arXiv subjects

Hou Ian

Publications and source records attributed to Hou Ian.

29 records · Page 2Linked to original sources

arXiv2019

A practicable guide to the quantum computation architectures

The primordial model of quantum computation was introduced over thirty years ago and the first quantum algorithms have appeared for over twenty years. Yet the exact architectures for quantum computer seem foreign to an undergraduate student major in computer science or engineering, even though the mass media has helped popularize the terminologies in the past decade. Despite being a cutting-edge technology from both the theoretical and the experimental perspectives, quantum computation is indeed imminent and it would be helpful to give the undergraduate students at least a skeleton understanding of what a quantum computer stands for. Since instruction-set architectures originated from classical computing models are familiar, we propose analogously a set of quantum instructions, which can be composed to implement renowned quantum algorithms. Albeit the similarity one can draw between classical and quantum computer architectures, current quantum instructions are fundamentally incommensurable from their classical counterparts because they lack the innate capability to implement logical deductions and recursions. We discuss this trait in length and illustrate why it is held responsible that current quantum computers not be considered general computers.

quant-ph↗

arXiv2017

Vacuum induced transparency and photon number resolved Autler-Townes splitting in a three-level system

We study the absorption spectrum of a probe field by a Λ-type three-level system, which is coupled to a quantized control field through the two upper energy levels. The probe field is applied to the ground and the second excited states. When the quantized control field is in vacuum, we derive a threshold condition to discern vacuum induced transparency (VIT) and vacuum induced Autler- Townes splitting (ATS). We also find that the parameter change from VIT to vacuum induced ATS is very similar to that from broken PT symmetry to PT symmetry. Moreover, we find the photon number resolved spectrum in the parameter regime of vacuum induced ATS when the mean photon number of the quantized control field is changed from zero (vacuum) to a finite number. However, there is no photon number resolved spectrum in the parameter regime of VIT even that the quantized control field contains the finite number of photons. Finally, we further discuss possible experimental realization.

quant-ph↗

arXiv2016

Numerical Studies on Antiferromagnetic Skyrmions in Nanodisks by Means of A New Quantum Simulation Approach

We employ a self-consistent simulation approach based on quantum physics here to study the magnetism of antiferromagnetic skyrmions formed on manolayer nanodisk planes. We find that if the disk is small and the Dzyaloshinsky-Moriya (DM) interaction is weak, a single magnetic vortex may be formed on the disk plane. In such a case, when uniaxial anisotropy normal to the disk plane is further considered, the magnetic configuration remains unchanged, but the magnetization is enhanced in that direction, and reduced in other two perpendicular orientations. Very similarly, a weak external magnetic field normal to the disk plane cannot obviously affect the spin structure of the nanodisk; however, when it is sufficiently strong, it can destroy the AFM skyrmion completely. On the other hand, by increasing DM interaction so that the disk diameter is a few times larger than the DM length, more self-organized magnetic domains, such as vortices and strips, will be formed in the disk plane. They evolve with decreasing temperature, however always symmetric about a geometric axis of the square unit cell. We further find that in this case introducing normal magnetic anisotropy gives rise to the re-construction of AFM single-vortex structure or skyrmion on the disk plane, which provides a way to create and/or stabilize such spin texture in experiment.

cond-mat.mes-hall↗

arXiv2015

Atom-driven multistability in an optomechanical cavity under broken PT-symmetry

An optical field inside a Fabry-Perot cavity would exhibit multistability either when an atomic medium, acting as a classical dielectric, is filled into the cavity or when the cavity becomes optomechanical where one reflecting end becomes movable. An external laser is essential in both cases to drive the cavity mode out of the zero equilibrium state. We study the equilibrium states of an atom-filled optomechanical cavity, where the atoms are considered a collective two-level systems as well as an active component that replaces the role of the driving laser. Multistability is found when the atomic ensemble is lossy, for which the complex atom-photon coupling breaks the \mathcal{PT}-symmetry of the system Hamiltonian and matches in a periodic pattern to the frequencies and the linewidths of the cavity mode and the collective bosonic mode of the atoms. We show an input-output hystersis cycle between the atomic mode and the cavity mode to demonstrate the driving role the atoms replace an extra-cavity laser.

quant-ph↗

arXiv2015

A self-consistent approach to the one-dimensional chain models for the ferro- and antiferro-magnetism of nanotubes

We employ a self-consistent simulation approach based on quantum theory to investigate the physical properties of a pair of ferromagnetic and antiferromagnetic nanotubes. It was observed that under the given conditions, no matter the external magnetic field was absent or applied along the easy longitudinal axis, the spins always ordered in that direction due to the special geometric shape of the tubes and the magnetic uniaxial anisotropy, so that the two sorts of nanosystems exhibit typical ferromagnetic and antiferromagnetic properties, which may find applications in modern technology, in strong contrast to the phenomena observed previously in nanoparticle, where an external magnetic field, applied parallel to the antiferromganetically coupled spins, is able to turn the spins off their original direction to form symmetric pattern around the field direction. This peculiar feature enable us to build up one-dimensional chain models for the two sorts of nanosystems. Considering their fast computational speed and simplicity, these theoretical models were then utilized to investigate the finite size effects of the nanosystems, and perform further analysis for long tubes. Especially, our results obtained with the theoretical models and the numerical approach are exactly identical, verifying the correctness and applicability of the computational methodology.

cond-mat.mes-hall↗

arXiv2015

Effects of Dzyaloshinsky-Moriya Interaction on magnetism in Nanodisks

We give a theoretical study on the magnetic properties of monolayer nanodisks with both Heisenberg exchange and Dzyaloshinsky-Moriya (DM) interactions. In particular, we survey the magnetic effects caused by anisotropy, external magnetic field, and disk size when DM interaction is present by means of a new quantum simulation method based on mean-field theory. This computational approach finds that uniaxial anisotropy and transverse magnetic field enhances the net magnetization as well as increases the transition temperature of the vortex phase while preserving the chiralities of the nanodisks. Whereas, when the strength of DM interaction is sufficiently strong for a given disk size, magnetic domains appear within the circularly bounded region, which vanish and gives in to a single vortex when a transverse magnetic field is applied. The latter confirms the magnetic skyrmions induced by the magnetic field as observed in the experiments.

cond-mat.mes-hall↗

arXiv2015

Dynamic entanglement transfer in a double-cavity optomechanical system

We give a theoretical study of a double-cavity system in which a mechanical resonator beam is coupled to two cavity modes on both sides through radiation pressures. The indirect coupling between the cavities via the resonator sets up a correlation in the optomechanical entanglements between the two cavities with the common resonator. This correlation initiates an entanglement transfer from the intracavity photon-phonon entanglements to an intercavity photon-photon entanglement. Using numerical solutions, we show two distinct regimes of the optomechanical system, in which the indirect entanglement either builds up and eventually saturates or undergoes a death-and-revival cycle, after a time lapse for initiating the cooperative motion of the left and right cavity modes.

quant-ph↗

arXiv2014

Cavity polariton in a quasi-lattice of qubits and its selective radiation

In a circuit quantum eletrodynamic system, a chain of N qubits inhomogeneously coupled to a cavity field forms a mesoscopic quasi-lattice, which is characterized by its degree of deformation from a normal lattice. This deformation is a function of the relative spacing, that is the ratio of the qubit spacing to the cavity wavelength. A polariton mode arise in the quasi-lattice as the dressed mode of the lattice excitation by the cavity photon. We show that the transition probability of the polariton mode is either enhanced or decreased compared to that of a single qubit by the deformation, giving a selective spontaneous radiation spectrum. Further, unlike a microscopic lattice with large-N limit and nearly zero relative spacing, the polariton in the quasi-lattice has uneven decay rate over the relative spacing. We show that this unevenness coincides with the cooperative emission effect expected from the superradiance model, where alternative excitations in the qubits of the lattice result in maximum decay.

quant-ph↗

arXiv2014

Thermodynamic cycle in a cavity optomechanical system

A cavity optomechanical system is initiated by a radiation pressure of a cavity field onto a mirror element acting as a quantum resonator. This radiation pressure can control the thermodynamic character of the mirror to some extent, such as cooling its effective temperature. Here we show that by properly engineering the spectral density of a thermal heat bath that interacts with a quantum system, the evolution of the quantum system can be effectively turned on and off. Inside a cavity optomechanical system, when the heat bath is realized by a multi-mode oscillator modeling of the mirror, this on-off effect translates to infusion or extraction of heat energy in and out of the cavity field, facilitating a four-stroke thermodynamic cycle.

quant-ph↗

arXiv2011

Excitation spectrum for an inhomogeneously dipole-field-coupled superconducting qubit chain

When a chain of superconducting qubits couples to a coplanar resonator in a cavity, each of its N qubits (equally-spaced with distance l) experiences a different dipole-field coupling strength due to the waveform of the cavity field. We find that this inhomogeneous coupling leads to a pair of l-dependent ladder operators for the angular momentum of the spin chain. Varying the qubit spacing l changes the transition amplitudes between the angular momentum levels. We derive an exact diagonalization of the general N-qubit Hamiltonian and, through the N=4 case, demonstrate how the l-dependent operators lead to a denser one-excitation spectrum and a probability redistribution of the eigenstates. Moreover, it will be shown that the variation of l between its two limiting values coincides with the crossover between Frenkel- and Wannier-type excitons in the superconducting spin chain.

quant-ph↗

arXiv2010

Tunable Electromagnetically Induced Transparency and Absorption with Dressed Superconducting Qubits

Electromagnetically induced transparency and absorption (EIT and EIA) are usually demonstrated by three-level atomic or atom-like systems. In contrast to the usual case, we theoretically study the EIT and EIA in an equivalent three-level system, which is constructed by dressing a superconducting two-level system (qubit) dressed by a single-mode cavity field. In this equivalent system, we find that both the EIT and the EIA can be tuned by controlling the level-spacing of the superconducting qubit and hence controlling the dressed system. This tunability is due to the dressed relaxation and dephasing rates which vary parametrically with the level-spacing of the original qubit and thus affect the transition properties of the dressed qubit and the susceptibility. These dressed relaxation and dephasing rates characterize the reaction of the dressed qubit to an incident probe field. We also use recent experimental data on superconducting qubits (charge, phase, and flux qubits) to demonstrate our approach and show the possibility of experimentally realizing this proposal.

quant-ph↗