Quantum mechanics is a fascinating realm filled with bizarre phenomena that challenge our understanding of the universe. One of the most peculiar aspects of this field is the role of measurement in the theory. When a measurement is made in a quantum system, it often disrupts the delicate “quantumness” of the system, serving as a mysterious bridge between the quantum and classical worlds.
In the realm of quantum information, specifically in large systems composed of quantum bits or “qubits,” measurements can have profound effects on the behavior and structure of the system. The interaction between qubits results in the creation of shared information through entanglement. However, the act of measuring the system disrupts this entanglement. The interplay between measurements and interactions gives rise to two distinct phases: one dominated by interactions with widespread entanglement, and another dominated by measurements with suppressed entanglement.
In a groundbreaking study by researchers from Google Quantum AI and Stanford University, published in the journal Nature, the crossover between these two phases, known as a “measurement-induced phase transition,” was observed in a system containing up to 70 qubits. This represents the largest system to date where the effects of measurements on quantum information have been explored.
Among the notable findings of the study was the discovery of a novel form of quantum teleportation that arose as a result of these measurements. Quantum teleportation involves transferring an unknown quantum state from one set of qubits to another. Through careful measurements, the researchers were able to observe signatures of this teleportation phenomenon, which has implications for the development of techniques relevant to quantum computing.
Visualizing the entanglement in a qubit system is a considerable challenge. The intricate web of connections between qubits is not directly observable. Instead, researchers must infer the existence of the entanglement by analyzing statistical correlations observed in measurement outcomes. Previous experiments tackling measurement-induced phase transitions faced limitations due to the need for numerous repetitive runs of the same experiment to unveil the entanglement pattern.
To overcome these challenges, the research team employed innovative experimental techniques. They rearranged the order of operations, allowing for all measurements to be conducted at the conclusion of the experiment. This streamlining reduced the complexity of the experiment. Additionally, they devised a new method of measuring specific features of the entanglement web using a single “probe” qubit. This enabled researchers to gather insights about the entanglement with fewer experiment runs than previously required.
Surprisingly, the susceptibility of the probe qubit to environmental noise, which is typically undesirable in quantum calculations, proved to be advantageous in this study. The researchers noted that the probe’s sensitivity to noise depended on the nature of the entanglement web surrounding it. By leveraging this correlation, they could infer the entanglement properties of the entire system.
By comparing noise sensitivity in different entanglement regimes, the researchers observed distinct behaviors. In the “disentangling phase,” where measurements dominated over interactions, the entanglement web remained relatively limited, with the probe qubit only being sensitive to the noise from its nearest qubits. In contrast, in the “entangling phase,” characterized by weaker measurements and widespread entanglement, the probe’s noise sensitivity extended throughout the entire system. The transition between these two contrasting behaviors served as a clear indicator of the measurement-induced phase transition.
In addition to shedding light on the nature of measurements in quantum systems, the experiments also revealed the potential of measurement-induced entanglement across long distances. By measuring all but two distant qubits in a weakly entangled state, the researchers generated stronger entanglement between those two distant qubits, effectively achieving quantum teleportation.
These findings have significant implications for the stability and robustness of entanglement in quantum computing. Understanding the role of measurements in driving new phases and physical phenomena is of fundamental interest to physicists. Stanford professor and study co-author, Vedika Khemani, emphasizes the vast potential for further exploration in this field, stating that incorporating measurements into dynamics opens up new avenues for studying non-equilibrium phases.
As quantum science continues to evolve, the transformative power of measurements holds great promise for the future of quantum information and computing.