The fundamental forces of physics govern the matter comprising the universe,
yet exactly how these forces work together is still not fully understood.
The existence of Hawking radiation—the particle emission from near black
holes—indicates that general relativity and quantum mechanics must
cooperate. But directly observing Hawking radiation from a black hole is
nearly impossible due to the background noise of the universe, so how can
researchers study it to better understand how the forces interact and
integrate into a "Theory of Everything?"

According to Haruna Katayama, a doctoral student in Hiroshima University's
Graduate School of Advanced Science and Engineering, since researchers
cannot observe Hawking radiation, Hawking radiation must be brought to the
researchers. She has proposed a quantum circuit that acts as a black hole
laser, providing a lab-bench black hole equivalent with advantages over
previously proposed versions. The proposal was published on Sept. 27
Scientific Reports.

"In this study, we devised a quantum-circuit laser theory using an analog
black hole and a white hole as a resonator," Katayama said.

A white hole is a theoretical partner of a black hole that emits light and
matter in equal opposition to light and matter a black hole consumes. In the
proposed electric circuit, a metamaterial engineered to allow
faster-than-light motion spans the space between horizons, near which
Hawking radiation is emitted.

"The property of superluminal speed is impossible in a normal medium
established in an ordinary circuit," Katayama said. "The metamaterial
element makes it possible for Hawking radiation to travel back and forth
between horizons, and the Josephson effect—which describes a continuous flow
of current that propagates without voltage—plays an important role in
amplifying the Hawking radiation through the mode conversion at the
horizons, mimicking the behavior between the white and black holes."

Katayama's proposal builds on previously proposed optical black hole lasers
by introducing the metamaterial that allows for superluminal speed and
exploiting the Josephson effect to amplify the Hawking radiation. The
resulting quantum circuit induces a soliton, a localized, self-reinforcing
waveform that maintains speed and shape until external factors collapse the
system.

"Unlike previously proposed black hole lasers, our version has a black
hole/white hole cavity formed within a single soliton, where Hawking
radiation is emitted outside of the soliton so we can evaluate it," Katayama
said.

Hawking radiation is produced as entangled particle pairs, with one inside
and one outside the horizon. According to Katayama, the observable entangled
particle bears the shadow of its partner particle. As such, the quantum
correlation between the two particles can be determined mathematically
without the simultaneous observation of both particles.

"The detection of this entanglement is indispensable for the confirmation of
Hawking radiation," Katayama said.

However, Katayama cautioned, the lab Hawking radiation differs from true
black hole Hawking radiation due to the normal dispersion of light in the
proposed system. The components of light split in one direction, like in a
rainbow. If the components can be controlled so that some can reverse and
bounce back, the resulting lab-made Hawking radiation would mirror the same
positive frequency of true black hole Hawking radiation. She is now
investigating how to integrate anomalous dispersion to achieve a more
comparable result.

"In the future, we would like to develop this system for quantum
communication between distinct spacetimes using Hawking radiation," Katayama
said, noting the system's scalability and controllability as advantages in
developing quantum technologies.

## Reference:

Haruna Katayama, Quantum-circuit black hole lasers, Scientific Reports
(2021).
DOI: 10.1038/s41598-021-98456-0

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