Towards violations of Local Friendliness with quantum computers

Abstract: Local Friendliness (LF) inequalities follow from seemingly reasonable assumptions about reality: (i) absoluteness of observed events'' (e.g., every observed event happens for all observers) and (ii)local agency'' (e.g., free choices can be made uncorrelated with other events outside their future light cone). Extended Wigner's Friend Scenario (EWFS) thought experiments show that textbook quantum mechanics violates these inequalities. Thus, experimental evidence of these violations would make these two assumptions incompatible. In [Nature Physics 16, 1199 (2020)], the authors experimentally implemented an EWFS, using a photonic qubit to play the role of each of the friends'' and measured violations of LF. One may question whether a photonic qubit is a physical system that counts as anobserver'' and thereby question whether the experiment's outcome is significant. Intending to measure increasingly meaningful violations, we propose using a statistical measure called the branch factor'' to quantify theobserverness'' of the system. We then encode the EWFS as a quantum circuit such that the components of the circuit that define the friend are quantum systems of increasing branch factor. We run this circuit on quantum simulators and hardware devices, observing LF violations as the system sizes scale. As errors in quantum computers reduce the significance of the violations, better quantum computers can produce better violations. Our results extend the state of the art in proof-of-concept experimental violations from branch factor 0.0 to branch factor 16.0. This is an initial result in an experimental program for measuring LF violations at increasingly meaningful branch factors using increasingly more powerful quantum processors and networks. We introduce this program as a fundamental science application for near-term and developing quantum technology.

• The paper introduces a branch factor metric to quantify how observer-like quantum superposition states are, enabling systematic tests of LF inequalities.
• The research encodes the Extended Wigner's Friend Scenario into quantum circuits, achieving potential LF violations with branch factors ranging from 0.0 to 16.0.
• Initial experimental evidence is provided, outlining future directions to advance quantum processor design and challenge classical notions of observed events.

Towards Violations of Local Friendliness with Quantum Computers

The paper presented by Zeng et al. introduces an innovative approach to testing the foundations of quantum mechanics through the violation of Local Friendliness (LF) inequalities using quantum computers. The concept of LF posits that every observed event is absolute for all observers (Absoluteness of Observed Events, AOE), and free choices are uncorrelated with distant events (Local Agency, LA). The Extended Wigner's Friend Scenario (EWFS) suggests that these inequalities, based on seemingly intuitive assumptions, can be violated in the field of quantum mechanics, challenging deeply held views on reality.

Key Contributions

This research proposes a strategy using quantum computers to create experiments increasingly close to human-like observers, thereby testing the LF inequalities' robustness. The primary contributions are as follows:

1. Quantification of Observerness: The paper introduces the "branch factor" metric, representing the complexity of quantum superposition states, especially in the context of distinguishing classical branches. This allows for quantifying how similar a system is to an observer. Higher branch factors imply more observer-like properties.
2. Quantum Circuit Implementation: An encoding of EWFS into a quantum circuit is devised, facilitating the role of observers played by quantum systems of varying branch factors. Through controlled quantum circuits, these setups strive to violate LF inequalities, advancing from a branch factor of 0.0 to 16.0.
3. Initial Experimental Evidence: Small-scale tests using current quantum processors demonstrate potential LF violations, serving as a precursor to more extensive tests as quantum technology advances.
4. Outlook on Quantum Technology Development: The authors present this framework not only as a test of foundational physics but as an application to push forward quantum hardware innovation, proposing a structured progression towards scalable, significant LF violations.

Implications and Future Directions

The implications of demonstrating LF violations are profound; they suggest that reality, as perceived in the macroscopic world, might not hold at the quantum level if validated with increasingly observer-like systems. This challenges both theoretical and philosophical interpretations of quantum mechanics, potentially leading to a reevaluation of concepts like superdeterminism, hidden variables, or separate realities for different observers.

Practically, advancing this line of inquiry sets a benchmark for developing quantum processors and networks. The pursuit of LF violations thus becomes interwoven with practical engineering goals, pushing toward more complex and reliable quantum computations.

For future research, one promising direction lies in creating specific quantum systems capable of validating LF violations at higher branch factors efficiently. Exploring systems beyond GHZ states, like controlled-random unitary states or Dicke states, could yield substantial strides with fewer resources. Moreover, addressing the challenge of validating these complex quantum states classically with efficiency will be critical.

In conclusion, this work opens a novel experimental frontier in quantum mechanics, marrying foundational physics questions with the advancement of quantum computational technology. It lays the groundwork for future experiments that could radically redefine our understanding of reality and the capabilities of quantum systems as we move toward constructing increasingly sophisticated quantum observers.