Quantum Entanglement and Spooky Action

January 6, 2025

Podcast about Quantum Entanglement and Spooky Action (AI generated)

Introduction

This is the first week of the “Smarter Year” series, in which I’ll be exploring a new topic every day. Today is Monday, so the theme is Quantum and Physical Sciences and this week’s theme is Foundations of Modern Understanding. We are starting this series off nice and light with this topic: Quantum Entanglement and Spooky Action.

What is Quantum Entanglement?

Quantum entanglement is the phenomenon of two or more particles being intrinsically linked to one another. If you take measurement of the state of one, you can know the state of the other as it is perfectly correlated with the other. Here’s an analogy: Imagine you have two coins that are magically linked. Whenever you flip one coin, the other coin always lands on the opposite side - heads vs tails. The spooky part of this, or as Einstein said, "spukhafte Fernwirkung", is that this can happen at any distance and instantaneously. Why is this spooky? Well it completely violates a concept called locality for three reasons:

  1. It seems like the effect could happen faster than the speed of light.
  2. There’s no way in classical physics in which the two particles could know about the state of each other.
  3. The act of measuring the state of one particle itself is enough to effect the other entangled particle.

How did we even discover Quantum Entanglement?

Entanglement first appeared around 1935. But lets go back, as every discovery comes about working on the work of our predecessors who in this instance were; Max Planck, Niels Bohr, Einstein, and Louis de Broglie - of whom I would say Quantum Theory is the birthchild of. They really laid the foundations in the very start of the 20th century.

Then in the period of 1925-34 more and more work was done on Quantum Theory, even though it was widely debated at the time, for example at the Fifth Solvay Conference. John von Neumann was a crucial player as in 1932 he published work that actually aligned quantum mechanics to a mathematical foundation. This alongside Heisenberg’s Matrix Mechanics in 1925, which described a mathematical framework that focused on what could be measured rather than describing electrons like orbiting planets, meant that more work could be done by more scientists around the world. I have strong opinions on the impact of good mathematics linking to our understanding of nature, maybe I’ll put them alongside the timeline in the footnotes.

Following this was the true birth of entanglement.

1935: The EPR (Einstein, Podolsky, and Rosen) Paper

This paper titled “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" was absolutely pivotal in the discovery of Quantum Entanglement. It argued that if quantum mechanics was correct, that measuring one particle would instantly affect another particle far away and that this violated our fundamental understanding of physics at the time (spooky action at a distance).

1935: Schrödinger's response

In the same year, Schrödinger responded with two crucial papers, “Discussion of Probability Relations Between Separated Systems” first and “The Present Situation in Quantum Mechanics” second.

The first was the introduction of the term entanglement, or “Verschränkung” in German. It showed how quantum states differ from classical states and how you cannot describe the state of two entangled systems independently of one another. It also explained how the measurement of one system affects the knowledge of the other system instantly.

The second, dubbed the “Cat Paper” was published in three parts and introduced the very famous Schrödinger's Cat thought experiment. It discussed how entanglement leads to apparently paradoxical situations.

1947: First experimental hints of entanglement in particles from cosmic rays

Real proof of entanglement is decades off. However, in 1947 some physicists studying cosmic rays accidentally discovered something interesting which turned the tide of the debates around quantum entanglement.

Physicists from the University of Bristol and the University of Calcutta observed pairs of photons being produced when cosmic rays hit the atmosphere. These photon pairs showed correlated properties in their polarization and the correlations were stronger than what classical physics could explain. These were not controlled experiments nor were the measurements accurate enough to be proof. However, these were hints as to what was to come.

Going forward and recent breakthroughs

I’m going to zoom through this so we can get to the fun stuff, full timeline in the footnotes.

Theoretical Developments (1950-1969)

Work by the likes of David Bohm, Hugh Everett and John Stewart Bell work on lots of ground-breaking theoretical work. I’ll go into the Hugh Everett’s many-worlds interpretation because it’s fun and John Stewart Bell’s theorem because it’s important.

Many-worlds interpretation by Hugh Everett

The idea is that when a quantum event occurs with many outcomes, they actually all happen and that reality splits into multiple worlds or universes. Pretty wild. An analogy that AI gave me to explain this which I think is pretty good is:

An Example with Schrödinger's Cat:

  • Traditional interpretation: The cat is both alive and dead until we look
  • Many-Worlds interpretation: The universe splits into two realities:
    • One where the cat is alive
    • One where the cat is dead
    • Both exist, but in separate, branching universes

So yeah, not massively helpful as a theorem as we can’t test it. But it is fun to think about.

Bell's Theorem (1964)

Einstein believed that there must be hidden variables that explain entanglement without the need of “Spooky Action”. Bell’s theorem allowed us a way to prove whether this was true or not, which was impossible before. He showed mathematically that if hidden variables exist, there's a mathematical limit to how correlated entangled particles can be, and that quantum mechanics predicts that correlations can exceed this limit. This later became known as Bell’s Inequality.

This theorem laid the groundwork for later experiments to prove that quantum mechanics was correct.

Experimental Verification

1972: First experimental tests of Bell's inequalities by Freedman and Clauser, showing that Bell’s Inequality was broken however, with loopholes.

1981-1982:

  • Alain Aspect performs first definitive Bell test experiments
  • Uses time-varying analyzers to close locality loophole

1985: First demonstration of quantum entanglement with photons

2015: First loophole-free Bell test performed 2017: First satellite-based quantum entanglement distribution

I’m not going into depth of these. Sorry, I love theory but as I learnt while doing labs on my physics degree I don’t find the experiments very interesting.

Tldr: they proved quantum entanglement.

The Future of Entanglement and Why Does It Matter?

Why is this cool, I’m going to write a list of just the cool stuff that this enables and I will be going into depth of all of them in the coming year:

  • Global quantum networks (Quantum Internet?!)
  • Un-hackable communication systems
  • Distributed quantum computing
  • Quantum computers
  • New forms of encryption (and breaking our current forms)
  • Quantum communication networks

Conclusion

In the last century or so we have made some incredible and mindboggling breakthroughs into how our world works and it’s unbelievably hard to understand. We are entering an age in which the things we interact with every day, our phones, computers, large logistics networks, etc. we just don’t have a grasp on. They work and we know they work but is it healthy that the average person does not know why? I think this is a big part of why I’m doing this series. I want to understand the world better and I want everybody to understand the world better. I don’t know if this has helped anybody else but, it’s helped me at least a little.

Any feedback is welcome at my email which can be found in my contact page. Thanks for reading.

Footnotes

1. The Discovery and Development of Quantum Entanglement: A Historical Timeline

Early Quantum Theory (1900-1924)

  • 1900: Max Planck introduces quantum theory with his work on black-body radiation
  • 1905: Einstein explains the photoelectric effect using light quanta
  • 1913: Bohr presents his model of the atom with quantized energy levels
  • 1924: Louis de Broglie proposes wave-particle duality

Foundation Years (1925-1934)

  • 1925: Werner Heisenberg develops matrix mechanics
  • 1926: Erwin Schrödinger publishes his wave equation
  • 1927:
    • Fifth Solvay Conference where quantum mechanics is intensely debated
    • Heisenberg formulates the uncertainty principle
    • Bohr presents the Copenhagen interpretation
  • 1932: von Neumann provides mathematical foundations for quantum mechanics

The Birth of Entanglement (1935-1949)

  • 1935:
    • Einstein, Podolsky, and Rosen publish the EPR paper in Physical Review
    • This paper challenges quantum mechanics' completeness using what we now call entanglement
    • Schrödinger coins the term "entanglement" (Verschränkung in German)
    • Schrödinger introduces his famous cat paradox
  • 1947: First experimental hints of entanglement in particles from cosmic rays

Theoretical Developments (1950-1969)

  • 1951: David Bohm reformulates the EPR argument using spin-1/2 particles
  • 1957: Hugh Everett proposes the many-worlds interpretation
  • 1964:
    • John Stewart Bell publishes his famous theorem
    • Bell's inequality provides first testable prediction distinguishing quantum mechanics from local hidden variable theories
  • 1967: John Clauser and others propose first experimental test of Bell's inequality

Experimental Verification Era (1970-1989)

  • 1972: First experimental tests of Bell's inequalities by Freedman and Clauser
  • 1981-1982:
    • Alain Aspect performs first definitive Bell test experiments
    • Uses time-varying analyzers to close locality loophole
  • 1985: First demonstration of quantum entanglement with photons
  • 1989: First IBM quantum teleportation experiment

Modern Applications (1990-2009)

  • 1991: Artur Ekert proposes entanglement-based quantum cryptography
  • 1993: First quantum teleportation proposal
  • 1997: First experimental quantum teleportation achieved
  • 1998: First three-particle entanglement demonstrated
  • 2004: First quantum key distribution using entangled photons
  • 2007: Entanglement of six photons achieved

Recent Breakthroughs (2010-Present)

  • 2012: Record-breaking distance for quantum teleportation (143 km)
  • 2015: First loophole-free Bell test performed
  • 2016:
    • China launches first quantum communications satellite
    • Record entanglement of 3,000 atoms achieved
  • 2017:
    • First satellite-based quantum entanglement distribution
    • Pan-Asian quantum communication network demonstrated
  • 2019: Demonstration of quantum advantage using entangled photons
  • 2020: First quantum teleportation over a distance of 44 km using fiber networks
  • 2021: Chinese scientists achieve record quantum communication distance
  • 2022: Major advances in quantum memory and repeater technology
  • 2023: Development of more robust entanglement-based quantum networks

Key Experimental Milestones

  1. First Bell test (1972)
  2. Aspect's definitive experiments (1981-82)
  3. First quantum teleportation (1997)
  4. First satellite-based entanglement (2017)
  5. Loophole-free Bell tests (2015-2018)

2. My opinion on mathematics as the fundamental science and how we understand the world.

These are my opinions take them with a large pinch of salt.

I speak English. It’s the only language I speak and I’m not even that good at it. Other people might speak Spanish, or French, or Mandarin, or Russian. This blog post could/would be absolute nonsense and gibberish to them. However, the mathematical foundation that gives us a way to describe how fundamental forces of nature work in physics would still make sense to them. The maths is the same in English, or Spanish, or French, or Mandarin, or Russian.

It gives us a language to understand the world. It’s a framework to predict the future, the past, to predict things we have never even perceived.

I’m not particularly good at mathematics either, truthfully, but it’s quite beautiful nonetheless.