Researchers from the Vienna University of Technology and the Free University of Berlin have, for the first time, experimentally observed how a quantum system “forgets” information, confirming a fundamental principle of physics: erasing information requires energy and increases entropy. The study, published in Nature Physics, marks an important step in understanding quantum mechanics and opens new prospects for quantum technologies.

What Did the Scientists Discover?

In 1961, physicist Rolf Landauer proposed that erasing information—such as deleting a bit from memory—is always accompanied by heat emission and an increase in entropy (a measure of disorder). This Landauer Principle remained theoretical for decades, as testing it in complex quantum systems was technically impossible. Now, an international team has experimentally confirmed it in a multi-particle quantum system for the first time.

The experiment demonstrated that:

  • Information loss in a quantum system is associated with energy costs and irreversible entropy growth.
  • When the system interacts with the environment (such as a measuring device), information becomes inaccessible, and the process becomes irreversible.
    “Erasing information is a physical process with a real cost,” said Professor Jörg Schmiedmayer of the Vienna University of Technology.

How Was the Experiment Conducted?

The team used a platform of ultracold atoms to observe a quantum system:

  1. System Preparation:
    • Thousands of rubidium atoms were cooled to near absolute zero (−273.15°C).
    • The atoms were held in optical traps on an atom chip—a device that creates magnetic and electric fields to manipulate atoms.
  2. Creating Interaction:
    • Two atom clouds were released from traps and collided, forming a quantum system with controlled information exchange.
    • The system was conditionally divided into two parts: the quantum system (one cloud) and the environment (the other cloud).
  3. Measurements:
    • Researchers measured the degree of quantum entanglement and correlations between the clouds using interferometry.
    • They tracked how information "leaked" into the environment, increasing the system's entropy.
    • One-dimensional Gross–Pitaevskii equations were used to model the behavior of the Bose–Einstein condensate.

The results showed that when interacting with the environment, information is lost irreversibly, and entropy increases—confirming Landauer’s Principle.

Why Is This Important?

The discovery has both fundamental and practical implications:

  1. Understanding Quantum Mechanics:
    • The experiment explains how measurement in a quantum system (such as observing atoms) leads to information loss, making the process irreversible.
    • It confirms the relationship between information, energy, and entropy proposed by Landauer.
  2. Quantum Technologies:
    • The results could help optimize quantum computers, where information loss (decoherence) limits computational performance. Understanding the energy cost of erasing a bit will help design more efficient quantum processors.
    • The ultracold atom platform may become a standard for testing quantum algorithms.
  3. Information Thermodynamics:
    • The study strengthens the connection between information physics and classical thermodynamics, paving the way for new theories about the nature of the universe.
    • “Information is a physical quantity, and its loss has measurable consequences,” said theorist Jens Eisert of the Free University of Berlin.
  4. Future Experiments:
    • The platform allows for the study of other quantum phenomena such as quantum chaos or entanglement dynamics with unprecedented precision.

Technical Details

  • System: A Bose–Einstein condensate of ~10,000 rubidium-87 atoms, cooled to ~50 nanokelvin.
  • Measurements: Cameras with up to 1 µm resolution were used to track atom trajectories and their correlations.
  • Entropy: The growth of von Neumann entropy was measured through quantum correlations between subsystems.
  • Energy: Heat release (~10⁻²³ J per bit) was confirmed by analyzing the kinetic energy of atoms.

The measurement error was less than 5%, a record for multi-particle quantum systems.

Limitations and Prospects

  • Scalability: The experiment was limited to a one-dimensional system with 10³–10⁴ atoms. More complex models are needed for real quantum computers with millions of qubits.
  • Temperature: Ultracold atoms require complex setups, which limits the technology’s accessibility.
  • Future Research:
    • Scientists plan to explore how information is lost in systems with quantum turbulence.
    • Integration with optical resonators could allow real-time observation of information loss.

Conclusion

Researchers from Vienna and Berlin have, for the first time, observed how a quantum system loses information, experimentally confirming Landauer’s Principle using ultracold rubidium atoms. The study, published in Nature Physics, proves that erasing information requires energy and increases entropy—even in complex quantum systems. This discovery deepens our understanding of quantum mechanics and promises improvements in quantum computing by reducing energy costs. The ultracold atom platform opens new horizons for exploring the nature of information, energy, and the universe. Stay tuned for more breakthroughs in quantum physics—we’re on the brink of revolutionary discoveries.