This science article describes a quantum optics breakthrough that distinguishes source radiation from vacuum fluctuations. Due to their unique causes, the researchers employed ultrafast laser pulses and nonlinear crystals to isolate these two phenomena, which were previously thought to be difficult to isolate in a lab. The scientists used phase-sensitive detection to show that certain radiation types correlate to certain light pulse quadratures, proving the fluctuation-dissipation theorem. These findings enable experiments on curved-space analogues, complex quantum processes, and entanglement harvesting. By making a long-standing thought experiment quantitative, we learn more about electromagnetic field-matter interactions.

Scientists Separate Quantum Coin Sides

Scientists at ETH Zürich have successfully detected and separated source radiation and vacuum fluctuations, a groundbreaking experiment in physics. In Nature Communications, the time-domain fluctuation-dissipation theorem is empirically proved at the quantum level for the first time, solving a century-old theoretical problem. A new doorway into sub-microscopic quantum electrodynamics was opened by the group using nonlinear crystals and ultrafast optics. This affects our understanding of black holes, the early cosmos, and quantum information.

The Empty Vacuum Mystery

To grasp this discovery's significance, define “empty” space. A vacuum is nothing in classical physics. Even at absolute zero, electromagnetic fields vary, and the vacuum is a sea of activity, according to quantum theory. Casimir effect, an attractive attraction between two uncharged metal surfaces, and Lamb shift, a minute energy change in an atom, are two well-known vacuum fluctuations-induced scientific phenomena.

Source radiation, or radiation response, happens with these changes. When an atom interacts with its radiative field, it "feels" its presence in the quantum field. Physicists have struggled to distinguish these effects for decades. The "two sides of the same quantum mechanical coin" were scientifically independent but empirically intertwined since they generally occur simultaneously and produce similar outcomes.

Relaunching Fermi's Thought Experiment

Fermi's 1932 Gedankenexperiment (thinking experiment) on the two-atom problem laid the groundwork for this finding. Fermi spotted two atoms in a vacuum—what if one “switched on” at any time?

Quantum theory describes two atom correlations. Initially, they may instantly interact with the surrounding field's vacuum oscillations over space. Second, a photon from one atom can reach the other at light speed. Despite its substantial discoveries, this thought experiment was impossible with actual atoms because to the difficulty of turning atomic interactions on and off quickly enough to see the time difference.

All-Optical Solution

Jérôme Faist and Alexa Herter led the ETH Zürich team in creating an all-optical duplicate of Fermi's setup to overcome these challenges. Instead of atoms, they used a zinc-telluride (ZnTe) nonlinear crystal and two ultra-short laser pulses.

The experiment was done in a cryostat frozen to 4 Kelvin to reduce thermal noise and ensure that only quantum signals were detected. Researchers were able to precisely “switch” the interaction on and off with pulses of 110 fs (quadrillionths of a second) and see causal effects.

The experiment was excellent because of phase-sensitive detection. Different laser pulse "quadratures" matched source radiation and vacuum fluctuations. They could switch between quarter-wave and half-wave plates in their detecting system to alter sensors to listen to source radiation or vacuum fluctuations separately.

Checking Fundamental Laws

Results stunningly confirmed quantum theory. When sensors were designed to detect vacuum fluctuations, correlations showed up symmetrically around zero-time delay to indicate their non-local, instantaneous nature. When set to source radiation, this radiation must follow causality rules and spread at a specific rate because the signal became asymmetric and only appeared at a certain time.

The results showed that the fluctuation-dissipation theorem may be experimentally validated. This fundamental law of physics states that a system's fluctuations (the vacuum's “noise”) are directly connected with their source radiation-induced dissipation (the “drag”). Source radiation signal was a Hilbert transform, or perfect mathematical reflection, of vacuum fluctuation signal, researchers found.

Black Holes to Quantum Computers: Why It Matters Distinguishing these two forces improves theoretical clarity and opens up new research avenues:

Scientists plan to “mine” the vacuum for quantum entanglement, needed for ultra-secure communication and quantum computers. This experiment shows that we can distinguish “harvested” entanglement from radiation exchange entanglement.

Curved Spacetime Analogies: The experimental setup can replicate an expanding cosmos or black hole event horizon conditions. Researchers could study Hawking radiation in a lab.

Quantum Information: Understanding the sub-picosecond periods during which information travels through the quantum vacuum will help scientists build noise-resistant quantum systems.

Vacuum fluctuations

In empty space, quantum physics causes vacuum fluctuations, tiny, unpredictable energy changes.

A plain explanation

A vacuum is a true empty space in classical physics.