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Frequency Comb Measures Quantum Interference

• Physics 15, 161

A multiwavelength laser supply often known as a frequency comb supplies a brand new method for atom interferometry, probably resulting in new assessments of basic physics.

C. Solaro/Sorbonne College

Interfering with atoms. Two frequency-comb laser beams, touring in reverse instructions, are used to separate and recombine wave packets of chilly rubidium atoms as they fall beneath gravity (left). Due to the totally different trajectories the wave packets comply with, they intervene once they recombine. When the researchers fluctuate the trajectories (additionally utilizing the frequency combs), the interference generates a set of oscillations (proper), or “fringes,” within the depth of sunshine emitted by the atoms, from which the acceleration attributable to gravity may be deduced with excessive precision.Interfering with atoms. Two frequency-comb laser beams, touring in reverse instructions, are used to separate and recombine wave packets of chilly rubidium atoms as they fall beneath gravity (left). Due to the totally different trajectories the wave packets fol… Present extra

In atom interferometry, researchers use the interference of quantum waves of matter, typically for high-precision experiments testing basic physics ideas. A analysis workforce has now demonstrated a brand new approach to produce matter-wave interference through the use of a frequency-comb laser—a comb-like set of spectral strains at recurrently spaced frequencies [1]. The comb allowed the workforce to generate interference in a cloud of chilly atoms. The strategy may finally be used to analyze variations between matter and antimatter.

Based on the weak equivalence precept, gravity should trigger each matter and antimatter to fall on the similar charge (see the graphical clarification, The Equivalence Precept beneath a MICROSCOPE). Deviations from this precept might level to explanations for the hitherto mysterious imbalance within the quantities of matter and antimatter within the Universe. Atom interferometry might present a take a look at of weak equivalence via exact measurements of the free fall of antihydrogen. Up to now, light-based management of atom interferometry has used continuous-wave (cw) lasers [2], which may’t simply be prolonged to the brief wavelengths within the excessive ultraviolet (XUV) which might be wanted for such research of antihydrogen.

Frequency combs supply a means round that impediment as a result of they’ll probably produce exactly tuned XUV gentle. They’re usually produced by a practice of ultrashort laser pulses that gives a spectrum of equally spaced laser frequencies (spectral strains) abruptly. Cyrille Solaro of Sorbonne College in Paris and his co-workers have now proven {that a} frequency comb can be utilized to generate interference inside a cloud of free-falling chilly rubidium atoms.

Within the experiment, the frequency comb creates a quantum superposition, that means that the atoms occupy two totally different states concurrently. The comb prepares every state as a “wave packet”—a brief burst of the respective particle waves—after which measures the interference of the 2 wave packets as they comply with totally different trajectories and are available again collectively.

The researchers used two comb beams with barely shifted frequencies propagating vertically in reverse instructions via the cloud to induce a so-called stimulated Raman transition. On this course of, an atom absorbs a photon from one beam and is then stimulated to decay by a photon of barely decrease frequency from the opposite beam, leaving the atom in a goal excited digital state. With cautious tuning of the experimental parameters, the researchers might place the atoms in a 50/50 superposition of the bottom state and the excited state. (Using a frequency comb to drive these Raman transitions in rubidium was demonstrated beforehand [3, 4].)

Crucially, the bottom state and the goal excited state have totally different kinetic energies as a result of the photons absorbed and emitted within the Raman transitions ship momentum kicks. These kicks would ship the wave packet of an excited atom on a free-fall trajectory totally different from that of a ground-state atom. Atoms within the superposition thus comply with each trajectories, which then come again collectively and intervene.

Solaro and colleagues measured interference between the bottom and excited wave packets by monitoring the resultant oscillations within the atoms’ gentle emission because the workforce different the frequency distinction between the 2 combs. From these oscillations, they decided the gravitational acceleration g with a precision of 1 half in 105. In a matter-antimatter comparability, one would examine the values of g decided for the 2 particle varieties, and a discrepancy would sign a violation of weak equivalence.

The precision the researchers achieved is inferior to may be obtained from atom interferometry excited by cw lasers. However cw lasers can’t be used at XUV wavelengths, for which frequency combs are actually being developed [5]. The researchers write that the precision demonstrated of their measurement, if it had been reproduced for an interferometric experiment utilizing antihydrogen, “would result in a stringent take a look at of the weak equivalence precept with antimatter.”

The brand new method “might open the door for introducing a extra numerous group of atoms with a bigger vary of vitality scales to precision measurement inside an atom interferometer,” says atomic physicist Jun Ye of the Nationwide Institute of Requirements and Know-how in Colorado. Though the frequency combs within the XUV area wanted for interferometric research of antihydrogen should not but absolutely developed, optical physicist Minhaeng Cho of Korea College in Seoul says that efforts to provide them are at the moment “vigorous.” Given the proof of precept established by the brand new outcomes, he thinks that such an interferometric experiment may be very more likely to be achievable.

–Philip Ball

Philip Ball is a contract science author in London. His newest ebook is The Trendy Myths (College of Chicago Press, 2021).


  1. C. Solaro et al., “Atom interferometer pushed by a picosecond frequency comb,” Phys. Rev. Lett. 129, 173204 (2022).
  2. M. Kasevich and S. Chu, “Atomic interferometry utilizing stimulated Raman transitions,” Phys. Rev. Lett. 67 (1991).
  3. Y. Fukuda et al., “Synchronized quantum beat spectroscopy utilizing periodic impression excitations with CW mode-locked laser pulses,” Choose. Commun. 38 (1981).
  4. C. Solaro et al., “Direct frequency-comb-driven Raman transitions within the terahertz vary,” Phys. Rev. Lett. 120 (2018).
  5. G. Porat et al., “Section-matched extreme-ultraviolet frequency-comb technology,” Nat. Photonics 12 (2018).

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