By Sophia Chen
In a lab in Boulder, Colorado, physicist Daniel Slichter plays an excruciatingly tiny version of pinballâ€”with an individual atom as the ball. He and his colleagues at the National Institute of Standards and Technology have built a chip about the size of a grain of rice, which they keep in a small freezer at around âˆ’430 degrees Fahrenheit. The chip, a square of gold-coated sapphire with metal wires bonded to it, holds a single magnesium ion. Confined by an electric force field, the ion hovers 30 microns above the surface of the chip. Outside the freezer, Slichterâ€™s team hits keys and turns knobs to bat the ion around with electric pulses.
Their game, though, is simpler than pinball. All they want to do is locate the ionâ€”to watch the motion of the ball as it jiggles back and forth on the chip.
Itâ€™s much more challenging than it sounds. Slichter is working with an object many thousands of times smaller than a bacterium. His team wants to pin down the location of the moving ion to less than a nanometer, a fraction of the ionâ€™s own diameter. At this level of precision, they inevitably brush against one of natureâ€™s unbreakable rules: Heisenbergâ€™s uncertainty principle.
The uncertainty principle basically says that you cannot measure or describe an object with absolute precision. This imprecision is no fault of the scientist or measurement device. Nature has innate mystery; its smallest building blocks simply are fuzzy and diffuse objects. â€œThe uncertainty principle means that you canâ€™t know everything about a certain system at any given time,â€� says Slichter.
The principle doesnâ€™t matter much in everyday life, because no one needs to bake a cake or even build a car with atomic precision. But itâ€™s a big deal for scientists like Slichter who work on the quantum scale. They want to study particles such as electrons, atoms, and molecules, which often entails cooling them to temperatures near absolute zero so they slow down to a more manageable speed. But nature dooms these scientists, always, to a level of imprecision.
So Slichter can never know his magnesium ion fully. At any particular moment, if he measures one property of the ion well, it comes at the cost of studying some other aspect of the ion. To him, the uncertainty principle is like a mandatory tax you have to pay to nature. â€œI think of it as â€˜Thereâ€™s no free lunch,â€™â€� says Slichter. For example, if he controls the ionâ€™s speed precisely, the particle will actually spread out so that itâ€™s harder for him to pinpoint its position.
But he can try to game the system. In a paper published today in Science, his team describes how to skirt the uncertainty principle to better measure the ionâ€™s position. Their method achieves 50 times more precision than the previous best techniques, which also means that they can make measurements 50 times faster than before. Now they can narrow down the particleâ€™s location to an atom-sized space in less than a second.
The key to their method is to accept the noisiness decreed by the uncertainty principle, and control where it manifests itself. To measure the ionâ€™s position, they basically transfer the uncertainty into its speed, a value they happen to care less about. They call this method â€œsqueezing,â€� because in a way, they â€œsqueezeâ€� uncertainty from one property to another.
To be clear, squeezing doesnâ€™t violate the uncertainty principle. Nothing can. Itâ€™s just that previously, physicists couldnâ€™t negotiate which property of the ion would contain the uncertainty in a particular moment. When the ion is left to its own devices, the fuzziness gets distributed evenly over various properties. With squeezing, â€œyouâ€™re putting the noise where it matters the least,â€� says physicist Nancy Aggarwal of Northwestern University, who was not involved in the experiment. Slichterâ€™s team still has to pay the same tax, but now they can tell nature which account to charge.
As the ion bounces around the chip, they reduce the uncertainty in the ionâ€™s position by periodically hitting it with an electric field. The reason this works is complicated, but roughly speaking, the temporary electric field restricts the ionâ€™s range of motion and corrals the particle into a smaller space. This makes measuring its position easier. â€œWhen the ion moves away from the center [of its trap], this electric field pushes it back,â€� says Slichter. Essentially, they push the ion from the center of the trap to let it jiggle; as it jiggles, they confine the ion briefly to reduce the position uncertainty. Then they release the ion, and repeat.
Bending the uncertainty principle has proven necessary as physicists probe subtler phenomena. For example, in its upgrade this year, the Laser Interferometer Gravitational-Wave Observatory, known as LIGO, has started using squeezing to improve its detection of gravitational waves, says Aggarwal, who helped develop the technique for the collaboration. To detect gravitational waves, LIGO tries to sense changes in length in its two 2.5-mile-long arms. So they beam a laser down each arm to pelt a mirror at the end with photons. If the photons take more or less time to reach the mirror, that could be evidence that space-time has stretched or shrunk, respectively. So LIGO has started using squeezing to more precisely control when the photons leave the laser. But in their Heisenberg tradeoff, they have to sacrifice control over the laserâ€™s brightness and allow a certain amount of flicker.
In addition, physicists studying dark matter also want to use squeezing, says physicist Daniel Allcock of the University of Oregon, one of Slichterâ€™s collaborators. Observations of faraway galaxies hint that an invisible dark matter makes up 85 percent of the universe, but researchers donâ€™t know exactly what the stuff is. Some theories posit that dark matter particles create extremely weak electric fields. These electric fields, if real, would push a magnesium ion ever so slightly, so their chip could be further developed to sense these dark matter particles.
Slichter and Allcock, though, want to use squeezing to engineer quantum technology. They developed their chip as a precursor to a quantum computer processor. A so-called trapped-ion quantum computer would consist of many ions arranged in a grid on a chip like theirs, and one potential scheme of this computer involves encoding information in each ionâ€™s motion. For example, they could define one type of ion wiggle as 1, and a different type of shimmy as 0. Because ions are electrically charged, the motion of one will disturb its neighborâ€™s position. If you can move the ions precisely, you can create a sort of quantum abacus, and squeezing is a fundamental step to monitoring and controlling an individual ionâ€™s motion.
Even if their planned technology doesnâ€™t pan out, Slichter and his team still have bragging rights. Their demonstration inches close to the edges of what nature allows, hinting at an ultimate limit to what human engineering can achieve. â€œWeâ€™re controlling matter with a precision beyond what is normally thought possible,â€� says Slichter. â€œAnd we do it by harnessing the laws of quantum mechanics to our advantage.â€� Physicists can never defy the laws of nature, but theyâ€™re figuring out ways to bend them.
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