Violation (and Restoration) of Einstein’s Equivalence Principle by the Hawking/Unruh effect

LC13069

- Einstein’s Equivalence Principle, the principle which forms the conceptual basis of General Relativity, says that an observer can't (locally) distinguish between acceleration or being in a gravitational field. Einstein imagined comparing an observer in an accelerating elevator with an observer in the same elevator at rest on the surface of the Earth; in each case the observer feels an identical downward force. In our work we’ve found that an observer confined to such an elevator can distinguish between the two cases by using two related quantum phenomena: the Hawking effect and the Unruh effect.

Due to quantum fluctuations of fields an observer in the gravitational field of a black hole will measure a temperature – the Hawking temperature. Similarly an accelerating observer will measure a temperature – the Unruh temperature. We have shown that if the two different observers measure the same local acceleration they will measure different temperatures – the observer in the gravitational field will measure a higher temperature.

Surprisingly in regions of stronger gravitational field the two temperatures approach the same value. Thus as the gravitational field becomes stronger the Equivalence Principle is restored, hinting that gravity and quantum mechanics are more compatible (not less) for strong gravitational fields.

Imaging atomic interactions

LE12909

- We have developed an atom-imaging technique that allows us to detect the
positions of individual Rydberg atoms, which are atoms with a highly
excited outer electron. Using this technique, we provide the first
spatially resolved images that demonstrate the “Rydberg blockade.” This
effect is at the core of proposals for a quantum computer architecture
based on neutral atoms

A Rydberg atom has such a tenuous grasp on its excited electron that the
atom is extremely sensitive to external electric and magnetic fields,
and interacts very strongly with other Rydberg atoms. The interaction
between Rydberg atoms is so strong that a Rydberg atom can “block” the
laser-excitation of another Rydberg atom by shifting the energy levels
of the second atom out of resonance with the laser. This is termed the
Rydberg blockade effect. This process leads to quantum entanglement,
which can be used in quantum computation algorithms.

A second Rydberg atom can only be excited if it is farther than a
“blockade radius” from the first atom. We directly measured this
blockade by laser-exciting Rydberg atoms in a cold atomic vapor and
measuring the Rydberg atom positions. We observe a blockade radius of
about 10 microns, which is about 100,000 times larger than the radius of
a ground state atom.

Information could escape from black holes after all

LB12889

- New research suggests that information could escape from black holes after all. The research, which appears in the latest issue of Physical Review Letters, uses the basic tenets of quantum mechanics to give a new description of information leaking from a black hole.

Our results didn’t need the details of a black hole’s curved space-time geometry. That lends support to recent proposals that space, time and even gravity itself may be emergent properties within a deeper theory. Our work subtly changes those proposals, by identifying quantum information theory as the likely candidate for the source of an emergent theory of gravity. The results actually extend the predictions made by standard techniques that rely on a detailed knowledge of space time and black hole geometry.

We cannot claim to have proven that escape from a black hole is truly possible, but that is the most straight-forward interpretation of our results. Our results suggest that quantum
information theory will play a key role in a future theory combining quantum mechanics and gravity.

ELECTROMAGNETIC WAVE PROPAGATION EXCEEDING THE SPEED OF LIGHT

LX12496B

- This work demonstrates theoretically and experimentally that the peak of a pulse can emerge from a metallic plate perforated with tiny holes and sandwiched between dielectrics before the pulse enters the sandwiched subwavelength-hole array. The effect known as superluminality arises because of the coupling interference between two states: one coming up from a mode supported by the grounded dielectric slab, and the other emerging from the periodic pattern. While being counterintuitive, this phenomenon is physical and does not violate the causality principle, which is defined with respect the pulse turn-on time occurring before the peak time. In contrast previous works, the presented superluminal mechanism is scalable to any wavelength regime from radio frequencies to visible light, it uses a simple structure without any special (e.g. gain or absorptive) materials, and it occurs for substantial values of the transmittance. This
work expands our fundamental understanding of periodic gratings and may find applications in pulse re-shaping, tunable delay element with canceled group velocity dispersion, and beam spectral analysis.