Quantum entanglement is an astonishing phenomenon in quantum mechanics that fascinates both scientists and the public.
In contrast to the predictable nature of classical physics, quantum mechanics thrives on unpredictability and often challenges our everyday perceptions. This is particularly evident in the behavior of entangled particles.
Understanding quantum entanglement
In quantum mechanics, particles can exist in multiple states simultaneously, a concept known as superposition. Entanglement takes this idea further: when particles are entangled, their states are linked.
A change in the state of one particle instantly affects the other particles, regardless of distance. Imagine two distant lightbulbs flickering in sync across the cosmos.
Entanglement suggests that information can be exchanged between particles instantaneously, contradicting the idea that nothing moves faster than light.
This “spooky action at a distance,” as Albert Einstein aptly put it, raises profound questions about the nature of space and time.
Challenges of measurement
Measurements in quantum mechanics present unique challenges. Observing a particle in an entangled pair determines the states of both particles, raising critical questions: What constitutes a “measurement” and how does it affect our understanding of reality?
The complex mathematics underlying quantum mechanics – which includes concepts such as Hilbert spaces, wave functions and operators – can seem intimidating and make entanglement less accessible to many.
Fascinating puzzle
Simply put, quantum entanglement is simply too complicated for most people to fully understand. It defies classical intuitions, requires sophisticated mathematics, and forces us to rethink our understanding of reality.
That is, until now.
This is where Professor Carl Kocher comes in, a pioneer in the study of quantum entanglement. He is currently a member of the Quantum Foundry at the University of California Santa Barbara and Professor Emeritus at Oregon State University.
Professor Kocher is the author of a new article in Frontiers in Quantum Science and Technology in which he explains his groundbreaking quantum entanglement experiments from 1964 to 1967 and discusses the quantum entanglement “paradox”.
In the following guest commentary, Professor Kocher helps us broaden our horizons to understand this seemingly paradoxical phenomenon.
Understanding quantum entanglement
My new article, “Quantum entanglement of optical photons: The first experiment, 1964–67,” aims to convey the spirit of a small research project venturing into uncharted territory.
The article breaks with tradition as it describes first-hand the strategy and challenges of the experiment, as well as an interpretation of the final result and its significance. In this guest commentary I will introduce the topic and also try to shed light on the question “What is a paradox?”
Let’s start with the gyroscope I bought at a joke and magic shop when I was eight years old. The rotating disk, resting at one end of its shaft, did not fall but moved slowly in a horizontal plane.
In the context of common experience, which excludes gyroscopes, this behavior seems mysterious or paradoxical, but in the context of Newtonian mechanics, it makes perfect sense, since Newtonian mechanics resolves the paradox by accurately predicting the behavior of gyroscopes.
Quantum theory, developed in the mid-1920s, has proven impressively successful in explaining the properties and interactions of atoms and molecules. In 1935, Einstein, Podolsky, and Rosen sparked controversy with a thought experiment in which two particles of common origin move away from each other. They found that quantum theory predicts correlations in subsequent measurements of their spins.
The correlation may seem quite puzzling, since a measurement on one of the particles appears to influence a subsequent measurement on the other, even when the particles do not interact with each other. In today’s terminology, these correlations are an example of entanglement, and the correlation phenomenon is known as the EPR paradox.
The puzzle has been the subject of much discussion and analysis, particularly because there was (and is) no known mechanism for measurements to communicate with each other.
Untangling quantum entanglement
In 1964, I was fascinated by this unknown effect and started thinking about how to actually perform the EPR experiment – or at least a version of it – to observe correlation and entanglement. It should be a low-energy experiment that could be done in a small laboratory.
For the experiment described here, the particles of interest are visible light photons that do not interact with each other and are emitted by excited calcium atoms in a two-step spontaneous emission process. The polarization states of the photons, which are related to their spins, can be easily measured using ordinary linear polarizers.
Photomultiplier detectors count individual photons #1 (green) and #2 (violet), and timing circuits allow pairs of photons from the same atom to be identified. A rotating linear polarizer is placed in front of each detector.
Put simply, the experiment involves counting the rate at which pairs of photons are detected as a function of the orientation of the polarizers. A pair of photons detected by the same atom is recorded as a “coincidence count.”
Theory and experiment agree
Quantum theory makes the following predictions:
- Each photon individually has a 50 percent chance of being passed through its polarizer, regardless of its orientation angle.
- If the polarizer axes are parallel, both photons from the same atom can pass through their polarizers and be counted. Coincidence counts are observed.
- If the polarizer axes are perpendicular to each other, it never happens that both photons pass through their polarizers. Therefore, no coincidence counts are observed.
Predictions #1 and #2 are not surprising since the green and violet light rays are unpolarized.
Prediction #3, which is further explained in my paper, is a quantum entanglement effect, which has no analogue in classical (non-quantum) physics. It is particularly interesting because it can be tested experimentally. I designed the experiment specifically for this purpose.
The results of the experiment, after almost three years of laboratory work, clearly show that coincidence counts are recorded when the polarizer axes are parallel and that no coincidence counts are recorded when the polarizers are perpendicular to each other. The agreement between theory and experiment is clear and striking.
So is there a paradox?
In our brief discussion of the gyroscope, we have not encountered any paradox, since Newton’s theory (classical dynamics) completely explains the motion of a gyroscope.
Furthermore, both the theory and the observed gyroscopic behavior are compatible with our life experience and our intuitive ability to understand natural processes in the classical domain.
In the case of entanglement, quantum theory explains the observed correlation of photon polarizations. But even if a theory predicts experimental results, a paradox can persist if intuition cannot.
Look again at predictions #1 and #3 above. Drawing on our experience of living in a non-quantum world, we may notice something very strange when the polarizers are “crossed” at 90 degrees.
If each photon has a 50 percent chance of passing through its polarizer, why are there no matches 25 percent of the time? Instead, we observe no matches at all.
Exercising the mind to solve the paradox
At first glance, this does indeed seem to be a paradox. One possible explanation could be a missing component of quantum theory – perhaps a causal mechanism that could allow one photon or measurement to communicate with another. However, despite extensive research, no evidence for such a mechanism has been found.
Since we do not live in a purely quantum world, classical phenomena can influence our thought processes – even when we venture into the realm of the quantum world. It may therefore remain a challenge to integrate entanglement into intuition.
I am convinced that this paradox can be at least partially resolved by further reflection and experience, such as the experiment considered here, which stimulates the mind to a point where it can better deal with entanglement and other quantum phenomena.
I now find these aspects of nature to be “strangely wonderful.”
The full article and study can be found at Frontiers of quantum science and technology.
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