Falling Dominos and an Army of Schrödinger’s Cats

Schrödinger’s Cat is one of the most famous thought experiments in quantum mechanics. Physicist Erwin Schrödinger’s thought experiment goes like this: suppose you have a cat sealed in a box with a contraption that may or may not go off to release a poison. The only way to confirm the cat’s fate is to open the box. Until the box is opened, the cat is both alive and dead.

Being in a superposition of two states at a time is an intrinsic property of quantum objects like atoms which, for example, can exist in two opposing spin states simultaneously: spin up and spin down. That makes superposition states really useful for physicists.

Quantum mechanics is bound by uncertainty; you can know with great certainty an object’s position but not its momentum, and vice versa. Atoms are also intrinsically fuzzy objects. You cannot know with full certainty which specific direction its spin is pointing. However, when arrays of atoms become entangled—like in a cat state—they can cancel out each other’s quantum noise and become less fuzzy.

“Cat states have been one of the states that reaches this quantum mechanical bound. You cannot be more precise, in principle, than a cat state,” JILA Fellow Ana Maria Rey explained.

But preparing cat states has been extremely difficult, especially with a large number of atoms. NIST’s Dave Wineland gained renown for generating a cat state in six atoms—a record number in 2005.

Mikhail Mamaev, a graduate student in the Rey Theory Group, developed a novel scheme to prepare a large number of atoms in cat states using the strontium clock in the Ye Lab at JILA. Not only can this new method prepare a large number of atoms in a cat state, they can be measured easily. These findings were published in Physical Review Letters on June 16.

“He starts with something that is very classical, and then step by step, he is converting this classical object into a highly entangled object using the exquisite resolution of the clock,” Rey said.

Falling dominoes

Typically, physicists prepared cat states by starting with all the atoms in the ground state—spins pointing down—and slowly start changing the parameters to change the energy of the entire system and create that superposition. But the more atoms you add, the longer that takes and the more difficult that process is, Rey said. And they’re delicate.

“It is harder to entangle many atoms because then you can kill the superposition. If you flip one [atom], it kind of disappears,” Rey said.

What Mamaev’s approach does is covert a local superposition—a local quantum state involving just two atoms—into a many-body cat state in a step-by-step fashion using the laser from the strontium clock as the flipping device, Rey explained. The process starts with a collection of atoms with their spins all pointing down. The atoms sit in an optical lattice, which can be thought of as an egg crate with each atom in an individual well. Starting in the corner of the array, Mamaev can use the clock’s laser to force one atom to tunnel into its neighbor’s well.

It costs the atoms too much energy to share their egg carton well with another atom. However, if the atom is illuminated by the laser, it acquires the sufficient energy to tunnel further, flipping itself as it moves.

“When it tunnels over, it flips itself…It’s in a quantum superposition,” Mamaev said. The cat is now both dead and alive.

They start with two atoms and from there, the rest of the array falls like dominoes. By using the laser as a pointer, the atoms can sequentially be flipped step-by-step. If the first atom was flipped up by the first step, so will the rest of the atoms after the sequence is done. If not, they’ll all stay down in their wells the whole time. Soon, the array is in a cat state—all equally up and down at the same time.

“It’s step by step. You can control which one to move by the precision of the clock or the frequency of the clock,” Rey said.

When all the dominoes have fallen, the atoms are entangled, and all that’s left is to measure that entanglement. Fortunately, there’s still doublons or pairs of atoms at the end of the array—the last domino. Using those leftover doublons, the team can determine if they successfully created a cat state.

“You measure only the corner and you can get the signal,” Rey said. And being able to do this with a single pair of atoms is a boon to the whole process.

Strength in an army of cats

This experiment also has a significant advantage over previous cat state-generating protocols. Unlike most of the prior works on this, this protocol makes not one cat, but an entire legion – an army of cats, coming from the different rows of the lattice. Each cat which can help provide a constructive signal for measurement, which can boost the performance of metrology tools that rely on atomic frequency, like optical atomic clocks.

This research was supported by the National Science Foundation’s Physics Frontier Center grant.

Written by Rebecca Jacobson