Measurements Of Fundamental Constants Research Articles

Varying alpha, blinding, and bias in existing measurements

ABSTRACT The high resolution spectrograph ESPRESSO on the VLT allows measurements of fundamental constants at unprecedented precision and hence enables tests for space–time variations predicted by some theories. In a series of recent papers, we developed optimal analysis procedures that expose and eliminate the subjectivity and bias in previous quasar absorption system measurements. In this paper, we analyse the ESPRESSO spectrum of the absorption system at zabs = 1.15 towards the quasar HE 0515-4414. Our goal here is not to provide a new unbiased measurement of Δα/α in this system (that will be done separately). Rather, it is to carefully examine the impact of blinding procedures applied in many previous measurements of the fine structure constant in quasar absorption systems. To do this, we emulate previous procedures, using supercomputer Monte Carlo AI calculations to generate a large number of independently constructed models of the absorption complex. Each model is obtained using ai-vpfit, with Δα/α fixed until a ‘final’ model for the absorption system is obtained, at which point Δα/α is then released as a free parameter for one final optimization. The results show that the value of Δα/α obtained in this way is systematically biased towards the initially fixed value i.e. this process produces measurements that are unrelated to the true value of Δα/α. The implication is straightforward: to avoid bias, all future measurements must include Δα/α as a free parameter from the beginning of the modelling process.

Avoiding Bias in Measurements of Fundamental Constants from High Resolution Quasar Spectra

Recent advances in spectroscopic instrumentation and calibration methods dramatically improve the quality of quasar spectra. Supercomputer calculations show that, at high spectral resolution, procedures used in some previous analyses of spacetime variations of fundamental constants are likely to generate spurious measurements, biased systematically towards a null result. Developments in analysis methods are also summarised and a prescription given for the analysis of new and forthcoming data.

Symmetric Bloch oscillations of matter waves

Cold atoms in an optical lattice provide an ideal platform for studying Bloch oscillations. Here, we extend Bloch oscillations to two superposed optical lattices that are accelerated away from one another, and for the first time show that these symmetric Bloch oscillations can split, reflect and recombine matter waves coherently. Using the momentum parity-symmetry of the Hamiltonian, we map out the energy band structure of the process and show that superpositions of momentum states are created by adiabatically following the ground state of the Hamiltonian. The relative phase and velocity of the two lattices completely determines the trajectories of different branches of the matter wave. Experimentally, we demonstrate symmetric Bloch oscillations using cold Cesium atoms where we form interferometers with up to $240\hbar k$ momentum splitting, one of the largest coherent momentum splittings achieved to date. This work has applications in macroscopic tests of quantum mechanics, measurements of fundamental constants, and searches for new physics.

Ramsey-comb precision spectroscopy in xenon at vacuum ultraviolet wavelengths produced with high-order harmonic generation

The remarkable progress in the field of laser spectroscopy induced by the invention of the frequency-comb laser has enabled many new high-precision tests of fundamental theory and searches for new physics. Extending frequency-comb based spectroscopy techniques to the vacuum (VUV) and extreme ultraviolet (XUV) spectral range would enable measurements in e.g. heavier hydrogen-like systems and open up new possibilities for tests of quantum electrodynamics and measurements of fundamental constants. The main approaches rely on high-harmonic generation (HHG), which is known to induce spurious phase shifts from plasma formation. After our initial report (Physical Review Letters 123, 143001 (2019)), we give a detailed account of how the Ramsey-comb technique is used to probe the plasma dynamics with high precision, and enables accurate spectroscopy in the VUV. A series of Ramsey fringes is recorded to track the phase evolution of a superposition state in xenon atoms, excited by two up-converted frequency-comb pulses. Phase shifts of up to 1 rad induced by HHG were observed at ns timescales and with mrad-level accuracy at $110$ nm. Such phase shifts could be reduced to a negligible level, enabling us to measure the $5p^6 \rightarrow 5p^5 8s~^2[3/2]_1$ transition frequency in $^{132}Xe$ at 110 nm (seventh harmonic) with sub-MHz accuracy. The obtained value is $10^4$ times more precise than the previous determination and the fractional accuracy of $2.3 \times 10^{-10}$ is $3.6$ times better than the previous best spectroscopic measurement using HHG. The isotope shifts between $^{132}Xe$ and two other isotopes were determined with an accuracy of $420$ kHz. The method can be readily extended to achieve kHz-level accuracy, e.g. to measure the $1S-2S$ transition in $He^+$. Therefore, the Ramsey-comb method shows great promise for high-precision spectroscopy of targets requiring VUV and XUV wavelengths.

Fundamental constant observational bounds on the variability of the QCD scale

Many physical theories beyond the Standard Model predict time variations of basic physics parameters. Direct measurement of the time variations of these parameters is very difficult or impossible to achieve. By contrast, measurements of fundamental constants are relatively easy to achieve, both in the laboratory and by astronomical spectra of atoms and molecules in the early universe. In this work measurements of the proton to electron mass ratio $\mu$ and the fine structure constant $\alpha$ are combined to place mildly model dependent limits on the fractional variation of the Quantum Chromodynamic Scale and the sum of the fractional variations of the Higgs Vacuum Expectation Value and the Yukawa couplings on time scales of more than half the age of the universe. The addition of another model parameter allows the fractional variation of the Higgs VEV and the Yukawa couplings to be computed separately. Limits on their variation are found at the level of less than $5 \times 10^{-5}$ over the past seven gigayears. A model dependent relation between the expected fractional variation of $\alpha$ relative to $\mu$ tightens the limits to $10^{-7}$ over the same time span. Limits on the present day rate of change of the constants and parameters are then calculated using slow roll quintessence. A primary result of this work is that studies of the dimensionless fundamental constants such as $\alpha$ and $\mu$, whose values depend on the values of the physics parameters, are excellent monitors of the limits on the time variation of these parameters.

Research progress of Al$^{+}$ ion optical frequency standard based on quantum logic technique

Optical atomic clocks have many applications in the definition of the time and the length, measurement of fundamental constants and their potential variations with time, testing of relativity, very long baseline interferometry, geodesy, and global navigation satellite systems. An optical clock consists of three parts: ultra-stable laser, frequency comb and trapped ions or atoms. The accuracy of optical atomic clocks can be affected by Doppler shift, Zeeman shift, Stark shift, collision shift, blackbody radiation (BBR) shift, etc. The BBR shift is very difficult to be characterized and controlled. Among all optical atomic clocks under current development, Al$^+$ optical clock has the smallest BBR shift at a level of $4.0\times10^{-19}$, therefore the Al$^+$ is a promising ion for high-accuracy optical frequency standard. In this article we give a review on the three parts of optical frequency standards, the basic working principle of Al$^+$ ion optical frequency standard based on quantum logic technique, and research progress of an Al$^+$ optical clock under development at Huazhong University of Science and Technology.

Precision test of the ac Stark shift in a rubidium atomic vapor

The ac Stark shift (also known as the ``light shift'') is one of the most important physical processes that arises in precision spectroscopy, affecting the basic understanding of field-matter interactions, measurements of fundamental constants, and even the atomic clocks onboard GPS satellites. Although the theory of the ac Stark shift was fully developed by the 1960s and 1970s, precision tests of theory have, for the most part, been few. Taking advantage of recent developments in atomic clock technology, specifically the pulsed approach to atomic signal generation, which allows frequency measurements with a resolution of $\ensuremath{\sim}{10}^{\ensuremath{-}15}$, we demonstrate a methodology for measuring the ac Stark shift. Here, we report results from a precision examination of the ac Stark shift in a vapor phase system, examining the resonant frequency of the $^{87}\text{Rb}$ 0-0 hyperfine transition for a perturbing laser tuned over a broad optical frequency range (18 GHz) around the ${D}_{1}$ absorption resonance. Over the full frequency range the agreement between semiclassical theory and experiment is very good (better than $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$), and in our experiments we test both the frequency dependence of the scalar and tensor components of the light shift.

Antihydrogen Physics at ALPHA/CERNThis paper was presented at the International Conference on Precision Physics of Simple Atomic Systems, held at University of Windsor, Windsor, Ontario, Canada on 21–26 July 2008.

Cold antihydrogen has been produced at CERN (Amoretti et al. (Nature, 419, 456 (2002)), Gabrielse et al. (Phys. Rev. Lett. 89, 213401 (2002))), with the aim of performing a high-precision spectroscopic comparison with hydrogen as a test of the CPT symmetry. Hydrogen, a unique system used for the development of quantum mechanics and quantum electrodynamics, has been continuously used to produce high-precision tests of theories and measurements of fundamental constants and can lead to a very sensitive search for CPT violation. After the initial production of cold antihydrogen atoms by the ATHENA group, the ALPHA Collaboration ( http://alpha.web.cern.ch/ ) has set forth on an experiment to trap and perform high-resolution laser spectroscopy on the 1S-2S transition of both atoms. In this contribution, we will review the motivations, goals, techniques, and recent developments towards this fundamental physics test. We present new discussion on predicted lineshapes for the 1S-2S spectroscopy of trapped atoms in a regime not discussed before.

COHERENT ATOM SOURCES FOR ATOM INTERFEROMETRY IN SPACE: THE ICE PROJECT

Atomic quantum sensors are a major breakthrough in the technology of time and frequency standards as well as ultraprecise sensing and monitoring of accelerations and rotations. They apply a new kind of optics based on matter waves. Today, atomic clocks are the standard for time and frequency measurement at the highest precisions. Inertial and rotational sensors using atom interferometers have already shown similar potential for replacing state-of-the-art sensors in other fields. With Bose–Einstein condensates, also referred as atom lasers, the traditional experiments with atom interferometers can be greatly improved. Testing of fundamental principles, studies of atomic properties, applications as inertial sensors, and measurements of fundamental constants can benefit from the brightness (intensity and small momentum spread) of these coherent sources. In addition, the coherence properties of condensates may allow BEC-based atom interferometers to approach the Heisenberg detection limit. This corresponds to a measurement precision which scales like 1/N for N atoms and not like [Formula: see text] as for independent measurements on N atoms. We present here the recent progress toward the acheivement of new coherent atomic sources, i.e. atom lasers, that will be use in space-based atom interferometers. We introduce new concepts of atom accelerometers and gyrometers that take advantage of the high collimation and coherence properties of atoms lasers and report on the development of a 0-g coherent atom interferometer (ICE) that will be used to test the ultimate performances of atom accelerometers in space.

Fundamental symmetries and interactions --Some aspects

In the framework of nuclear physics and at nuclear physics facilities a large number of different experiments can be performed which render the possibility to investigate fundamental symmetries and interactions in nature. In particular, the precise measurements of properties of fundamental fermions, searches for new interactions in β-decays, and violations of discrete symmetries have a robust discovery potential for physics beyond standard theory. Precise measurements of fundamental constants can be carried out as well. Low energy experiments allow probing of New Physics models at mass scales far beyond the reach of present accelerators or such planned for the future in the domain of high energy physics and at which predicted new particles could be produced directly.

A Simple Laboratory Experiment to Measure e/k

The measurement of fundamental constants is common practice in instructional laboratories. A number of the equipment manufacturers have developed apparatus for such applications, e.g., the determination of e by the Millikan oil drop method or the determination of the speed of light with fiber optics. Other experiments determine not a single constant, but a combination of constants, e.g., e/m by electron beam deflection in a magnetic field or h/e by the photoelectric effect. About 30 years ago Carl E. Miller and I1 proposed a method of measuring e/k, the ratio of the electron charge to Boltzmann's constant, that was reasonably simple but not necessarily inexpensive because it involved the use of a sensitive electrometer. In recent years, however, inexpensive digital multimeters (DMM), many costing less than $30, have found their way into the physics laboratory. The purpose of this paper is to suggest the use of two DMMs, one operating as a voltmeter and the other as an ammeter, in a simple circuit involving a junction transistor and a variable potential source. Even the potential source can be quite simple, a 1.5-V battery and a 1-kΩ potentiometer, as shown in Fig. 1. If available, a variable dc power supply replacing the battery and potentiometer would be more convenient.

Precision studies in traps: Measurement of fundamental constants and tests of fundamental theories

Experiments on single atomic particles confined in Penning ion traps have contributed significantly to the improvements of fundamental constants and to tests of the theory of Quantum Electrodynamics for free and bound electrons. The most precise value of the fine structure constant as well as the electron mass have been derived from trap experiments. Numerous atomic masses of interest for fundamental questions have been determined with precisions of 10−9 or below. Further progress is envisaged in the near future.

Rabi-Lorentzian profile of an atomic resonance obtained with Gaussian beams.

We demonstrate that the interaction between atoms and a Gaussian beam changes the Lorentzian resonance into a so-called Rabi-Lorentzian resonance profile. The existence of such profiles is experimentally evidenced in a dark resonance situation for metastable (4) He, in the absence of any external perturbation. A topological mechanism is shown theoretically to account for the measured profiles. We observe a 3% systematic residual with respect to a pure Lorentzian profile, which could play a role in the search of high precision in the measurement of fundamental constants.

Putting an electron to rest

Cyclotrons are well known as the first useful particle accelerators. Less familiar is their use for high-precision measurements of fundamental constants, such as the electron magnetic moment. An experiment that records the fundamental quantum limit of a cyclotron opens the way to even better measurements of the electron magnetic moment.

X-ray and Neutron Interferometry and the Measurement of Fundamental Constants

X-ray and Neutron Interferometry and the Measurement of Fundamental Constants

Laser spectroscopy of muonic atoms

Laser spectroscopy of muonic atoms is well suited for testing the Quantum Electrodynamics of bound states, for precise measurements of fundamental constants, for investigating properties of hadronic nuclei, and for searching for anomalous muon interactions. Successful experiments have been reported for the 2S-2P transitions in muonic helium ion and for the 1S-2S two-photon transition in muonium. Experiments on muonic hydrogen are coming up. A source of very slow positive muons is being developed which employs laser photoionization of muonium. Polarized muonic helium was obtained by spin transfer from laser polarized Rubidium. The field is open for addressing a variety of interesting physics problems.

Present state of the electrical units

The situation with regard to the basic electrical SI units has changed remarkably over the past five years. Emphasis has shifted from the definition of the ampere and its realisation using a current balance to realisation of the electrical watt in terms of the mechanical watt, or the volt in terms of mechanical potential energy, because these latter routes offer sub-part-per-million accuracy. Measurements of fundamental constants which involve a mixture of electrical and mechanical units have provided another accurate way of relating electrical units to the metre, kilogram and second. Moreover, once they have been realised in these new ways, the electrical units can be maintained, using the Josephson and quantum Hall effects, with 1 in 108 precision. The paper describes the hundredfold improvement in the accuracy with which the electrical SI units can now be realised, conserved and disseminated and which culminated in the change in the values of the maintained reference standards which took place on 1st January 1990.

LASER SPECTROSCOPY, PAST, PRESENT, AND PERHAPS FUTURE

Historically, observations of the spectra of simple atoms have led to much of our understanding of the laws of physics. Recently, with tunable lasers, we are able to extend these observations with enormously increased resolution, sensitivity, and precision. There are a number of methods for overcoming the Doppler broadening of spectral lines. As little as a single atom can be observed and studied, by resonance fluorescence or photoionization. Complex spectra can be simplified in systematic ways by laser labeling of chosen levels. These and other advances in techniques are being applied to improve the measurement of fundamental constants, and to test the basic laws of physics.

Near zero, a frontier of physics

By reducing several of the variables of physics, such as temperature, electric field, magnetic field, gravitational field, and electric charge, very nearly to zero, new regions of physics can be explored. This makes possible the investigation of many fundamental problems in physics such as the dragging of inertial frames in general relativity; the search for gravitational waves from collapsing stars; the search for fraction electric charge on matter; the search for monopoles; the force of gravity on electrons and positrons; the electric dipole moment on 3He; the equivalence principle; a more accurate determination of the λ transition in zero gravitational field; the search for a new gravitational particle by checking the inverse square law of gravity; and the measurement of fundamental constants h/m, α.

Near zero, a frontier of physics

By reducing several of the variables of physics, such as temperature, electric field, magnetic field, gravitational field, and electric charge, very nearly to zero, new regions of physics can be explored. This makes possible the investigation of many fundamental problems in physics such as the dragging of inertial frames in general relativity; the search for gravitational waves from collapsing stars; the search for fraction electric charge on matter; the search for monopoles; the force of gravity on electrons and positrons; the electric dipole moment on 3 He; the equivalence principle; a more accurate determination of the λ transition in zero gravitational field; the search for a new gravitational particle by checking the inverse square law of gravity; and the measurement of fundamental constants h/m , α.