Quantum States Of Atoms Research Articles

Pound–Drever–Hall feedforward: laser phase noise suppression beyond feedback

Pound–Drever–Hall (PDH) laser frequency stabilization is a powerful technique widely used for building narrow linewidth lasers. This technique is, however, ineffective in suppressing high-frequency (>100kHz) laser phase noise detrimental for many applications. Here, we introduce an effective method that can greatly enhance its high-frequency performance. The idea is to recycle the residual PDH signal of a laser locked to a cavity by feedforwarding it directly to the laser output field after a delay fiber. Using this straightforward method, we demonstrate a phase noise suppression capability about four orders of magnitude better than just using the usual PDH feedback for noise around a few MHz. We further find that this method exhibits noise suppression performance equivalent to cavity filtering. This method holds great promise for applications demanding highly stable lasers with diminished phase noise up to tens of MHz (e.g., precise and high-speed control of atomic and molecular quantum states).

Atomically precise engineering of spin–orbit polarons in a kagome magnetic Weyl semimetal

Atomically precise defect engineering is essential to manipulate the properties of emerging topological quantum materials for practical quantum applications. However, this remains challenging due to the obstacles in modifying the typically complex crystal lattice with atomic precision. Here, we report the atomically precise engineering of the vacancy-localized spin–orbit polarons in a kagome magnetic Weyl semimetal Co3Sn2S2, using scanning tunneling microscope. We achieve the step-by-step repair of the selected vacancies, leading to the formation of artificial sulfur vacancies with elaborate geometry. We find that that the bound states localized around these vacancies undergo a symmetry dependent energy shift towards Fermi level with increasing vacancy size. As the vacancy size increases, the localized magnetic moments of spin–orbit polarons become tunable and eventually become itinerantly negative due to spin–orbit coupling in the kagome flat band. These findings provide a platform for engineering atomic quantum states in topological quantum materials at the atomic scale.

Tomography of a Number-Resolving Detector by Reconstruction of an Atomic Many-Body Quantum State.

The high-fidelity analysis of many-body quantum states of indistinguishable atoms requires the accurate counting of atoms. Here we report the tomographic reconstruction of an atom-number-resolving detector. The tomography is performed with an ultracold rubidium ensemble that is prepared in a coherent spin state by driving a Rabi coupling between the two hyperfine clock levels. The coupling is followed by counting the occupation number in one level. We characterize the fidelity of our detector and show that a negative-valued Wigner function is associated with it. Our results offer an exciting perspective for the high-fidelity reconstruction of entangled states and can be applied for a future demonstration of Heisenberg-limited atom interferometry.

Metamaterials With Analogous Electromagnetically Induced Transparency and Related Sensor Designs—A Review

Electromagnetically induced transparency (EIT) originates from quantum physics, where a narrow transparent peak appears in the opaque band due to the destructive interference between quantum states of atoms and molecules. Similar phenomena can be realized based on strong-coupling resonators with a similar spectrum of transmission peaks and abrupt dispersion variations. These classical systems, ranging from elastic to optical, are named analogs of EIT. The sharp resonant peaks with high-quality factors in the spectrum exhibit powerful potentials in sensors with ultra-high sensitivity. In order to better understand the development history of EIT-like metamaterials and their specific applications in the field of sensors, this paper makes a brief review of the EIT-like phenomenon in metamaterials. Firstly, we conduct the universal mathematical formulation based on the coupling oscillator model. Then, we classify specific metamaterial designs and practical applications of EIT-like devices in acoustic, electromagnetic, and optical waves, respectively. We also summarize the recent technologies of dynamic modulations of EIT-like metamaterials and discuss future research directions.

GRASIAN: towards the first demonstration of gravitational quantum states of atoms with a cryogenic hydrogen beam

At very low energies, a light neutral particle above a horizontal surface can experience quantum reflection. The quantum reflection holds the particle against gravity and leads to gravitational quantum states (gqs). So far, gqs were only observed with neutrons as pioneered by Nesvizhevsky and his collaborators at ill. However, the existence of gqs is predicted also for atoms. The Grasian collaboration pursues the first observation and studies of gqs of atomic hydrogen. We propose to use atoms in order to exploit the fact that orders of magnitude larger fluxes compared to those of neutrons are available. Moreover, recently the q-Bounce collaboration, performing gqs spectroscopy with neutrons, reported a discrepancy between theoretical calculations and experiment which deserves further investigations. For this purpose, we set up a cryogenic hydrogen beam at 6 hbox {K}. We report on our preliminary results, characterizing the hydrogen beam with pulsed laser ionization diagnostics at 243 hbox {nm}.Graphical abstract

Shock-Tube Measurements of Atomic Nitrogen Collisional Excitation in 8000-12000 K Partially Ionized Nitrogen-Argon Mixtures.

We report on shock-tube experiments measuring the collisional excitation of atomic nitrogen using tunable diode laser absorption spectroscopy (TDLAS). Conditions behind the reflected shocks ranged from 8000 to 12000 K and 0.1 to 1.1 atm in mixtures of 1 or 2% molecular nitrogen (N2) in argon (Ar). Absorption from the transition between atomic nitrogen quantum states 4P to 4D at 868 nm was used to monitor the formation of electronically excited nitrogen. Population measurements of the 4P state were made at a rate of 50 kHz. In connection with these measurements, a multitemperature kinetic mechanism is proposed for nitrogen excitation. Measurements suggest a multistage process. In early test times, a period of induction due to N2 dissociation is followed by a rise via heavy particle excitation. The dominant channel causing this excitation is believed to be N + N ↔ N(4P) + N with a measured forward rate constant of 3.65 × 10-18 exp(-119892/T) [m3/s]. As test time evolves, the excitation of 4P, referred to as N*, is subsequently interrupted and then resumes, as the kinetic environment becomes increasingly electron-dominated. The most impactful reactions of the mechanism are optimized to reduce the residual between simulations and the measurements. The measured N* populations indicate strong, though indirect, sensitivity to adjacent processes, including the excitation of metastable nitrogen, and ionization channels.

Optical reconstruction of the collective density matrix of a qutrit

Reliable tomography of a quantum state of atoms in room-temperature vapors offers interesting applications in quantum-information science. To step toward the applications, here, we theoretical investigate a technique of reconstruction of a collective density matrix of atoms in a state of a total angular momentum equal to unity. By studying transformation of density matrix due to magnetic-field pulses and deriving explicit relations between optical signals and specific density-matrix elements, we demonstrate full reconstruction of quantum states. Numerical simulations allow us to demonstrate that the reconstruction fidelity of pure and mixed states may exceed 0.95 and that the technique is robust against noise and uncertainties.

Manipulation of Dirac band curvature and momentum-dependent g factor in a kagome magnet

The Zeeman effect describes the energy change of an atomic quantum state in a magnetic field. The magnitude and direction of this change depend on the dimensionless Landé g factor. In quantum solids, the response of the Bloch electron states to the magnetic field also exhibits the Zeeman effect, with an effective g factor that was theoretically predicted to depend on the momentum1,2,3, and which may be particularly strong in topological and magnetic systems. However, the momentum dependence of the g factor is difficult to extract and it has not been directly measured. Here we report the experimental discovery of a momentum-dependent g factor in the kagome magnet YMn6Sn6. We map the evolution of a massive Dirac band in the vicinity of the Fermi level as a function of the magnetic field. We find that electronic states at different lattice momenta exhibit different Zeeman energy shifts, giving rise to an anomalous g factor that peaks around the Dirac point. Our work provides a momentum-resolved visualization of Dirac band curvature manipulated by a magnetic field, which will be relevant to other topological kagome magnets.

Attosecond Optical and Ramsey-Type Interferometry by Postgeneration Splitting of Harmonic Pulse

Time domain Ramsey-type interferometry is useful for investigating spectroscopic information of quantum states in atoms and molecules. The energy range of the quantum states to be observed with this scheme has now reached more than 20 eV by resolving the interference fringes with a period of a few hundred attoseconds. This attosecond Ramsey-type interferometry requires the irradiation of a coherent pair of extreme ultraviolet (XUV) light pulses, while all the methods used to deliver the coherent XUV pulse pair until now have relied on the division of the source of an XUV pulse in two before the generation. In this paper, we report on a novel technique to perform attosecond Ramsey-type interferometry by splitting an XUV high-order harmonic (HH) pulse of a sub-20 fs laser pulse after its generation. By virtue of the postgeneration splitting of the HH pulse, we demonstrated that the optical interference emerging at the complete temporal overlap of the HH pulse pair seamlessly continued to the Ramsey-type electronic interference in a helium atom. This technique is applicable for studying the femtosecond dephasing dynamics of electronic wavepackets and exploring the ultrafast evolution of a cationic system entangled with an ionized electron with sub-20 fs resolution.

An Advanced Platform for Thermodynamics Education. Part two: Monomer Quantum Volume in Pure Fluides

The paper presents a remarkable application of the advanced thermodynamics education platform to the molar Gibbs energy G for basic particles in neat fluids. From the G, named also as the chemical potential, and the monomer fraction density Dm functions the Monomer Quantum Volume Vq (T) may be computed. The Vq (T) variable reflects the quantum uncertainty of basic particles positions in atomic and molecular fluids. It has proven to be universal for all fluid’s densities at a fixed temperature T. An extraordinary precision of modern thermophysical databases, such as the NIST Webbook, permits an estimation of the Monomer Quantum Volume values millions times lower than the atom’s volume! By studying the Vq (T) function the students can estimate the quantum uncertainty effects in pure fluids for a total range of their existence up to thousands of Kelvin! The enthalpy data normalization has proven to be very efficient. The advanced platform is highly useful and informative for thermodynamics education. For students it is very educative to study and utilize the author’s computer aided big thermophysical data analysis method to form their vision of the atomic and molecular quantum states distribution in pure fluids.

Soliton-like unipolar objects in nonequilibrium dissipative media

For electromagnetic pulses without a carrier frequency, propagating in a system of multilevel atoms with a fast irreversible relaxation of the induced dipole moment, but a slow relaxation of the populations of quantum states, the nonlinear integro-differential equations of the ‘reaction–diffusion’ type are derived. One-dimensional soliton-like solutions of these equations in the form of unipolar pulses are found and analyzed. It is shown that such pulses can be formed in a nonequilibrium medium. These pulses transfer a medium to other metastable states, which depend on the input conditions. The propagation of these soliton-like signals is accompanied by a change in the populations of quantum states of atoms in the modes of switching waves with the memory of input conditions.

Pair Correlations and Photoassociation Dynamics of Two Atoms in an Optical Tweezer.

We investigate the photoassociation dynamics of exactly two laser-cooled ^{85}Rb atoms in an optical tweezer and reveal fundamentally different behavior to photoassociation in many-atom ensembles. We observe nonexponential decay in our two-atom experiment that cannot be described by a single rate coefficient and find its origin in our system's pair correlation. This is in stark contrast to many-atom photoassociation dynamics, which are governed by decay with a single rate coefficient. We also investigate photoassociation in a three-atom system, thereby probing the transition from two-atom dynamics to many-atom dynamics. Our experiments reveal additional reaction dynamics that are only accessible through the control of single atoms and suggest photoassociation could measure pair correlations in few-atom systems. It further showcases our complete control over the quantum state of individual atoms and molecules, which provides information unobtainable from many-atom experiments.

Rabi Oscillations, Entanglement and Teleportation in the Anti-Jaynes-Cummings Model

This paper provides a scheme for generating maximally entangled qubit states in the anti-Jaynes-Cummings interaction mechanism, so called entangled anti-polariton qubit states. We demonstrate that in an initial vacuum-field, Rabi oscillations in a cavity mode in the anti-Jaynes-Cummings interaction process, occur in the reverse sense relative to the Jaynes-Cummings interaction process and that time evolution of entanglement in the anti-Jaynes-Cummings interaction process takes the same form as in the Jaynes-Cummings interaction process. With the generated anti-polariton qubit state as one of the initial qubits, we present quantum teleportation of an atomic quantum state by applying entanglement swapping protocol achieving an impressive maximal teleportation fidelity~$F_\rho=1$.

Experimental study of coherent manipulation in <sup>87</sup>Rb Bose-Einstein condensate with phase difference of double stimulated Raman adiabatic passage

In this paper, we develop a new method to adjust the Raman coupling strength by using the relative phase between two pairs of Raman lasers. The stimulated Raman transition process is highly controllable and has the characteristics of multiple degrees of freedom. In experiments on ultracold atoms, the populations of atomic energy levels can be adjusted by taking an appropriate Raman light intensity and interaction time, and by detuning the two-photon frequency. The intensity of the Raman laser is usually changed to adjust the Raman coupling strength. Based on two-level atoms, a new method of accurately controlling the Raman coupling strength by using the relative phase between two pairs of Raman light beams is developed. This technology can achieve coherent manipulation of atomic quantum states, which greatly broadens the ability of ultracold atoms to perform quantum simulations. First, the <sup>87</sup>Rb Bose-Einstein condensate is realized by using an optical dipole trap. Then, the two pairs of Raman lasers are designed with a special optical path to keep the relative phase of the two pairs of Raman lasers stable in the transmission process, and can be controlled accurately. Then the two pairs of Raman light beams act on the two ground state hyperfine energy levels <inline-formula><tex-math id="Z-20210731120833-1">\begin{document}$ |1, 1\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20210173_Z-20210731120833-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20210173_Z-20210731120833-1.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="Z-20210731120833-2">\begin{document}$ |1, 0\rangle $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20210173_Z-20210731120833-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20210173_Z-20210731120833-2.png"/></alternatives></inline-formula> of the <sup>87</sup>Rb atom. In the experiment, we observe the relation between the percentage of atoms in the two quantum states and the relative phase between the two pairs of Raman light beams. This method provides a unique control parameter for ultracold atom quantum simulation experiments, which is the laser phase. It is hoped that this technology can be used to manipulate the interaction between light and atoms in the future to achieve more abundant physical phenomena.

Breakdown of the Einstein’s Equivalence Principle for a quantum body

We review our recent theoretical results about inequivalence between passive gravitational mass and energy for a composite quantum body at a macroscopic level. In particular, we consider macroscopic ensembles of the simplest composite quantum bodies — hydrogen atoms. Our results are as follows. For the most ensembles, the Einstein’s Equivalence Principle is valid. On the other hand, we discuss that for some special quantum ensembles — ensembles of the coherent superpositions of the stationary quantum states in the hydrogen atoms (which we call Gravitational demons) — the Equivalence Principle between passive gravitational mass and energy is broken. We show that, for such superpositions, the expectation values of passive gravitational masses are not related to the expectation values of energies by the famous Einstein’s equation, i.e. [Formula: see text]. Possible experiments at the Earth’s laboratories are briefly discussed, in contrast to the numerous attempts and projects to discover the possible breakdown of the Einstein’s Equivalence Principle during the space missions.

Visualizing spin degrees of freedom in atoms and molecules

In this work we show how constructing Wigner functions of heterogeneous quantum systems leads to new capability in the visualization of quantum states of atoms and molecules. This method allows us to display quantum correlations (entanglement) between spin and spatial degrees of freedom (spin-orbit coupling) and between spin degrees of freedom, as well as more complex combinations of spin and spatial entanglement. This is important as there is growing recognition that such properties affect the physical characteristics, and chemistry, of atoms and molecules. Our visualizations are sufficiently accessible that, with some preparation, those with a nontechnical background can gain an appreciation of subtle quantum properties of atomic and other systems. By providing insights and modeling capability, our phase-space representation will be of great utility in understanding aspects of atomic physics and chemistry not available with current techniques.

Quantum sensors probe uncharted territories, from Earth’s crust to the human brain

The quantum realm is often portrayed as weird and nebulous, far removed from our everyday world. Subatomic particles can be in many places at once or linked across vast distances by the intimate connection known as entanglement. But researchers are now using this quantum fuzziness to forge a new set of practical tools. In this entanglement-assisted magnetometer, lenses, and other optical devices steer a laser beam to the small glass cell filled with cesium vapor, and a photodetector above records the effect of magnetic fields on light emitted by the cesium atoms. Image credit: Ola J. Joensen (Niels Bohr Institute, Copenhagen, Denmark). Although quantum computers gain the most headlines, a quieter revolution in the world of sensors may be just as profound. Quantum states, such as the spin of an electron, have a peculiar delicacy that is being exploited to detect tiny variations in gravity and magnetism, opening up a range of applications that could give us a clearer view of the world beneath our feet and provide deep insights into our own biology. This has become possible thanks to continuing advances in components such as lasers and cooling systems that allow researchers and engineers to manipulate the quantum states of atoms and measure how they are affected by their environment. “We are at this sweet spot where we have quantum systems under sufficient control,” says Kai Bongs, who leads the Quantum Sensors and Metrology Hub (1), a collaboration between academics and industry centered at the University of Birmingham in the United Kingdom. Some of the large vacuum systems and magnetic traps now used to hold cold atoms are being replaced with chip-scale devices, while researchers are developing compact, low-power lasers that could make quantum sensors much more accessible. “The early devices are still clumsy, but now we can start …

Atom interferometers with weak-measurement path detectors and their quantum mechanical analysis**Project supported by the National Key Research and Development Program of China (Grant No. 2018YFA 0306200), the National Natural Science Foundation of China (Grant No. 11434017), and the Guangdong Innovative and Entrepreneurial Research Team Program, China (Grant No. 2016ZT06C594).

According to the orthodox interpretation of quantum physics, wave-particle duality (WPD) is the intrinsic property of all massive microscopic particles. All gedanken or realistic experiments based on atom interferometers (AI) have so far upheld the principle of WPD, either by the mechanism of the Heisenberg’s position-momentum uncertainty relation or by quantum entanglement. In this paper, we propose and make a systematic quantum mechanical analysis of several schemes of weak-measurement atom interferometer (WM-AI) and compare them with the historical schemes of strong-measurement atom interferometer (SM-AI), such as Einstein’s recoiling slit and Feynman’s light microscope. As the critical part of these WM-AI setups, a weak-measurement path detector (WM-PD) deliberately interacting with the atomic internal electronic quantum states is designed and used to probe the which-path information of the atom, while only inducing negligible perturbation of the atomic center-of-mass motion. Another instrument that is used to directly interact with the atomic center-of-mass while being insensitive to the internal electronic quantum states is used to monitor the atomic center-of-mass interference pattern. Two typical schemes of WM-PD are considered. The first is the micromaser-cavity path detector, which allows us to probe the spontaneously emitted microwave photon from the incoming Rydberg atom in its excited electronic state and record unanimously the which-path information of the atom. The second is the optical-lattice Bragg-grating path detector, which can split the incoming atom beam into two different directions as determined by the internal electronic state and thus encode the which-path information of the atom into the internal states of the atom. We have used standard quantum mechanics to analyze the evolution of the atomic center-of-mass and internal electronic state wave function by directly solving Schrödinger’s equation for the composite atom-electron-photon system in these WM-AIs . We have also compared our analysis with the theoretical and experimental studies that have been presented in the previous literature. The results show that the two sets of instruments can work separately, collectively, and without mutual exclusion to enable simultaneous observation of both wave and particle nature of the atoms to a much higher level than the historical SM-AIs, while avoiding degradation from Heisenberg’s uncertainty relation and quantum entanglement. We have further investigated the space–time evolution of the internal electronic quantum state, as well as the combined atom–detector system and identified the microscopic origin and role of quantum entanglement, as emphasized in numerous previous studies. Based on these physics insights and theoretical analyses, we have proposed several new WM-AI schemes that can help to elucidate the puzzling physics of the WPD of the atoms. The principle of WM-AI scheme and quantum mechanical analyses made in this work can be directly extended to examine the principle of WPD for other massive particles.

Reaction blockading in a reaction between an excited atom and a charged molecule at low collision energy.

Recent advances have enabled studies of atom-ion chemistry at unprecedentedly low temperatures, allowing precision observation of chemical reactions and novel chemical dynamics. So far, these studies have primarily involved reactions between atoms and atomic ions or non-polar molecular ions, often in their electronic ground state. Here, we extend this work by studying an excited atom-polar-molecular-ion chemical reaction (Ca* + BaCl+) at low temperature in a hybrid atom-ion trapping system. The reaction rate and product branching fractions are measured and compared to model calculations as a function of both atomic quantum state and collision energy. At the lowest collision energy we find that the chemical dynamics differ dramatically from capture theory predictions and are primarily dictated by the radiative lifetime of the atomic quantum state instead of the underlying excited-state interaction potential. This reaction blockading effect, which greatly suppresses the reactivity of short-lived excited states, provides a means for directly probing the reaction range and also naturally suppresses unwanted chemical reactions in hybrid trapping experiments.

Ground-State Phase Diagram of a Spin-Orbital-Angular-Momentum Coupled Bose-Einstein Condensate.

By inducing a Raman transition using a pair of Gaussian and Laguerre-Gaussian laser beams, we realize a ^{87}Rb condensate whose orbital angular momentum (OAM) and its internal spin states are coupled. By varying the detuning and the coupling strength of the Raman transition, we experimentally map out the ground-state phase diagram of the system for the first time. The transitions between different phases feature a discontinuous jump of the OAM and the spin polarization, and hence are of first order. We demonstrate the hysteresis loop associated with such first-order phase transitions. The role of interatomic interaction is also elucidated. Our work paves the way to explore exotic quantum phases in the spin-orbital-angular-momentum coupled quantum gases.