Pei-Chen Kuan

Influence of the Coriolis force in atom interferometry.

In a light-pulse atom interferometer, we use a tip-tilt mirror to remove the influence of the Coriolis force from Earths rotation and to characterize configuration space wave packets. For interferometers with a large momentum transfer and large pulse separation time, we improve the contrast by up to 350% and suppress systematic effects. We also reach what is to our knowledge the largest space-time area enclosed in any atom interferometer to date. We discuss implications for future high-performance instruments.

A Clock Directly Linking Time to a Particle's Mass

Linking Mass and Time The precision of atomic clocks is based on the transitions between two well-defined energy levels—the frequency of oscillation. We know from relativity that mass and energy are equivalent and from quantum mechanics that energy relates to frequency. Therefore, the ticking of a clock can be related, in principle, to the mass of a particle. The oscillation frequency of a particle is known as its Compton frequency and, because of the high frequency involved and stability of atoms, it has been argued that a clock linking mass and time would offer very high precision. Ordinarily, the Compton frequency is extremely high and not accessible to direct excitation. Lan et al. (p. 554, published online 10 January; see the Perspective by Debs et al.) demonstrate the operation of a Compton clock exploiting a related parameter, the phase accumulation rate of cold cesium atoms. Using an atom interferometer and an optical frequency comb to bring the Compton frequency into an experimentally accessible regime, mass and time could be directly linked. A clock is demonstrated wherein the ticks are related to the mass of a cesium atom. [Also see Perspective by Debs et al.] Historically, time measurements have been based on oscillation frequencies in systems of particles, from the motion of celestial bodies to atomic transitions. Relativity and quantum mechanics show that even a single particle of mass m determines a Compton frequency ω0 = mc2/ℏ, where c is the speed of light and ℏ is Plancks constant h divided by 2π. A clock referenced to ω0 would enable high-precision mass measurements and a fundamental definition of the second. We demonstrate such a clock using an optical frequency comb to self-reference a Ramsey-Bordé atom interferometer and synchronize an oscillator at a subharmonic of ω0. This directly demonstrates the connection between time and mass. It allows measurement of microscopic masses with 4 × 10−9 accuracy in the proposed revision to SI units. Together with the Avogadro project, it yields calibrated kilograms.

High-Resolution Atom Interferometers with Suppressed Diffraction Phases.

We experimentally and theoretically study the diffraction phase of large-momentum transfer beam splitters in atom interferometers based on Bragg diffraction. We null the diffraction phase and increase the sensitivity of the interferometer by combining Bragg diffraction with Bloch oscillations. We demonstrate agreement between experiment and theory, and a 1500-fold reduction of the diffraction phase, limited by measurement noise. In addition to reduced systematic effects, our interferometer has high contrast with up to 4.4×10(6) radians of phase difference, and a resolution in the fine structure constant of δα/α=0.25  ppb in 25 h of integration time.

Large Fizeau's light-dragging effect in a moving electromagnetically induced transparent medium.

As one of the most influential experiments on the development of modern macroscopic theory from Newtonian mechanics to Einsteins special theory of relativity, the phenomenon of light dragging in a moving medium has been discussed and observed extensively in different types of systems. To have a significant dragging effect, the long duration of light travelling in the medium is preferred. Here we demonstrate a light-dragging experiment in an electromagnetically induced transparent cold atomic ensemble and enhance the dragging effect by at least three orders of magnitude compared with the previous experiments. With a large enhancement of the dragging effect, we realize an atom-based velocimeter that has a sensitivity two orders of magnitude higher than the velocity width of the atomic medium used. Such a demonstration could pave the way for motional sensing using the collective state of atoms in a room temperature vapour cell or solid state material.

An atomic velocity sensor based on the light-dragging effect

A velocity sensor (or velocimeter) is a device used to measure the rate of change of a moving object’s position. Such devices (which have important applications in, e.g., navigation and manufacturing) are typically based on measuring the first-order Doppler shift of electromagnetic waves that are reflecting or scattering off of a moving object. In the quantum regime, the velocity measurements of particles are important for studying fundamental physics. As an example, when a photon is absorbed by an atom, the atom will gain a recoil energy, or recoil velocity. By measuring this recoil velocity from the spectral shift of the atomic resonance, the fine-structure constant can be determined and the theory of quantum electrodynamics tested.1 Another example of its usefulness is in the measurement of the local gravitational acceleration of two different species of free-falling atoms (to test Einstein’s equivalence principle).1 All atom-based sensors rely on measuring the first-order Doppler shift of the atomic transition. By using Dopplersensitive methods to detect the population of atomic states, the velocity can be measured precisely. However, due to the thermal distribution of an atomic ensemble, the uncertainty of the measurement is limited by the Doppler width of the ensemble. Thus, to determine its center-of-mass motion, one usually needs to map or truncate the velocity distribution of the ensemble. This approach complicates the process and lowers the data rate.1 In our experiment, we demonstrate the light-dragging effect (i.e., the deviation of the phase velocity of an electromagnetic wave from the speed of light in a moving medium) and use it to directly sense the center-of-mass motion of an atomic ensemble. The light-dragging effect was first observed by Fizeau in a flowing-water experiment for the study of ether, before the era of Einstein’s special theory of relativity. It was later explained by the Lorentz addition to the first order of velocity in the equation related to Einstein’s theory.2 The effect (illustrated in Figure 1) Figure 1. Illustration of the light-dragging effect in a moving medium. The phase velocity (Vp) of light is modified by an additional term, Fd V (where Fd is the dragging coefficient and V is the velocity of the moving medium). The dragged light has a phase shift of ̊ compared to a reference light. c: The speed of light in a vacuum.