano-electromechanical shuttle

Contents: We have a plan to have a discussion meeting for an establishing research topic related to the superconducting shuttle. The electromechanical coupling introduces the new degrees of freedom in electric and Josephen currents. Our discussion will provide an endurable and robust qubit for quantum computing.

Participants: Hee Chul Park (PCS), Sang-Jun Choi (University of Wurzburg, aiehtle2@gmail.com), Junho Suh (KRISS, junho.suh@kriss.re.kr)

Date: Apr 6, 2021

Title: The effect of decoherence to the evolution of the entangled cat-state

Contents: We consider that the evolution of the Cat-state is sensitive to the decoherence of the qubit while the mechanical vibration of CPB is insensitive to the decoherence. We consider the qubit as a superposition of two current-carrying states, the eigenstates of s_1 . Moreover, we focus on the phase dephasing as a primary source of decoherence. The phase dephasing causes a random rotation of a qubit about its z -axis, i.e., J=s_1.  We note that the effect of phase decoherence preserves the symmetry of Hamiltonian P=(-1)^n s_1, which is useful in reducing the computational cost. It might be interesting to study the effect of such a symmetry-preserving dephasing in future. When we limit the maximum number of vibrons n_max= 80, a direct numerical approach to the above Lindblad equation solves 25600 of coupled differential equations for hundreds of time periods. The tremendously large computational requirement is circumvented using the concept of superoperator.  We discuss the evolution of cat-state in depth in terms of (i) an entanglement measure (ii) Wigner distribution. With a pure state, the entanglement entropy is a good measure of the entanglement. But when it comes to a mixed state, the entanglement entropy captures only the quantum correlation.  Wigner distribution W(x,p ) provides the better visualization of the cat-state. Due to the immobile state localized around x=0, Wigner distribution exhibits the interference fringes, which is absent in the case without decoherence. 

Participants: Hee Chul Park (PCS), Dr. Junho Suh (KRISS, junho.suh@kriss.re.kr), Robert Shekhter (Gothenburg Univ., Robert.Shekhter@physics.gu.se)

Date: Feb 17, 2021

Title: Nanomechanical cat states generated by a dc voltage-driven Cooper pair box qubit

Contents: We study a nanoelectromechanical system consisting of a Cooper pair box qubit performing nanomechanical vibrations between two bulk superconductors. We demonstrate that a bias voltage applied to the superconductors may generate states represented by entanglement between qubit states and quantum ’cat states’, i.e. a superposition of the coherent states of the nanomechanical oscillator. We characterize the formation and development of such states in terms of the corresponding Wigner function and entropy of entanglement. Also, we propose an experimentally feasible detection scheme for the effect, in which the average current that attains the specific features created by the entanglement is measured.

Participants: Hee Chul Park (PCS), Olha M. Bahrova (B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, olha.bahrova@gmail.com), Robert Shekhter (Gothenburg Univ., Robert.Shekhter@physics.gu.se), Leonid Gorelik (Chalmers Univ., gorelik@chalmers.se)

Date: Jan 20, 2021

Title: Entanglement between charge qubit states and coherent states of nanomechanical resonator generated by AC Josephson effect

Contents: We considered a nanoelectromechanical system consisting of a movable Cooper pair box qubit which is subject to an electrostatic field, and coupled to the two bulk superconductors via tunneling processes. We suggest that qubit dynamics is described by the dynamics of a quantum oscillator and demonstrate that a bias voltage applied between superconductors generates states represented by the entanglement of qubit's states and coherent states of the oscillator if certain resonant conditions are fulfilled. It is shown that a structure of this entanglement may be controlled by the bias voltage in a way that gives rise to the entanglement incorporating so-called cat-states - the superposition of coherent states. We characterize the formation and development of such states analyzing the entropy of entanglement and corresponding Wigner function. The experimental detection of the effect by measuring the average current is also considered.