Google's latest masterpiece! "Measurement" successfully generates quantum entanglement and invisible transmission states

ICV ꄲ QUANTUM-news ꄲ Google's latest masterpiece! "Measurement" successfully generates quantum entanglement and invisible transmission states

Quantum mechanics is full of strange phenomena, but perhaps none stranger than the role that measurement plays in the theory: since measurement tends to destroy the "quantum nature" of a system, it seems to be the mysterious link between the quantum and classical worlds. And in a large system consisting of quantum bits of information, the effect of measurement induces surprising new behaviors and even drives the emergence of entirely new phases of quantum information.

This happens when two competing effects, interaction and measurement, are present. In quantum systems, when quantum bits interact with each other, their information is shared non-locally in an "entangled state"; but if the system is measured, the entangled state is destroyed. The struggle between measurements and interactions leads to two distinct phases: one in which interactions dominate and entanglement is widespread, and another in which measurements dominate and entanglement is suppressed.

On Oct. 18, Nature reported that researchers at Google Quantum Artificial Intelligence and Stanford University observed a "crossover" between the two states in a system of up to 70 quantum bits - the "measurement-induced phase transition". "measurement-induced phase transition", the largest system to explore measurement-induced effects to date.

The experiment was observed on a 70-qubit system Sycamore quantum processor

The researchers also discovered a new form of quantum invisible transmission state, an unknown indication of the transfer of a quantum state from one set of quantum bits to another; in the experiments, this quantum invisible transmission state was the result of these measurements: studies that could help to inspire new technologies for quantum computing.

Quantum entanglement: an intricate "web"

Researchers at Google Quantum Artificial Intelligence and Stanford University have explored how measurements could fundamentally change the structure of quantum information in space-time

We can think of entanglement in a system of quantum bits as an intricate web of connections. When we measure an entangled system, its effect on the "web" depends on the strength of the measurement: it may destroy the web completely, or it may cut and trim some parts of the web, but leave others intact.

It is difficult to see this entangled web for real in an experiment. The entangled web itself is invisible, so researchers can only infer its existence by seeing statistical correlations between measurements of quantum bits. To infer a pattern of entangled webs, multiple identical experiments are required.

This and other challenges have plagued past experiments and limited research into measuring induced phase transitions to very small system scales. This time, the Google team observed this phase transition by varying the relative strengths between interactions and measurements.

Three New Techniques for Observing "Measurement-Induced Phase Transitions"

To address these challenges, the researchers used a number of experimental techniques. First, they rearranged the order of operations so that all measurements could be taken at the end of the experiment, rather than staggered, thus reducing the complexity of the experiment; second, they developed a new method for measuring certain features of the network with individual "probe" quantum bits. In this way, they can learn more about entangled networks in fewer experiments than before.

Technique 1: Interchange space and time. This conversion turns the one-dimensional spatial circuits that one would otherwise want to study into two-dimensional circuits; moreover, since all measurements are now made at the ends of the circuits, it is possible to adjust the relative strengths of the measurements and the entanglement interactions by varying the number of entanglement operations in the circuits.

Tip 2: Overcome the post-selection bottleneck. The experimental team developed a new "decoding" protocol that compares each instance of an entangled real "net" with the same instance in a classical simulation. This avoids post-selection and is sensitive to features common to all nets.

Finally, "probes", like all quantum bits, are susceptible to unwanted noise in the environment. This is usually seen as a bad thing, as noise can disrupt quantum computation, but the researchers turned this shortcoming into a feature by noting that the probe's sensitivity to noise depends on the nature of the entangled web around it. As a result, they can use the probe's sensitivity to noise to infer the entanglement of the entire system.

What do you see?

The team first investigated the difference in sensitivity to noise in the two entangled states and found very different behaviors: when the measurement dominates the interaction ("unentangled phase"), the strands of the net remain relatively short, and the probe quantum bits are sensitive only to the noise of their nearest quantum bits.

In contrast, when the measurement is weak and entanglement is more prevalent ("entanglement phase"), the probes are sensitive to the noise of the entire system. The "crossover" between these two distinct behaviors is a sign of the sought-after "measurement-induced phase transition".

The team also demonstrated a new form of quantum invisible transmission that arises naturally during the measurement process: by measuring all but two distant quantum bits in a weakly entangled state, a stronger entanglement is created between the two distant quantum bits. Due to the ability to generate measurement-induced distant entanglement, the stealthy transmission state observed in the experiment is realized.

During the entanglement phase, the stability of entanglement to measurement can inspire new schemes that make quantum computation more robust to noise; the role of measurement in driving new phases and physical phenomena is also of fundamental interest to physicists.

Order parameters versus gate density (number of entanglement operations) for different numbers of quantum bits. When the number of entanglement operations is low, measurements play a greater role in limiting entanglement throughout the system. When the number of entanglement operations is high, entanglement is pervasive, which leads to a dependence of the order parameter on the size of the system (inset).

Agent entropy versus gate density for two widely separated subsystems (pink and black quantum bits) when measuring all other quantum bits. There is a finite size crossover at ~0.9. Above this gate density, the probe quantum bit is entangled with a quantum bit on the other side of the system, which characterizes the invisible transmission phase

This experiment demonstrates the effect of measurements on quantum circuits. The experiment shows that by adjusting the intensity of the measurement, the team can induce a shift to a new phase of quantum entanglement within the system, and even produce an emerging form of quantum invisible state transfer.

In response, Vedika Khemani, co-author of the study and professor at Stanford University, commented, "Incorporating measurements into dynamics introduces a whole new arena for many-body physics, where many fascinating new types of nonequilibrium phases can be discovered. We have explored some of these compelling and counter-intuitive measurement-induced phenomena in this work, but there are many more rich phenomena to be discovered in the future."

Reference Links:

[1]https://phys.org/news/2023-10-generate-quantum-entanglement-teleportation.html

[2]https://cns.utexas.edu/news/research/peering-inside-quantum-computer-creates-new-phases-information

[3]https://blog.research.google/2023/10/measurement-induced-entanglement-phase.html