Quantum mechanics is full of weird phenomena, but perhaps none as weird as the role measurement plays in the theory. Since a measurement tends to destroy the “quantumness” of a system, it seems to be the mysterious link between the quantum and classical world. And in a large system of quantum bits of information, known as “qubits,” the effect of measurements can induce dramatically new behavior, even driving the emergence of entirely new phases of quantum information.
This happens when two competing effects come to a head: interactions and measurement. In a quantum system, when the qubits interact with one another, their information becomes shared nonlocally in an “entangled state.” But if you measure the system, the entanglement is destroyed. The battle between measurement and interactions leads to two distinct phases: one where interactions dominate and entanglement is widespread, and one where measurements dominate, and entanglement is suppressed.
A measurement-induced phase transition in a system of 70 qubits
As reported in the journal Nature, researchers at Google Quantum AI and Stanford University have observed the crossover between these two regimes—known as a “measurement-induced phase transition”—in a system of up to 70 qubits. This is by far the largest system in which measurement-induced effects have been explored.
The researchers used a superconducting quantum processor called Sycamore, which consists of 54 qubits arranged in a two-dimensional grid. Each qubit can be controlled individually by applying microwave pulses, and can interact with its nearest neighbors by turning on a coupling device. The qubits can also be measured by applying a readout pulse and detecting the resulting signal.
The researchers prepared an initial state where each qubit was in a superposition of two states, denoted by |0> and |1>. Then they applied a sequence of operations that consisted of three steps: interaction, measurement, and reset. In the interaction step, they turned on the coupling device for a short time, allowing the qubits to exchange information and create entanglement. In the measurement step, they randomly measured some of the qubits by applying a readout pulse and recording the outcome. In the reset step, they applied another pulse to bring the measured qubits back to their initial state.
By repeating this sequence many times, the researchers were able to study how the system evolved under different measurement rates. They found that when the measurement rate was low, the system remained in a phase where entanglement was high and spread across many qubits. But when the measurement rate was high, the system entered a phase where entanglement was low and localized to small clusters of qubits. The transition between these two phases occurred at a critical measurement rate that depended on the strength of the interaction.
A novel form of quantum teleportation induced by measurements
The researchers also saw signatures of a novel form of “quantum teleportation”—in which an unknown quantum state is transferred from one set of qubits to another—that emerges as a result of these measurements. Quantum teleportation is usually achieved by using a pair of entangled qubits as a quantum channel, and performing a joint measurement on one qubit from the pair and another qubit that holds the state to be teleported. The outcome of this measurement determines how to manipulate the other qubit from the pair to recover the teleported state.
However, in this experiment, there was no pre-existing entanglement between any pair of qubits. Instead, the researchers found that when they measured some of the qubits in their system, stronger entanglement was generated between those two distant qubits that were not measured. The ability to generate measurement-induced entanglement across long distances enables the teleportation observed in this experiment.
The researchers demonstrated this phenomenon by preparing an unknown quantum state on one qubit at one corner of their processor, and measuring some of the other qubits in their system. Then they verified that the state was teleported to another qubit at another corner of their processor by comparing their outcomes.
Implications for quantum computing and beyond
These studies could help inspire new techniques useful for quantum computing. For example, measurement-induced entanglement could be used to create long-range connections between distant qubits, which are essential for implementing error correction schemes. Measurement-induced teleportation could also be used to transfer quantum information across different parts of a quantum processor, or even between different processors, without requiring physical wires or optical fibers.
Moreover, these studies could shed light on fundamental questions in quantum physics, such as the nature of quantum measurement, the emergence of classicality, and the role of entanglement in quantum phase transitions. The researchers hope that their work will stimulate further exploration of measurement-induced phenomena in larger and more complex quantum systems.
