arXiv subjects

Sang Jae Yun

Publications and source records attributed to Sang Jae Yun.

7 recordsLinked to original sources

arXiv2016

An analytic formula determining quantum pointer basis and its application to a flying bullet

We propose a general scheme determining the quantum pointer basis of a macroscopic object interacting with environment. Unlike the decoherence program, our pointer basis does not require robustness against entanglement, but require robustness on the object state itself. The proposed pointer basis does not rely on master equations, and so is easily calculable since it has a time-local analytic formula. We show that, as a convincing example, the proposed formula gives an satisfactory pointer basis of a macroscopic flying body such as a bullet whose pointer states should have well-defined momentum as well as highly-localized position.

quant-ph↗

arXiv2016

A theory addressing the quantum measurement problem: collapse occurs when the entangling speed reaches a threshold

To resolve the quantum measurement problem, we propose an objective collapse theory in which both the wavefunction and the process of collapse are regarded as ontologically objective. The theory, which we call the entangling-speed-threshold theory, postulates that collapse occurs when the entangling speed of a system reaches a threshold, and the collapse basis is determined so as to eliminate the entangling speed and to minimize its increasing rate. Using this theory, we provide answers to the questions of where and when collapse occurs, how the collapse basis is determined, what systems are (in other words, what the actual tensor product structure is), and what determines the observables. We also explain how deterministic classical dynamics emerges from indeterministic quantum collapse, explaining the quantum-to-classical transition. In addition, we show that the theory guarantees energy conservation to a high accuracy. We apply the theory to a macroscopic flying body such as a bullet in the air, and derive a satisfactory collapse basis that is highly localized in both position and momentum, consistent with our everyday observation. Finally, we suggest an experiment that can verify the theory.

quant-ph↗

arXiv2015

Superconducting qubit without Josephson junctions manipulated by the orbital angular momentum of light

Conventional superconducting qubits have used Josephson junctions as an essential part to provide anharmonicity for well-separated energy-level spacings. However, because a superconducting ring without Josephson junctions has intrinsically well-separated energy-level spacings, Josephson junctions are not necessary as long as one can achieve single-qubit operations. We show that the orbital angular momentum of light can be adopted as a qubit-control means and can eliminate the need for Josephson junctions. The feasibility study reveals that the proposed qubit has many advantages over the previous ones. The lifetime of the qubit is extended due to the lack of the junction resistance. Very fast (sub-nanosecond) qubit manipulation in both single- and two-qubit gate is achieved. Without Josephson junctions, the fabrication process is simple and the deviations in the characteristics of each qubit can be improved.

quant-ph↗

arXiv2015

Quantum Phase Analysis of Field-Free Molecular Alignment

We present quantum mechanical explanations for unresolved phenomena observed in field-free molecular alignment by a femtosecond laser pulse. Quantum phase analysis of molecular rotational states reveals the physical origin of the following phenomena: strong alignment peaks appear periodically, and the temporal shape of each alignment peak changes in an orderly fashion depending on molecular species; the strongest alignment is not achieved at the first peak; the transition between aligned and anti-aligned states is very fast compared to the time scale of rotational dynamics. These features are understood in a unified way analogous to that describing a carrier-envelope-phase-stabilized mode-locked laser.

physics.optics↗

arXiv2015

Ising interaction between two qubits composed of the highest magnetic quantum number states through magnetic dipole-dipole interaction

In quantum information processing, one of the most useful interaction between qubits is the Ising type interaction. We propose a scheme to implement the exact Ising interaction through magnetic dipole-dipole interaction. Although magnetic dipolar interaction is Heisenberg type in general, this interaction can bring about the exact mathematical form of the Ising interaction if qubit levels are chosen among the highest magnetic quantum number states. Real physical systems to which our scheme can be applied include rotational states of molecules, hyperfine states of atoms, or electronic states of nitrogen-vacancy centers in diamond. We analyze the feasibility of our scheme for these systems. For example, when the hyperfine levels of rubidium 87 atoms are chosen as qubits and the distance of the two atoms is 0.1 micrometer, controlled-Z gate time will be 8.5 ms. We suggest diverse search and study to achieve optimal implementation of this scheme.

quant-ph↗

arXiv2015

Molecular quantum rotors in gyroscopic motion with a nonspreading rotational wavepacket

We provide a way of generating and observing molecular quantum gyroscopic motion that resembles gyroscopic motion of classical rotors. After producing a nonspreading rotational wavepacket called a cogwheel state, one can generate a gyroscopic precession motion by applying an external magnetic field interacting through a rotational magnetic dipole moment. The quantum rotors, realized with linear nonparamagnetic ionic molecules trapped in an ion trap, can keep their gyroscopic motion for a long time in a collectively synchronized fashion. A Coulomb-explosion technique is suggested to observe the gyroscopic motion. Despite limited molecular species, the observation of the gyroscopic motion can be adopted as a method to measure rotational g factors of molecules.

quant-ph↗

arXiv2015

Quantum Computation with Rotational States of Nonpolar Ionic Molecules

We propose a quantum computer architecture which is robust against decoherence and scalable. As a qubit, we adopt rotational states of a nonpolar ionic molecule trapped in an ion-trap. It is revealed that the rotational-state qubits are much more immune to decoherence than the conventional electronic-state qubits of atomic ions. A complete method set for state preparation, single-qubit gate, controlled-NOT gate, and qubit-readout suitable for the rotational-state qubits is provided. Since the ionic molecules can be transported in an array of ion traps, the rotational-state qubits are expected to be a promising candidate to build a large-scale quantum computer.

quant-ph↗