[Draft] [“Quantum foundations” series] [“What’s changed in the last ~50 years” series]
TABLE OF CONTENTS
THE BUZZ
Much buzz on Tuesday October 4, 2022, as media marked the 2022 Nobel Prize in physics.
So many articles – a hopeful indication regarding communicating science. An annual celebration at least. Hopefully not continuing overused sci-fi tropes, eh.
I’ve cited some of the many articles below; but here I’ll mainly note how journalists describe the physics of the actual research on “a feature of the universe that baffled Albert Einstein” – aka “spooky action at a distance.”
But first, here’re some notes which I’ve been working on before the announcement this week. Notes which I had considered as a comment on my prior post about quantum entanglement.
PROVING THAT QUANTUM ENTANGLEMENT IS REAL
The road from thought experiments and mathematical models (quantum theory, Bell’s theorem, etc.) to practical experiments (Bell tests and closing loopholes) to real-world applications …
How do you measure entanglement?
• Caltech > News > “Proving that Quantum Entanglement is Real” (September 20, 2022) – A Q&A with Caltech alumnus John Clauser on his first experimental proof of quantum entanglement [1]
(quote) I really didn’t know what to expect except that I would finally determine who was right – Bohr or Einstein. I admittedly was betting in favor of Einstein but did not actually know who was going to win. – John Clauser
Various labs have now entangled and violated the CHSH inequality with photon pairs, beryllium ion pairs, ytterbium ion pairs, rubidium atom pairs, whole rubidium-atom cloud pairs, nitrogen vacancies in diamonds, and Josephson phase qubits. (end quote)
A historial recap …
… no experimental evidence for or against quantum entanglement of widely separated particles was available then [in the 1930’s]. Experiments have since proven that entanglement is very real and fundamental to nature. Moreover, quantum mechanics has now been proven to work, not only at very short distances but also at very great distances. Indeed, China’s quantum-encrypted communications satellite, Micius, relies on quantum entanglement between photons that are separated by thousands of kilometers.
The very first of these experiments was proposed and executed by Caltech alumnus John Clauser (BS ’64) in 1969 and 1972, respectively. His findings are based on Bell’s theorem, devised by CERN theorist John Bell. In 1964, Bell ironically proved that EPR’s argument actually led to the opposite conclusion from what EPR had originally intended to show. Bell showed that quantum entanglement is, in fact, incompatible with EPR’s notion of locality and causality.
In 1969, while still a graduate student at Columbia University, Clauser, along with Michael Horne, Abner Shimony, and Richard Holt, transformed Bell’s 1964 mathematical theorem into a very specific experimental prediction via what is now called the Clauser–Horne–Shimony–Holt (CHSH) inequality.
Clauser’s work earned him the 2010 Wolf Prize in physics. He shared it with Alain Aspect of the Institut d’ Optique and Ecole Polytechnique and Anton Zeilinger of the University of Vienna and the Austrian Academy of Sciences “for an increasingly sophisticated series of tests of Bell’s inequalities, or extensions thereof, using entangled quantum states,” according to the award citation.
(Clauser) In the 1960s and 70s, experimental testing of quantum mechanics was unpopular at Caltech, Columbia, UC Berkeley, and elsewhere. My faculty at Columbia told me that testing quantum physics was going to destroy my career. While I was performing the 1972 Freedman–Clauser experiment at UC Berkeley, Caltech’s Richard Feynman was highly offended by my impertinent effort and told me that it was tantamount to professing a disbelief in quantum physics. He arrogantly insisted that quantum mechanics is obviously correct and needs no further testing! My reception at UC Berkeley was lukewarm at best and was only possible through the kindness and tolerance of Professors Charlie Townes [PhD ’39, Nobel Laureate ’64] and Howard Shugart [BS ’53], who allowed me to continue my experiments there.
Part of the reason that I wanted to test the ideas was because I was still trying to understand them. I found the predictions for entanglement to be sufficiently bizarre that I could not accept them without seeing experimental proof.
Regarding the short-lived and refuted theory of local realism, which highlights what quantum mechanics is NOT, Clauser states [labels A, B, C, … are mine]:
A. (Clauser) Local Realism assumes that nature consists of stuff, of objectively real objects, i. e., stuff you can put inside a box. (A box here is an imaginary closed surface defining separated inside and outside volumes.)
B. It further assumes that objects exist whether or not we observe them. Similarly, definite experimental results are assumed to obtain, whether or not we look at them. We may not know what the stuff is, but we assume that it exists and that it is distributed throughout space.
C. Stuff may evolve either deterministically or stochastically.
D. Local Realism assumes that the stuff within a box has intrinsic properties, and that when someone performs an experiment within the box, the probability of any result that obtains is somehow influenced by the properties of the stuff within that box. If one performs say a different experiment with different experimental parameters, then presumably a different result obtains.
E. Now suppose one has two widely separated boxes, each containing stuff. Local Realism further assumes that the experimental parameter choice made in one box cannot affect the experimental outcome in the distant box.
F. Local Realism thereby prohibits spooky action-at-a-distance. It enforces Einstein’s causality that prohibits any such nonlocal cause and effect.
Surprisingly, those simple and very reasonable assumptions are sufficient on their own to allow derivation of a second important experimental prediction limiting the correlation between experimental results obtained in the separated boxes. That prediction is the 1974 Clauser–Horne (CH) inequality.
• Phys.org > “Nobel physics winner wanted to topple quantum theory he vindicated” by Issam Ahmed (October 5, 2022)
According to quantum mechanics, two or more particles can exist in what’s called an entangled state – what happens to one in an entangled pair determines what happens to the other, no matter their distance.
“My thesis advisor thought it was a distraction from my work in astrophysics,” he [Clauser] recalled, but undeterred, he wrote to Bell, who encouraged him to take up the idea.
They focused a laser on calcium atoms, making it emit particles of entangled photon pairs that shot off in opposite directions, and used filters set to the side to measure whether they were correlated.
After hundreds of thousands of runs, they found the pairs correlated more than Einstein would have predicted, proving the reality of “spooky action” with hard data.
At the time, leading lights of the field were unimpressed, said Clauser, including the renowned physicist Richard Feynman who told him the work was “totally silly, you’re wasting everybody’s time and money” and threw him out his office.
MEDIA ARTICLES
• LA Times (digital edition) > “Californian and 2 others share Nobel in physics” by Seth Borenstein, Maddie Burakoff and Frank Jordans [Associated Press] (Oct 5, 2022) – prize awarded for proving that tiny particles [emphasis on only tiny] could retain a connection with each other even when separated, a phenomenon being explored for potential real-world applications
(photo caption in article) JOHN F. CLAUSER of Walnut Creek, Calif., developed quantum theories into a practical experiment. (Terry Chea Associated Press)
A Northern California scientist and two European researchers won the Nobel Prize in physics Tuesday … for experiments proving the “totally crazy” field of quantum entanglements to be all too real. They demonstrated that unseen particles, such as photons, can be linked — or “entangled” — with each other even when they are separated by large distances.
Quantum entanglement “has to do with taking these two photons and then measuring one over here and knowing immediately something about the other one over here,” said David Haviland, chair of the Nobel Committee for Physics. “This allows us to do things like secret communication, in ways which weren’t possible to do before.”
“Why this happens I haven’t the foggiest,” Clauser said during a Zoom interview from his home in Walnut Creek. “I have no understanding of how it works, but entanglement appears to be very real.”
His fellow laureates also said they can’t explain the how and why behind this effect. But each performed ever-more-intricate tests that prove it just is.
• Quanta Magazine > “Pioneering Quantum Physicists Win Nobel Prize in Physics” by Charlie Wood (October 4, 2022) – Alain Aspect, John Clauser and Anton Zeilinger have won the 2022 Nobel Prize in Physics for groundbreaking experiments with entangled particles.
This long article includes a somewhat technical historical recap. And a useful timeline diagram.
(quote) Their experiments collectively established the existence of a bizarre quantum phenomenon known as entanglement, where two widely separated particles appear to share information despite having no conceivable way of communicating.
“I would not call entanglement ‘one,’ but rather ‘the’ trait of quantum mechanics,” Thors Hans Hansson, a member of the Nobel committee, quoted Schrödinger as writing in 1935. He observed, “The experiments performed by Clauser and Aspect opened the eyes of the physics community to the depth of Schrödinger’s statement, and provided tools for creating and manipulating and measuring states of particles that are entangled although they are far way.”
Initially, physicists including Richard Feynman discouraged Clauser from pursuing the experiment, arguing that quantum mechanics needed no further experimental proof. But Bell personally encouraged Clauser to see the research through, …
Entanglement is what it seems: The pair of particles is one unified system. For each individual particle, properties like spin and polarization really are undefined until the moment of measurement. In other words, reality has no fixed and predetermined state until you measure it. It’s a dramatic conclusion that most researchers accept but still struggle to fully grasp.
• BBC > “Physics Nobel rewards ‘spooky science’ of entanglement” by Jonathan Amos (October 4, 2022) – This year’s Nobel Prize in Physics rewards research into quantum mechanics – the science that describes nature at the smallest scales.
Quantum mechanics describes the behaviour of sub-atomic particles. … It concerns something called “entanglement” in which two or more quantum particles – usually photons, the particles of light – can be strongly connected when very far apart even though they are not physically linked. Their shared state might be their energy or their spin.
TAKEAWAYS
• Elusive visualization – it is what it is, with “paradigm-shattering philosophical implications [Charlie Wood’s characterization]”
(The BBC article includes an interesting art image – a Getty image, which often is used to depict entanglement.)
As noted above in the LA Times article (as elsewhere), visualization eludes even the experts – a quantum mystery not just for mere mortals.
“Why this happens I haven’t the foggiest,” Clauser said during a Zoom interview from his home in Walnut Creek. “I have no understanding of how it works, but entanglement appears to be very real.”
His fellow laureates also said they can’t explain the how and why behind this effect. But each performed ever-more-intricate tests that prove it just is.
• Shortfalls of the famous – a cautionary tale of the halo effect
As noted above in the Caltech News article, even Richard Feynman exhibited triumphalism.
(Clauser) In the 1960s and 70s, experimental testing of quantum mechanics was unpopular at Caltech, Columbia, UC Berkeley, and elsewhere. My faculty at Columbia told me that testing quantum physics was going to destroy my career. While I was performing the 1972 Freedman–Clauser experiment at UC Berkeley, Caltech’s Richard Feynman was highly offended by my impertinent effort and told me that it was tantamount to professing a disbelief in quantum physics. He arrogantly insisted that quantum mechanics is obviously correct and needs no further testing!
The Phys.org article characterized the context as: “Questioning the foundation of quantum mechanics was deemed unnecessary.”
(quote) At the time, leading lights of the field were unimpressed, said Clauser, including the renowned physicist Richard Feynman who told him the work was “totally silly, you’re wasting everybody’s time and money” and threw him out his office.
“We did not prove what quantum mechanics is – we proved what quantum mechanics isn’t,” he [Clauser] said, “and knowing what it is not then has practical applications.”
• What entanglement is good for
Charlie Wood notes the potential in quantum communication networks using unbreakable cryptography.
Zeilinger also developed a procedure called entanglement swapping, involving the emission of two entangled Bell pairs, for a total of four particles. When you perform a particular measurement on two of the particles that are not entangled, the remaining two become entangled with each other. Swapping entanglement from particle to particle in this way could help link nodes in a quantum communication network. In a landmark 1998 publication, Zeilinger and his collaborators demonstrated the ability to swap entanglement between photons that had never been in contact with each other.
The BBC article notes: “Their work should pave the way to a new generation of powerful computers and telecommunications systems that are impossible to break into.”
(quote) This is useful for military and banking, etc, in secure communications,” John Clauser said. “The biggest application to my knowledge is the Chinese who launched a satellite several years ago that they use for secure communications over thousands of kilometres.”
Notes
[1] See also this Caltech article, which includes photos and a brief bio.
• Caltech > About > News> “Caltech Alum Wins Nobel Prize in Physics” by Whitney Clavin (October 4, 2022) – “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”
“Clauser’s contributions were the seed from which a whole field of physics has grown,” says Carver Mead (BS ’56, MS ’57, PhD ’60), the Gordon and Betty Moore Professor of Engineering and Applied Science, Emeritus, at Caltech, referring to entanglement’s applications to quantum cryptography, quantum computing, quantum information science, and more.
John Preskill, Caltech’s Richard P. Feynman Professor of Theoretical Physics and the Allen V. C. Davis and Lenabelle Davis Leadership Chair of the Institute for Quantum Information and Matter (IQIM), explains Bell’s theorem as follows: “In the 1960s, John Bell described a game played by two parties, let’s call them Alice and Bob, such that if they play the best possible strategy, Alice and Bob can win the game with a higher probability of success if they share entangled qubits [quantum information bits] compared to when they share only classical bits.
“John Clauser was among the first to appreciate the profound implications of Bell’s work,” says Preskill. “With Freedman, Clauser did the first experiment confirming that Alice and Bob can really win Bell’s game with the higher success probability that quantum physics allows.“
Related posts
• Communicating what physics says — The Science Asylum
• Communicating what physics says — Domain of Science YouTube channel
• What the heck is quantum entanglement?
• Spooky action vs. spooky communication
Here’s an interesting recap by Paul Sutter of how spooky-action plays into the notion of a multiverse.
• Space.com > “‘Spooky action at a distance’ can lead to a multiverse. Here’s how.” by Paul Sutter (Oct 23, 2022) – A new reality might be produced by every possible quantum interaction.
BACKGROUND
1. Assumptions – non-relational quantum mechanics
Energy waves (wave packets) can be spread / smeared out over space vs. localized so-called point particles.
A wavefunction of an electron is a complete description of that “thing.” As well as energy topology.
2. Experiments
Electron scattering – particle scattering is more like wave interaction.
3. The measurement problem – when measured, an electron lands “as a single, compact particle, so it couldn’t be physically extended in space” [hmm].
4. The problem of wavefunction reality
5. Reifying the wavefunction – “the electron doesn’t know it’s being measured.”
THE FORK IN THE ROAD
6. Tangling all the way everywhere
7. Mounding to a universal gestalt
8. Spinoffs – probabilities of particle’s state
MY TAKE
Not only may something be lost in the translation of the mathematical model to a description for mere mortals, but by “in principle” extrapolations. Perhaps “there be dragons” in assumptions.
What is entanglement?
In this article, Whitney Clavin’s popsci tone may have overstated what entanglement entails.
Perhaps referencing correlation would make the description too obscure? – too technical or too mathematical. His use of the phrase “direct contact” also is misleading. Just too many classical (Newtonian) physics allusions, eh.
With additional nuance in general and for the term “particle” (as always), I’d prefer something like this:
Entanglement is a phenomenon in quantum physics where particles’ properties remain correlated even though they are widely separated.
• Caltech > News > “Randomness in Quantum Machines Helps Verify Their Accuracy” by Whitney Clavin (January 24, 2023)
Is Charlie Wood’s reference to entanglement in this article any better?
• Quanta Magazine > “How Quantum Physicists ‘Flipped Time’ (and How They Didn’t)” by Charlie Wood, Staff Writer (January 27, 2023)
An illustration which zooms in on a complex set of states within an apparently smooth quantum system. Credit: Adam Shaw/Caltech