University of Chicago physicists have invented a "quantum flute" that, like
the Pied Piper, can coerce particles of light to move together in a way
that's never been seen before.
Described in two studies published in Physical Review Letters and Nature
Physics, the breakthrough could point the way towards realizing quantum
memories or new forms of error correction in quantum computers, and
observing quantum phenomena that cannot be seen in nature.
Assoc. Prof. David Schuster's lab works on quantum bits—the quantum
equivalent of a computer bit—which tap the strange properties of particles
at the atomic and sub-atomic level to do things that are otherwise
impossible. In this experiment, they were working with particles of light,
known as photons, in the microwave spectrum.
The system they devised consists of a long cavity made in a single block of
metal, designed to trap photons at microwave frequencies. The cavity is made
by drilling offset holes—like holes in a flute.
"Just like in the musical instrument," Schuster said, "you can send one or
several wavelengths of photons across the whole thing, and each wavelength
creates a 'note' that can be used to encode quantum information." The
researchers can then control the interactions of the "notes" using a master
quantum bit, a superconducting electrical circuit.
But their oddest discovery was the way the photons behaved together.
In nature, photons hardly ever interact—they simply pass through each other.
With painstaking preparation, scientists can sometimes prompt two photons to
react to each other's presence.
"Here we do something even weirder," Schuster said. "At first the photons
don't interact at all, but when the total energy in the system reaches a
tipping point, all of a sudden, they're all talking to each other."
To have so many photons "talking" to one another in a lab experiment is
extremely strange, akin to seeing a cat walking on hind legs.
"Normally, most particle interactions are one-on-one—two particles bouncing
or attracting each other," Schuster said. "If you add a third, they're
usually still interacting sequentially with one or the other. But this
system has them all interacting at the same time."
Their experiments only tested up to five "notes" at a time, but the
scientists could eventually imagine running hundreds or thousands of notes
through a single qubit to control them. With an operation as complex as a
quantum computer, engineers want to simplify everywhere they can, Schuster
said: "If you wanted to build a quantum computer with 1,000 bits and you
could control all of them through a single bit, that would be incredibly
valuable."
The researchers are also excited about the behavior itself. No one has
observed anything like these interactions in nature, so the researchers also
hope the discovery can be useful for simulating complex physical phenomena
that can't even be seen here on Earth, including perhaps even some of the
physics of black holes.
Beyond that, the experiments are just fun.
"Normally quantum interactions take place over length and time scales too
small or fast to see. In our system, we can measure single photons in any of
our notes, and watch the effect of the interaction as it happens. It's
really quite neat to 'see' a quantum interaction with your eye," said
UChicago postdoctoral researcher Srivatsan Chakram, the co-first author on
the paper, now an assistant professor at Rutgers University.
Reference:
Srivatsan Chakram et al, Seamless High- Q Microwave Cavities for Multimode
Circuit Quantum Electrodynamics, Physical Review Letters (2021).
DOI: 10.1103/PhysRevLett.127.107701
Srivatsan Chakram et al, Multimode photon blockade, Nature Physics (2022).
DOI: 10.1038/s41567-022-01630-y
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Physics