# Research Highlights

**Published:**

In the quiet halls of the Duane Physics building at the University of Colorado Boulder, two JILA researchers, postdoctoral research associate Catie LeDesma and graduate student Kendall Mehling, combine machine learning with atom interferometry to create the next generation of quantum sensors. Because these quantum sensors can be applied to various fields, from satellite navigation to measuring Earth’s composition, any advancement has major implications for numerous industries.

As reported in a recent article preprint, the researchers successfully demonstrated how to build a quantum sensor using atoms moving through crystals made entirely of laser light. They applied accelerated forces to atoms along multiple directions and, using this sensor, measured the results, which closely matched values predicted by quantum theory. LeDesma and Mehling also showed that their device could accurately detect accelerations from just one run of their experiment, a feat that is very difficult to accomplish with traditional cold atom interferometry.

**PI:**Murray Holland

**Published:**

Precisely measuring the energy states of individual atoms has been a historical challenge for physicists due to atomic recoil. When an atom interacts with a photon, the atom “recoils” in the opposite direction, making it difficult to measure the position and momentum of the atom precisely. This recoil can have big implications for quantum sensing, which detects minute changes in parameters, for example, using changes in gravitational waves to determine the shape of the Earth or even detect dark matter.

In a new paper published in the *Science,* JILA and NIST Fellows Ana Maria Rey and James Thompson, JILA Fellow Murray Holland, and their teams proposed a way to overcome this atomic recoil by demonstrating a new type of atomic interaction called momentum-exchange interaction, where atoms exchanged their momentums by exchanging corresponding photons.

Using a cavity—an enclosed space composed of mirrors—the researchers observed that the atomic recoil was dampened by atoms exchanging energy states within the confined space. This process created a collective absorption of energy and dispersed the recoil among the entire population of particles.

**PI:**Ana Maria Rey |

**PI:**James Thompson |

**PI:**Murray Holland

**Published:**

Quantum sensors help physicists understand the world better by measuring time passage, gravity fluctuations, and other effects at the tiniest scales. For example, one quantum sensor, the LIGO gravitational wave detector, uses quantum entanglement (or the interdependence of quantum states between particles) within a laser beam to detect distance changes in gravitational waves up to one thousand times smaller than the width of a proton!

LIGO isn’t the only quantum sensor harnessing the power of quantum entanglement. This is because entangled particles are generally more sensitive to specific parameters, giving more accurate measurements.

While researchers can generate entanglement between particles, the entanglement may only be useful sometimes for sensing something of interest. To measure the “usefulness” of quantum entanglement for quantum sensing, physicists calculate a mathematical value, known as the Quantum Fisher Information (QFI), for their system. However, physicists have found that the more quantum states in the system, the harder it becomes to determine which QFI to calculate for each state.

To overcome this challenge, JILA Fellow Murray Holland and his research team proposed an algorithm that uses the Quantum Fisher Information Matrix (QFIM), a set of mathematical values that can determine the usefulness of entangled states in a complicated system.

Their results, published in *Physical Review Letters* as an Editor’s Suggestion, could offer significant benefits in developing the next generation of quantum sensors by acting as a type of “shortcut” to find the best measurements without needing a complicated model.

**PI:**Murray Holland

**Published:**

Atomic clocks have been heavily studied by physicists for decades. The way these clocks work is by having atoms, such as rubidium or cesium, that are "ticking" (that is, oscillating) between two quantum states. As such, atomic clocks are extremely precise, but can be fragile to shaking or other perturbations, like temperature fluctuations. Additionally, these clocks need a special laser to probe the clock. Both factors can make atomic clocks imprecise, difficult to study, and expensive to make.

A team of physicists are proposing a new type of laser that could change the future path of atomic clocks. In this team, JILA Fellow Murray Holland and Research Associate Simon Jäger theorized a new type of laser system in a paper recently published in *Physical Review Letters. *

**PI:**Murray Holland

**Published:**

A large fraction of JILA research relies on laser cooling of atoms, ions and molecules for applications as diverse as world-leading atomic clocks, human-controlled chemistry, quantum information, new forms of ultracold matter and the search for new details of the origins of the universe. JILAns use laser cooling every day in their research, and have mastered arcane details of the process.

**PI:**James Thompson |

**PI:**Murray Holland

**Published:**

The Kapteyn/Murnane group has measured how long it takes an electron born into an excited state inside a piece of nickel to escape from its birthplace. The electron’s escape is related to the structure of the metal. The escape is the fastest material process that has been measured before in the laboratory––on a time scale of a few hundred attoseconds, or 10^{-18} s. This groundbreaking experiment was reported online in *Science*on June 2, 2016. Also in *Science* on July 1, 2016, Uwe Bovensiepen and Manuel Ligges offered important insights into the unusual significance of this work.

**PI:**Henry Kapteyn |

**PI:**Margaret Murnane |

**PI:**Murray Holland

**Published:**

Move over, single-atom laser cooling! The Holland theory group has just come up with a stunning idea for a new kind of laser cooling for use with ensembles of atoms that all “talk” to each other. In other words, the theory looks at laser cooling not from the perspective of cooling a single atom, but rather from the perspective of many atoms working together to rapidly cool themselves to a miniscule fraction of a degree above absolute zero.

**PI:**John (Jinx) Cooper |

**PI:**Murray Holland

**Published:**

Dynamical phase transitions in the quantum world are wildly noisy and chaotic. They don’t look anything like the phase transitions we observe in our everyday world. In Colorado, we see phase transitions caused by temperature changes all the time: snow banks melting in the spring, water boiling on the stove, slick spots on the sidewalk after the first freeze. Quantum phase transitions happen, too, but not because of temperature changes. Instead, they occur as a kind of quantum “metamorphosis” when a system at zero temperature shifts between completely distinct forms.

**PI:**James Thompson |

**PI:**Murray Holland

**Published:**

There’s exciting news from JILA’s ultracold molecule collaboration. The Jin, Ye, Holland, and Rey groups have come up with new theory (verified by experiment) that explains the suppression of chemical reactions between potassium-rubidium (KRb) molecules in the KRb quantum simulator. The main reason the molecules do not collide and react is continuous measurement of molecule loss from the simulator.

**PI:**Ana Maria Rey |

**PI:**Deborah Jin |

**PI:**Jun Ye |

**PI:**Murray Holland

**Published:**

The quantum world is not quite as mysterious as we thought it was. It turns out that there are highways into understanding this strange universe. And, graduate students Minghui Xu and David Tieri with Fellow Murray Holland have just discovered one such superhighway that has been around since the 1950s. Traveling along this superhighway has made it possible to understand the quantum behavior of hundreds of atoms inside every laser used in JILA, including the superradiant laser in the Thompson lab that works entirely differently from all the others.

**PI:**Murray Holland

**Published:**

To be the best they can be, optical atomic clocks need better clock lasers — lasers that remain phase coherent a hundred times longer than the very best conventional lasers. For instance, light from the clock laser in Fellow Jun Ye’s lab can travel around the Earth 10 times before it loses coherence. However, realizing the potential of the lab’s optical clock requires that the laser light remain coherent for 1000 trips around the Earth. The brute force solution to this problem would be to operate the clock laser at 4 K. This approach would increase the cost, complexity, and size of the optical clock as well as rendering it impractical for space exploration and travel.

**PI:**Murray Holland |

**PI:**Jun Ye