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It's not just the same particles that can get entangled, even those with fundamentally different properties can interfere with each other.
This plot shows the connectivity of two entangled particles. At the beginning of 2023, entanglement between different particles (positive and negative mesons) was demonstrated for the first time not only exist, but also can be measured, exploited and used to probe the internal structure of atomic nuclei.
Key takeaways
- One of the most bizarre quantum phenomena discovered so far is quantum entanglement: two particles are in one state, and the properties of one depend on the other.
- If the properties of the process are uncertain, the state of the quantum particle cannot be measured, and whenever it is done it will "break" the entanglement.
- Often observed with the same particles, entanglement has just been demonstrated between particles with opposite charges, and using this property shows us atomic nuclei that have never been seen before.
In the quantum universe, things behave very differently than our usual experience reveals. In the macroscopic world we are familiar with, any object we can measure seems to have intrinsic properties that have nothing to do with whether we observe it or not. We can measure things like mass, position, motion, duration, etc. without worrying about whether the object will be affected by what we measure; Reality exists completely independently of the observer. But in the quantum world, this is clearly not true. The behavior of a measurement system fundamentally changes its properties in an irreversible way.
One of the strangest quantum properties is entanglement: multiple quanta have indeterminate intrinsic properties, but none of the properties of each quantum is independent of the other. We've seen proofs of photons, electrons, and a variety of identical particles before, allowing us to test and explore the fundamental and surprising nature of reality. In fact, the 2022 Nobel Prize in Physics was awarded precisely for the study of this phenomenon.
But quantum entanglement between different particles has been demonstrated for the first time in a novel experiment, and the technique has been used to observe atomic nuclei in unprecedented ways.
Illustration of two entangled particles that are separated in space and each has uncertain properties before being measured. Experiments have established that none of the members of the entangled pair exists in a particular state until the measurement occurs at the critical moment of occurrence: this is a key aspect of enabling many modern quantum technologies.
In principle, quantum entanglement is an easy-to-understand concept that is built on the idea of quantum indeterminism. Imagine that you pull a ball out of a hat and there is a 50/50 chance that the ball has one of two attributes.
- Maybe the color: the ball can be black or white.
- Maybe it's quality: either you pull a light ball, or a heavy ball.
- Maybe it's the direction in which it rotates: the ball may "spin up" or "spin down".
If you only have one ball, you might be thinking: does it always have these characteristics when you pull it out to check the ball, even before you look at it? Or does the ball have an indeterminate set of parameters in which mixed:
- black and white,
- Light and heavy,
- Up and down mixing rotation,
Is that only determined when you make critical measurements?
As evidenced by famous experiments such as the double-slit experiment and the Stern-Gerlach experiment, this is one of the important insights of quantum mechanics. Both are worth explaining.
Results of the "masked" double-slit experiment. Note that when the first slit (P1), the second slit (P2), or the two slits (P12) open, you see a very different pattern depending on whether one or both slits are available.
If you take a barrier with two slits and what happens when you send waves to it? The answer is simple: you'll see a wavy pattern behind an obstacle, with the parts of the wave that pass through each slit interfering with each other, causing a pattern of peaks and troughs on the other side.
Conversely, what happens if you send a series of particles to the barrier? The answer is also simple: you get a particle-like pattern behind the barrier, where the particles either pass through slit #1 or through slit #2, so you only get two piles on the other side.
But in quantum mechanics, when you let quantum particles pass through double slits, if you don't measure which slit each particle goes through, you get a wavy pattern, but if you make a measurement, you get a particle-like pattern. Even if you send quanta one at a time, this is correct, as if they are interfering with themselves. The behavior of observation—making key measurements—and whether you do so determines which pattern you see. As we have observed, reality depends on interactions that occur or do not occur before critical observation.
When a group of particles passes through a single Stern-Gerlach magnet, they deflect based on spin. If you let them go through the second vertical magnet, they will split again in the new direction. If you then use the third magnet to go back to the first direction, they split again, proving that the previously identified information was randomized by the most recent measurement.
Similarly, the Stern-Gerlach experiment stems from passing quantum particles with intrinsic properties called "spin" through a magnetic field, which implies intrinsic angular momentum. These particles align the deflection with or against the field: up or down relative to the direction of the field.
If you try to deflect a particle whose spin has already been determined by such a magnetic field, it will not change: the rising particle will still rise; Those that go down will still go down.
But if you let it pass through a magnetic field in a different direction—in one of the other two spatial dimensions—it splits again: left-right or forward-backward, instead of up and down. What's even weirder now is that once you separate it left and right or back and forth, if you let it pass through an up and down magnetic field again, it will oppose the split at once. It's as if the last measurement you made deleted all previous measurements and any deterministic determination of the quantum state that exists in that dimension.
The entangled pairs of quantum mechanics can be likened to a machine that throws balls of opposite colors in opposite directions. When Bob catches a ball and sees that it's black, he immediately knows that Alice catches a white ball. In a theory that uses hidden variables, the ball always contains hidden information about what color to show. However, according to quantum mechanics, the balls remain gray until someone observes, at which point one randomly turns white and the other randomly turns black. Bell's inequality indicates that there are experiments that can distinguish between these cases. These experiments proved that the description of quantum mechanics was correct.
It's a bit quantum weird, but it has nothing to do with entanglement. Entanglement occurs when you have two or more particles that both exhibit some of this quantum uncertainty, but together in an interconnected way. In entangled quantum systems, the quantum state of one particle is related to the quantum state of another particle. Individually, each quantum state appears (and is measured as) completely random.
But if you put these two quanta together, you'll find a correlation between the combined properties of the two: if you only measure one of them, you can't know that. You can assume
- Either apply standard quantum mechanics,
- Or the state of two particles exists independently of whether they are observed or not,
And two different predictions are derived. Part of the 2022 Nobel Prize in Physics is to prove that when you actually do these experiments and measure two quantum states, you'll find that the correlation only coincides with standard quantum mechanics, and does not correspond to the idea that the states of the two particles exist independently of each other.
The experimentally measured ratio R(φ)/R_0 as a function of the angle φ between the polarizer shafts. The solid line is not a fitting of the data point, but a polarization correlation of quantum mechanical predictions; As it happens, the data coincides with theoretical predictions with astonishing accuracy and cannot be explained by a local true correlation between two photons (which would result in a straight line instead of a curve in the prediction).
It is for this reason that quantum entanglement is often described as creepy and counterintuitive.
However, quantum entanglement experiments usually involve photons: particles into which light, electromagnetic radiation are quantized. These entangled photons are usually produced by passing a single photon through a so-called downconversion crystal, with one photon coming in and two photons coming out. These photons will have all the normal properties of regular photons—including spin, wavelengths defined by their energy, no charge, and all the standard quantum behavior that quantum electrodynamics brings—but will also have properties that correlate between them: correlation that goes beyond quantum predictions for a single isolated particle, and is specific to the set of entangled particles.
For a long time, this was the only way to experiment with entangled quantum particles: to have two particles with the same properties, that is, they are the same kind of quantum particles. But in the first experiment, a new kind of quantum entanglement has just been observed: entanglement between two fundamentally different particles, which even have opposite electric charges!
The STAR detector itself, about the size of a house, is the first detector sensitive enough to measure the entanglement properties of daughter particles produced by relativistic heavy ion "misses" interactions. This early 2023 result demonstrates for the first time entanglement between two different particles.
In particle physics, you can produce new, heavy, unstable particles, as long as you meet all the quantum requirements (i.e., you don't violate any conservation laws) and you also have enough energy (via Einstein's E = mc²) to be available for the particles to be created. From collisions involving protons and/or neutrons (i.e., quark-containing particles), the particles most likely to produce are called mesons, which are quark-antiquark combinations. The lightest mesons involving only upquarks, downquarks, and odd quarks (and antiquarks) are:
- π particles (pion), which can be up-antidown, down-antiup, or neutral (a superposition of up-antiup and down-antidown),
- K particles (mesons), which involve a strange quark (or antiquark) and an upper or lower antiquark (or quark),
- η particles (ETAS), containing a mixture of up-inverse upquarks, down-inverse-down quarks, and singular anti-chirks,
- and ρ particles (RHOS), which, together with ω (omega) particles, consist of up and down quarks and antiquarks, but their spins are aligned rather than anti-homogeneous like other mesons.
They are the only mesons lighter than protons (and neutrons) and are responsible for carrying the nuclear force within the nucleus. They are both short-lived and both decay into lighter particles, but neutral π meson (π 0) particles always decay into two photons, while neutral rho (ρ 0) particles always decay into two positively charged (π +) and negatively charged (π – ) mesons.
Theoretically, the RHO meson can decay into a pair of mesons through either a strong interaction (left) or a weak interaction (right). Due to the relative strength of these interactions and the mass of the W boson, the strong decay channel is the only channel relevant for our experiment.
You might not be surprised that some properties of photons produced by neutral π meson decay can be entangled: photons are the same particle, and the two particles come from the decay of a single quantum particle. But the shocking discovery that two charged mesons produced by the decay of neutral RHO are also entangled with each other, marking the first time that two different, different particles have been found to exhibit entanglement properties. Particles such as mesons and rho can not only be produced from two protons colliding with each other, but also from the gluon field interactions of these two protons to produce sufficiently high energies.
The way entanglement is identified is excellent: when two RHO particles are produced in the nuclei of two adjacent protons, they decay into these two charged mesons almost immediately, respectively. Because they are close together in space, two positively charged (π +) mesons and two negatively charged (π – ) mesons interfere with each other, producing their own superposition and their own wave function.
This schematic shows how rho particles are produced and how they decay, and how this signal appears in the STAR detector in Brookhaven. This experiment is the first to measure a new type of quantum entanglement.
The interference patterns observed between positively and negatively charged mesons are key evidence to reveal the inevitable but strange conclusion: the oppositely charged mesons produced in the decay of each RHO particle—π+ and π—must be entangled with each other.
These observations are possible because the resulting rho particles have surprisingly short lifetimes: the average lifetime is only about 4 seconds, or one-seventh of a second of 4 seconds. Even at the speed of light, these particles decay very quickly compared to the distance between them, making the overlap of the π meson wavelet function very large.
Most importantly, this new form of entanglement has produced immediate applications: measuring the radius and structure of heavy nuclei that collide with each other almost (but not completely) in these experiments. The spin interference pattern that emerged came from the overlap of these two wave functions, allowing the researchers to determine what radius is to describe the interaction of the gluon field of each nucleus, for gold (Au-197) and uranium (U-238). The results of 6.53 ± 0.06 fm for gold and 7.29 ± 0.08 fm for uranium are both significantly larger than the radius you would expect to use to measure each nucleus using charge characteristics.
The close passage from two high-energy heavy nuclei produces two short-lived mesons, resulting in two π meson pairs, which proves an unprecedented form of entanglement: between oppositely charged particles.
For the first time, an experiment has been able to show that not only the same quantum particles can be entangled, but also particles with opposite charges. (π + and π – are, in terms of their value, antiparticles of each other. The technique of passing two heavy nuclei very close together at speeds close to the speed of light allows photons to be generated from the electromagnetic field of each nucleus, interacting with another nucleus, occasionally forming a Rho particle that decays into two π mesons. When two nuclei do this at the same time, entanglement is visible, and the radius of the nucleus can be measured.
It is also worth noting that measuring the size of the nucleus by this method of using a strong force rather than an electromagnetic force differs from the result obtained using the nuclear charge radius, which is much greater. James Brandenburg, lead author of the study, said: "Now we can take a picture where, given an angle and radius, we can really distinguish the density of gluons. The images are so precise that we can even begin to see differences between where protons and neutrons are distributed in these large nuclei. We now have a promising way to probe the internals of these complex, heavy nuclei, and there is no doubt that it will have more applications soon.