arXiv subjects

Suotang Jia

Publications and source records attributed to Suotang Jia.

At least 127 records · Page 7Linked to original sources

arXiv2017

Nonlinear Zeeman effect, line shapes and optical pumping in electromagnetically induced transparency

We perform Zeeman spectroscopy on a Rydberg electromagnetically induced transparency (EIT) system in a room-temperature Cs vapor cell, in magnetic fields up to 50~Gauss and for several polarization configurations. The magnetic interactions of the $\vert 6S_{1/2}, F_g=4 \rangle$ ground, $\vert 6P_{3/2}, F_e=5 \rangle$ intermediate, and $\vert 33S_{1/2} \rangle$ Rydberg states that form the ladder-type EIT system are in the linear Zeeman, quadratic Zeeman, and the deep hyperfine Paschen-Back regimes, respectively. Starting in magnetic fields of about 5~Gauss, the spectra develop an asymmetry that becomes paramount in fields $\gtrsim40$~Gauss. We use a quantum Monte Carlo wave-function approach to quantitatively model the spectra. Simulated spectra are in good agreement with experimental data. The asymmetry in the spectra is, in part, due to level shifts caused by the quadratic Zeeman effect, but it also reflects the complicated interplay between optical pumping and EIT in the magnetic field. Relevance to measurement applications is discussed. %The simulations are also used to study optical pumping in the magnetic field and to investigate the interplay between optical pumping and EIT, which reduces photon scattering and optical pumping.

physics.atom-ph↗

arXiv2017

Enhanced transmission capacity for laser communication at the single-photon level using the multi-channel frequency coding scheme

The statistical properties of a radiation sources are commonly characterized by second-order-correlation or Mandel parameter. Our research found that the single photons modulation spectrum provides us another optional way which is more sensitive to the high frequency information contained in the photon sequence. In this paper, we present direct laser communication by using a multi-channel frequency coding scheme based on the single photons modulation spectrum in which the multi-frequency modulation makes the transmission capacity efficiently enhanced. The modulation frequencies could be operated in a wide band without frequency aliasing due to the inherent randomness of photons arrival time of weak coherent light. The error rate less than 10-5 has been achieved experimentally when the mean signal photon count is 80 kcps. The modulated coherent light field shows nonlinear effects of single photons modulation spectrum. The studies of statistical properties of the single photons modulation spectrum, including the dependence of mean noise photon count, integration time, channel spacing and the number of frequency component, helped us to optimize the error rate and transmission capacity.

quant-ph↗

arXiv2017

Fano effect in an ultracold atom-molecule coupled system

The Fano effect or Fano resonance with a characteristically asymmetric line shape originates from quantum interference between direct and indirect transition pathways in continuum-bound coupled systems, and is a ubiquitous phenomenon in atomic, molecular, nuclear and solid-state physics. In optical nanoscale structures, the Fano effect has wide-ranging applications that include optical filtering, sensing, all-optical switching, quantum interferometry and nonlinear optics, and this opens new avenues for photonic devices. The emergent area of ultracold atomic and molecular gases presents an ideal platform for studying Fano resonances, since the physical parameters of these gases can be extensively tuned with high precision using external fields. However, an experimental demonstration of the Fano effect in hybridized atom-molecular coupled systems has remained elusive. Here, we report on observations of the Fano effect in molecular spectra obtained by photoassociation near a d-wave Feshbach resonance. This effect occurs due to quantum interference in PA transitions involving the continuum of atom-atom scattering states, the underlying Feshbach and photoassociated excited bound molecular states. We measure the variation in atom loss rate with an external magnetic field close to the Feshbach resonance in the presence of PA laser, and thereby clearly demonstrate the Fano effect. Our results further reveal that the Fano effect has significant influence on spectral shifts. Based on Fano's method, we develop a theory that explains the observed experimental results relatively well. Our theoretical formulation takes into account quantum interference between or among multiple transition pathways and between inelastic channels. Our results present a novel method for tuning the collisional interaction strength with laser light using Fano resonance.

quant-ph↗

arXiv2017

Atom-based radio-frequency field calibration and polarization measurement using cesium $nD_J$ Floquet states

We investigate atom-based electric-field calibration and polarization measurement of a 100-MHz linearly polarized radio-frequency (RF) field using cesium Rydberg-atom electromagnetically induced transparency (EIT) in a room-temperature vapor cell. The calibration method is based on matching experimental data with the results of a theoretical Floquet model. The utilized 60$D_J$ fine structure Floquet levels exhibit $J$- and $m_j$-dependent AC Stark shifts and splittings, and develop even-order RF-modulation sidebands. The Floquet map of cesium 60$D_J$ fine structure states exhibits a series of exact crossings between states of different $m_j$, which are not RF-coupled. These exact level crossings are employed to perform a rapid and precise ($\pm 0.5\%$) calibration of the RF electric field. We also map out three series of narrow avoided crossings between fine structure Floquet levels of equal $m_j$ and different $J$, which are weakly coupled by the RF field via a Raman process. The coupling leads to narrow avoided crossings that can also be applied as spectroscopic markers for RF field calibration. We further find that the line-strength ratio of intersecting Floquet levels with different $m_j$ provides a fast and robust measurement of the RF field's polarization.

physics.atom-ph↗

arXiv2017

Superfluid-Mott-insulator phase transition of light in a two-mode cavity array with ultrastrong coupling

In this paper we construct a new type of cavity array, in each cavity of which multiple two-level atoms interact with two independent photon modes. This system can be totally governed by a two-mode Dicke-lattice model, which includes all of the counter-rotating terms and therefore works well in the ultrastrong coupling regime achieved in recent experiments. Attributed to its special atom-photon coupling scheme, this model supports a global conserved excitation and a continuous $U(1)$ symmetry, rather than the discrete $Z_{2}$ symmetry in the standard Dicke-lattice model. This distinct change of symmetry via adding an extra photon mode strongly impacts the nature of photon localization/delocalization behavior. Specifically, the atom-photon interaction features stable Mott-lobe structures of photons and a second-order superfluid-Mott-insulator phase transition, which share similarities with the Jaynes-Cummings-lattice and Bose-Hubbard models. More interestingly, the Mott-lobe structures predicted here depend crucially on the atom number of each site. We also show that our model can be mapped into a continuous $XX$ spin model. Finally, we propose a scheme to implement the introduced cavity array in circuit quantum electrodynamics. This work broadens our understanding of strongly-correlated photons.

quant-ph↗

arXiv2017

Quantum phases of a two-dimensional polarized degenerate Fermi gas in an optical cavity

In this paper we analytically investigate the ground-state properties of a two-dimensional polarized degenerate Fermi gas in a high-finesse optical cavity, which is governed by a generalized Fermi-Dicke model with tunable parameters. By solving the photon-number dependent Bogoliubov-de-Gennes equation, we find rich quantum phases and phase diagrams, which depend crucially on the fermion-photon coupling strength, the fermion-fermion interaction strength, and the atomic resonant frequency (effective Zeeman field). In particular, without the fermion-fermion interaction and with a weak atomic resonant frequency, we find a mixed phase that the normal phase with two Fermi surfaces and the superradiant phase coexist, and reveal a first-order phase transition from this normal phase to the superradiant phase. With the intermediate fermion-fermion interaction and fermion-photon coupling strengths, we predict another mixed phase that the superfluid and superradiant phases coexist. Finally, we address briefly how to detect these predicted quantum phases and phase diagrams in experiments.

cond-mat.quant-gas↗

arXiv2016

Electric-field-induced interferometric resonance of a one-dimensional spin-orbit-coupled electron

We consider a one-dimensional spin-orbit-coupled nanowire quantum dot, driven by external electric and magnetic fields, and theoretically formulate an electric mechanism to interfere its electron orbits. Owing to the existence of spin-orbit coupling and a pulsed electric field, different spin-orbit states are shown to interfere with each other, generating intriguing interference-resonant patterns. We also reveal that an in-plane magnetic field does not affect the strength interval of any neighboring resonant peaks, but contributes a weak shift of each peak, which is sensitive to the direction of the magnetic field. We find that this proposed external-field-controlled scheme should be regarded as a new type of quantum-dot-based interferometry. Finally, this interferometry has an important application in precisely measuring relative experimental parameters, such as the Rashba and Dresselhaus spin-orbit-coupling strengths, as well as the Lande-g factor.

quant-ph↗

arXiv2016

Spectroscopy of cesium Rydberg atoms in strong radio-frequency fields

We study Rydberg atoms modulated by strong radio-frequency (RF) fields with a frequency of 70 MHz. The Rydberg atoms are prepared in a room temperature cesium cell, and their level structure is probed using electromagnetically induced transparency (EIT). As the RF field increases from the weak- into the strong-field regime, the range of observed RF-induced phenomena progresses from AC level shifts through increasingly pronounced and numerous RF-modulation sidebands to complex state-mixing and level-crossings with high-l hydrogen-like states. Weak anharmonic admixtures in the RF field generate clearly visible modifications in the Rydberg-EIT spectra. A Floquet analysis is employed to model the Rydberg spectra, and good agreement with the experimental observations is found. Our results show that all-optical spectroscopy of Rydberg atoms in vapor cells can serve as an antenna-free, atom-based and calibration-free technique to measure and map RF electric fields and to analyze their higher-harmonic contents.

physics.atom-ph↗

arXiv2015

Unconventional pairings of spin-orbit coupled attractive degenerate Fermi gas in a one dimensional optical lattice

Understanding novel pairings in attractive degenerate Fermi gases is crucial for exploring rich superfluid physics. In this report, we reveal unconventional pairings induced by spin-orbit coupling (SOC) in a one-dimensional optical lattice, using a state-of-the-art density-matrix renormalization group method. When both bands are partially occupied, we find a strong competition between the interband Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and intraband Bardeen-Cooper-Schrieffer (BCS) pairings. In particular, for the weak and moderate SOC strengths, these two pairings can coexist, giving rise to a new phase called the FFLO-BCS phase, which exhibits a unique three-peak structure in pairing momentum distribution. For the strong SOC strength, the intraband BCS pairing always dominates in the whole parameter regime, including the half filling. We figure out the whole phase diagrams as functions of filling factor, SOC strength, and Zeeman field. Our results are qualitatively different from recent mean-field predictions. Finally, we address that our predictions could be observed in a weaker trapped potential.

cond-mat.quant-gas↗

arXiv2015

Spin-orbit coupled repulsive Fermi atoms in a one-dimensional optical lattice

Motivated by recent experimental development, we investigate spin-orbit coupled repulsive Fermi atoms in a one-dimensional optical lattice. Using the density-matrix renormalization group method, we calculate momentum distribution function, gap, and spin-correlation function to reveal rich ground-state properties. We find that spin-orbit coupling (SOC) can generate unconventional momentum distribution, which depends crucially on the filling. We call the corresponding phase with zero gap the SOC-induced metallic phase. We also show that SOC can drive the system from the antiferromagnetic to ferromagnetic Mott insulators with spin rotating. As a result, a second-order quantum phase transition between the spin-rotating ferromagnetic Mott insulator and the SOC-induced metallic phase is predicted at the strong SOC. Here the spin rotating means that the spin orientations of the nearest-neighbor sites are not parallel or antiparallel, i.e., they have an intersection angle $θ\in (0,π)$. Finally, we show that the momentum $k_{\mathrm{peak}}$, at which peak of the spin-structure factor appears, can also be affected dramatically by SOC. The analytical expression of this momentum with respect to the SOC strength is also derived. It suggests that the predicted spin-rotating ferromagnetic ($k_{\mathrm{peak}% }<π/2$) and antiferromagnetic ($π/2<k_{\mathrm{peak}}<π$) correlations can be detected experimentally by measuring the SOC-dependent spin-structure factor via the time-of-flight imaging.

cond-mat.quant-gas↗

arXiv2015

Experimental quantum secure direct communication with single photons

Quantum communication holds promise for absolutely security in secret message transmission. Quantum secure direct communication is an important mode of the quantum communication in which secret messages are securely communicated over a quantum channel directly. It has become one of the hot research areas in the last decade, and offers both high security and instantaneousness in communication. It is also a basic cryptographic primitive for constructing other quantum communication tasks such as quantum authentication, quantum dialogue and so on. Here we report the first experimental demonstration of quantum secure direct communication with single photons. The experiment is based on the DL04 protocol, equipped with a simple frequency coding. It has the advantage of being robust against channel noise and loss. The experiment demonstrated explicitly the block data transmission technique, which is essential for quantum secure direct communication. In the experiment, a block transmission of 80 single photons was demonstrated over fiber, and it provides effectively 16 different values, which is equivalent to 4 bits of direct transmission in one block. The experiment has firmly demonstrated the feasibility of quantum secure direct communication in the presence of noise and loss.

quant-ph↗

arXiv2015

Photon Devil's staircase: photon long-range repulsive interaction in lattices of coupled resonators with Rydberg atoms

The realization of strong coherent interactions between individual photons is a long-standing goal in science and engineering. In this report, based on recent experimental setups, we derive a strong photon long-range repulsive interaction, by controlling the van der Waals repulsive force between Cesium Rydberg atoms located inside different cavities in extended Jaynes-Cummings-Hubbard Lattices. We also find novel quantum phases induced by this photon long-range repulsive interaction. For example, without photon hopping, a photon Devil's staircase, induced by the breaking of long-range translation symmetry, can emerge. If photon hopping occurs, we predict a photon-floating solid phase, due to the motion of particle-and hole-like defects. More importantly, for a large chemical potential in the resonant case, the photon hopping can be frozen even if the hopping term exists. We call this new phase the photon-frozen solid phase. In experiments, these predicted phases could be detedted by measuring the number of polaritons via resonance fluorescence.

quant-ph↗

arXiv2015

Atom-interferometric measurement of Stark level splittings

Multiple adiabatic/diabatic passages through avoided crossings in the Stark map of cesium Rydberg atoms are employed as beam splitters and recombiners in an atom-interferometric measurement of energy-level splittings. We subject cold cesium atoms to laser-excitation, electric-field and detection sequences that constitute an (internal-state) atom interferometer. For the read-out of the interferometer we utilize state-dependent collisions, which selectively remove atoms of one kind from the detected signal. We investigate the dependence of the interferometric signal on timing and field parameters, and find good agreement with time-dependent quantum simulations of the interferometer. Fourier analysis of the interferometric signals yield coherence frequencies that agree with corresponding energy-level differences in calculated Stark maps. The method enables spectroscopy of states that are inaccessible to direct laser-spectroscopic observation, due to selection rules, and has applications in field metrology.

physics.atom-ph↗

arXiv2015

Dipolar Rydberg-atom gas prepared by adiabatic passage through an avoided crossing

The passage of cold cesium 49S$_{1/2}$ Rydberg atoms through an electric-field-induced multi-level avoided crossing with nearby hydrogen-like Rydberg levels is employed to prepare a cold, dipolar Rydberg atom gas. When the electric field is ramped through the avoided crossing on time scales on the order of 100~ns or slower, the 49S$_{1/2}$ population adiabatically transitions into high-\emph{l} Rydberg Stark states. The adiabatic state transformation results in a cold gas of Rydberg atoms with large electric dipole moments. After a waiting time of about $1~μ$s and at sufficient atom density, the adiabatically transformed highly dipolar atoms become undetectable, enabling us to discern adiabatic from diabatic passage behavior through the avoided crossing. We attribute the state-selectivity to $m$-mixing collisions between the dipolar atoms. The data interpretation is supported by numerical simulations of the passage dynamics and of binary $m$-mixing collisions.

physics.atom-ph↗

arXiv2015

V-type electromagnetically induced transparency and saturation effect at the gas-solid interface

We theoretically study electromagnetically induced transparency (EIT) in reflection spectra of V-type system at the gas-solid interface. In addition to a narrow dip arising from the EIT effect, we find the other particular saturation effect induced by pump field, which does not exist in $Λ$ or $Ξ$ -type system reflection spectra. The saturation effect only induces an intensity decrement in the reflection spectra, and there is no influence on the narrow dip arising from the EIT effect. We detailedly calculate and analyze the dependence of V-type system reflection spectra on probe field intensity, pump field intensity, coherent decay rate, and the initial population after the collision between atoms and the interface.

physics.atom-ph↗

arXiv2015

Two-axis spin squeezing in two cavities

Ultracold atoms in an ultrahigh-finesse optical cavity are a powerful platform to produce spin squeezing since photon of cavity mode can induce nonlinear spin-spin interaction and thus generate a one-axis twisting Hamiltonian $H_{\text{OAT}}=qJ_{x}^{2}$, whose corresponding maximal squeezing factor scales as $N^{-2/3}$, where $N$ is the atomic number. On the contrary, for the other two-axis twisting Hamiltonian $H_{\text{TAT}}=q(J_{x}^{2}-J_{y}^{2})$, the maximal squeezing factor scales as $N^{-1}$, approaching the Heisenberg limit. In this paper, inspired by recent experiments of cavity-assisted Raman transitions, we propose a scheme, in which an ensemble of ultracold six-level atoms interacts with two quantized cavity fields and two pairs of Raman lasers, to realize a tunable two-axis spin Hamiltonian $%H=q(J_{x}^{2}+χJ_{y}^{2})+ω_{0}J_{z}$. For proper parameters, the above one- and two- axis twisting Hamiltonians are recovered, and the scaling of $N^{-1}$ of the maximal squeezing factor can occur naturally. On the other hand, in the two-axis twisting Hamiltonian, spin squeezing is usually reduced when increasing the effective atomic resonant frequency $ω_{0}$. Surprisingly, we find that by combined with the dimensionless parameter $χ(>-1)$, the effective atomic resonant frequency $ω_{0}$ can enhance spin squeezing largely. These results are benefit for achieving the required spin squeezing in experiments.

quant-ph↗

arXiv2014

SU(2) Ginzburg-Landau theory for degenerate Fermi gases with synthetic non-Abelian gauge fields

The non-Abelian gauge fields play a key role in achieving novel quantum phenomena in condensed-matter and high-energy physics. Recently, the synthetic non-Abelian gauge fields have been created in the neutral degenerate Fermi gases, and moreover, generate many exotic effects. All the previous predictions can be well understood by the microscopic Bardeen-Cooper-Schrieffer theory. In this work, we establish an SU(2) Ginzburg-Landau theory for degenerate Fermi gases with the synthetic non-Abelian gauge fields. We firstly address a fundamental problem how the non-Abelian gauge fields, imposing originally on the Fermi atoms, affect the pairing field with no extra electric charge by a local gauge-field theory,and then obtain the first and second SU(2) Ginzburg-Landau equations. Based on these obtained SU(2) Ginzburg-Landau equations, we find that the superfluid critical temperature of the intra- (inter-) band pairing increases (decreases) linearly, when increasing the strength of the synthetic non-Abelian gauge fields. More importantly, we predict a novel SU(2) non-Abelian Josephson effect, which can be used to design a new atomic superconducting quantum interference device.

cond-mat.quant-gas↗

arXiv2014

Quantum phases in circuit QED with a superconducting qubit array

Circuit QED on a chip has become a powerful platform for simulating complex many-body physics. In this report, we realize a Dicke-Ising model with an antiferromagnetic nearest-neighbor spin-spin interaction in circuit QED with a superconducting qubit array. We show that this system exhibits a competition between the collective spin-photon interaction and the antiferromagnetic nearest-neighbor spin-spin interaction, and then predict four quantum phases, including: a paramagnetic normal phase, an antiferromagnetic normal phase, a paramagnetic superradiant phase, and an antiferromagnetic superradiant phase. The antiferromagnetic normal phase and the antiferromagnetic superradiant phase are new phases in many-body quantum optics. In the antiferromagnetic superradiant phase, both the antiferromagnetic and superradiant orders can coexist, and thus the system possesses $Z_{2}^{z}\otimes Z_{2}$\ symmetry. Moreover, we find an unconventional photon signature in this phase. In future experiments, these predicted quantum phases could be distinguished by detecting both the mean-photon number and the magnetization.

quant-ph↗