Manufacturers want precise vacuum measurements

Space is a giant nothing. It’s not a perfect vacuum — as far as astronomers know, the concept only exists in theoretical calculations and Hollywood thrillers. But aside from the rest of the hydrogen atom floating around, it is a vacuum.

This is important because here on Earth much of the modern world silently relies on partial vacuum. The machine-based environments are more than just a place for physicists to conduct fun experiments. They are critical to the manufacture of many electronic components in state-of-the-art phones and computers. But to actually measure a vacuum — and understand how good it will be when manufactured — engineers rely on relatively simple technology left over from the days of old-school vacuum tubes.

[Related: What happens to your body when you die in space?]

Now some teams are working on an upgrade. Recent research has brought a novel technique — one that relies on the coolest new atomic physics (as cool as -459 degrees Fahrenheit) — one step closer to being used as a standardized method.

“This is a new way of measuring vacuum, and I think it’s really revolutionary,” says Kirk Madison, a physicist at the University of British Columbia in Vancouver.

The NIST mass-in-vacuum precision mass comparator. NIST

What’s in a vacuum

It might seem difficult not to quantify anything, but what you’re actually doing is reading the gas pressure in a vacuum — in other words, the force that all the remaining atoms are exerting on the chamber. So measuring vacuum is really about calculating pressures with far more accuracy than your local weather forecaster can provide.

Today engineers can do this with a tool called an ion gauge. It consists of a helical wire from which electrons erupt when inserted into a vacuum chamber; The electrons collide with any gas atoms inside the spiral, turning them into charged ions. The meter then reads the number of ions remaining in the chamber. But to interpret that number, you need to know the composition of the different gases you’re measuring, which isn’t always easy.

Ion gauges are technological cousins ​​of vacuum tubes, the components that powered ancient radios and the colossal computers that filled rooms and science fiction stories before the advent of the silicon transistor. “They are very unreliable,” says Stephen Eckel, a physicist at the National Institute for Standards and Technology (NIST). “They require constant recalibration.”

There are other vacuum gauges, but ion gauges are the best for getting pressure readings down to billionths of pascals (the standard unit for pressure). While this may seem unnecessarily precise, many high-tech manufacturers want to read the nothing as accurately as possible. Some common techniques for manufacturing electronic components and devices, such as lasers and nanoparticles, rely on the fine layering of materials in vacuum chambers. These techniques require pure matter voids to work well.

The purer the voir, the more difficult it is to identify the remaining atoms, making ion gauges even less reliable. This is where frozen atoms come into play.

Playing snooker with atoms

For decades, physicists have taken atoms, pulsed them with a finely tuned laser, and locked them in a magnetic cage to keep them trapped at temperatures just a fraction of a degree above absolute zero. The cold forces atoms that would otherwise be flying around to sit virtually still so physicists can watch how they behave.

In 2009, Madison and other physicists at several British Columbia institutions were observing trapped atoms of cooled rubidium — an element with psychrophilic properties — when a new arrangement dawned on them.

Suppose you place a trap full of ultracold atoms in a vacuum chamber at room temperature. They would face a constant barrage of hotter, more energetic atoms left in the vacuum. Most of the speeding particles would slip through the magnetic trap without notice, but some would collide with the trapped atoms and eject them from the trap.

It’s not a perfect measurement—not all collisions would successfully knock an atom out of the trap. But if you know the “depth” (or temperature) of the trap and a number called the atomic cross-section (essentially a measure of the likelihood of a collision), you can figure out how many atoms are entering the plane fairly quickly. Based on that, you can see the pressure, along with the amount of matter remaining in the vacuum, Madison explains.

Such a method could have some advantages over ion gauges. For one, it would work for all types of gases that exist in vacuum since no chemical reactions are taking place. Mainly because you are making calculations from the behavior of the atoms, nothing needs to be calibrated.

At first, few people in the physics community noticed the breakthrough made by Madison and his collaborators. “Nobody believed that our work would have an impact,” he says. But in the 13 years since, other groups have picked up the technology themselves. In China, the Lanzhou Institute of Physics has started building its own version. So did an agency in the German government.

NIST is the newest test subject on the list. It is the US agency responsible for establishing the country’s official weights and measures, such as the official kilogram (yes, even the US government uses the SI system). For decades, one of NIST’s jobs was calibrating these finicky ion gauges because manufacturers kept sending them in. The British Columbia researchers’ new approach offered an appealing shortcut.

NIST engineer in red polo shirt and glasses tests silver cold atom vacuum chamber
As part of a project testing the method of vacuum measurement using ultracold atoms, NIST scientist Stephen Eckel stands behind a pCAVS unit (silver cube, left of center) connected to a chamber (cylinder, right). C. Suplee/NIST

A new standard for nothing

NIST’s system isn’t exactly like the one Madison’s group developed. For one, the agency uses lithium atoms, which are much smaller and lighter than rubidium. Eckert, who was involved with the NIST project, says those atoms are far less likely to remain trapped after the collision. But it uses the same underlying principles as the original experiment, which reduces the amount of work because it doesn’t need to be calibrated again and again.

“When I go out and build something like this, it better measures the pressure correctly,” says Eckel. “Otherwise it’s not standard.”

NIST has been testing its system for the past two years. To make sure it worked, they built two identical cold-atom devices and ran them in the same vacuum chamber. When they turned on the devices, they were dismayed to find that both provided different readings. As it turned out, the vacuum chamber had developed a leak, allowing atmospheric gases to seep in. “Once we fixed the leak, they agreed,” says Eckel.

Now that their system appears to be working against itself, NIST researchers want to compare the ultra-cooled atoms to ion gauges and other old-fashioned techniques. If these also give the same measurement, engineers could soon be approaching nothing themselves.

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