What is Quantum Entanglement?

Please note that this article is taken from chapter 13 in Part 2 of a Journey into Modern Physics and represents improved descriptions of the physics associated with entanglement, as well as more up-to-date facts. In particular, as an example of entanglement, the decay of a Higgs boson has been used rather than an unidentified boson with spin 0.

Einstein referred to quantum entanglement as ‘spooky action at a distance'.  Quantum entanglement occurs when two or more particles are described by a single quantum state and cannot be assigned independent quantum states of their own. Measurements performed on one particle are then correlated with measurements on the others in a way that cannot be explained by classical physics.

These correlations persist even when the particles are separated by large distances, making entanglement one of the most remarkable features of quantum mechanics.

What is the meaning of entanglement?

Basically, if you measure one of the properties of a particle that is entangled with another, then whatever property is measured, this is immediately reflected in the property of the other particle in the entangled system. 

Experiments show that the correlations appear regardless of the distance separating the particles and no measurable delay has ever been observed. However, no information is transmitted between the particles and relativity remains intact.

This is what really upset Einstein, the fact that the outcome of entanglement is instantaneous regardless of how far apart the entangled particles are when one of them undergoes an interaction.

Is entanglement just a result of conservation laws? 

This is quite a common statement: entanglement is a result of conservation laws, but is it?

If the effect of influencing the state that entangled particles fall into isn’t transmitted as information from one particle to another, then it must be intrinsic to the quantum nature of the entangled system, which may well involve a conservation law (and usually does), but it doesn't have to.

Let’s look at a simple example where a Higgs boson suddenly decays into two new particles.  The original Higgs particle had a spin of 0 and the two product particles are quarks which are fermions. 

Ignoring all other quantum properties for the moment, there must be conservation of spin before and after the interaction. The two particles are produced in a quantum state whose total angular momentum is zero. Measurements along the same axis will always produce opposite results, one fermion must have a spin of ½ (i.e., an UP-spin), but the other must have a spin of -½ (i.e., a DOWN-spin), making a total spin of 0. 

Here is the fundamental fact;

Entangled particles are not defined by individual wave functions, but by a single wave-function, which contains all the possible outcomes of the constituent particles in a superposition of states.

If we collapse this wave function by observing one of the entangled fermions and measure spin ½, then the spin of the other particle must show -½ if measured in the same direction (important point).  Alternatively, if the observed particle shows a spin of -½, then the other will show ½. 

The entanglement, defined by a single wave function, ensures that in this particular case, the total spin property before and after the decay is always zero.

Measuring the two particles' spins in the same direction is very important. If the first particle's spin is measured in the vertical alignment, so too must the second particle's spin and this gives a 100% result that the two spins are opposite. The same applies in the horizontal.

If one spin is made in the vertical direction and the other in the horizontal, there is not a 100% correlation. It doesn't matter what direction is used, vertical, horizontal or any angle in between, as long as the same direction is used for both measurements.

Whatever direction is chosen, the effect that ensures that the second particle has a spin in the opposite sense to the first is always found to be the case, instantaneously, regardless of separation of the two particles.

So, is entanglement just a fancy way of describing the outcome of conservation laws? Conservation laws often lead to entangled states, but entanglement is a more general quantum phenomenon that can exist even when no simple conservation law is involved.

Example of Entanglement without conservation of a law

This is a regular occurrence when dealing with quantum computers and qubits, each of which can be in one of three possible states, [0}, [1] or [0 and 1], unlike conventional computer bits which are either [0] or [1].

Two entangled qubits can be created so that their entangled states are defined as;

{ [0] and [0] OR [1} and [1] }

On measurement, if one qubit is [0], the other will be [0] as well. Alternatively, if one is [1], so too will be the other. Each result is a 50% possibility. The states might be derived, for example, from possible energy states of a pair of atoms. In this case both atoms might be in the lower energy state (regarded as [0] and [0] ) or both atoms are in the higher energy state (regarded as [1] and [1] ). No other energy states are possible.

The entangled state can be prepared only if the preparation apparatus participates appropriately in the overall energy accounting, but the observed correlation itself - “same result at both locations” - is not a consequence of a fixed conserved total energy.

All possible quantum states 

With respect to the Higgs boson, once it has decayed, neither of the two resultant particles possesses a definite spin value along a chosen measurement axis before measurement.

Particles often become entangled when they are created in the same physical process, but this is not a requirement. Entanglement can also be generated later through suitable interactions between previously independent particles.

What about the Higgs' Electric and Colour charges?

The Higgs boson has no electric charge, no colour charge, no spin and when it decays, the resultant electric charge, colour charge and spin must all be 0 to comply with the conservation of electric charge, colour charge and spin.

One of the common decay channels for a Higgs boson is to decay into a bottom quark and its anti-quark.

H → bbˉ

The electrically neutral state of the Higgs is preserved in the production of one bottom quark and its anti-quark as they possess opposite electrical charge.

From Chromodynamics, the colour charge of the bottom quark can be red, green or blue and the colour charge on the anti-quark must be the anti-colour, anti-red, anti-green or anti-blue. The quark and antiquark are produced in a colour-neutral quantum state that is a superposition of all allowed colour–anticolour combinations.

Hidden variables v spooky action

Einstein was so incensed by this idea of what he called 'spooky action at a distance' that he co-authored a paper on an alternative.  Rather than the outcome of the entangled particles being instantaneous when the wave function describing them collapsed, he believed that each outcome had somehow already been encoded into the wave function at birth.

In 1964, Physicist John Bell came up with a thought experiment to see which theory described reality best. In 1981, based on Bell's work, a test was devised to see which was correct, hidden variables or spooky action. 

Without going into detail (link below if you are interested) Bell-test experiments strongly favour the predictions of quantum mechanics and rule out broad classes of local hidden-variable theories. So, no hidden variables created when the entanglement wave function comes into being. We have a single wave function that describes the entangled quantum particles.

The wave function encodes the probabilities and correlations associated with all possible measurement outcomes. When the wave function collapses (measurement made on one of the particles) one outcome becomes dominant for all the entangled particles.

If you want to know more about the test of hidden variable versus spooky quantum action, go to YouTube and hunt out Fermi Lab's Don Lincoln's video on 'quantum entanglement: spooky action at a distance'. Don Lincoln will explain all with good graphics.

Einstein was adamant that without an adequate explanation for how entanglement worked, quantum physics was not complete and could not be said to describe reality in full. His hidden variables, inherent in the combined wave function, provided his possible solution to the dilemma at the time. 

There is a new theory about how the outcome of entanglement is immediate, branded as ER=EPR. You can read all about this theory in chapter 13 of Part 2 of A Journey into Modern Physics. This, as yet unverified theory, also has profound implications for how gravity is works.

Polarisation

There is another property that relates to EM radiation and this is polarisation. 

Light waves, consist of electric and magnetic oscillations at a right angle to each other. Polarisation usually refers to the orientation of the electric field oscillation. The magnetic field remains perpendicular to both the electric field and the direction of propagation.

Polarised glasses make use of this property to cut down glare.  Light reflected off surfaces tends to be selectively polarised in the direction parallel to the surface, i.e., in the horizontal plane. 

Polarised glasses have very fine lines cut into them in the opposite orientation to reflected light.  The result is that the glasses cut down glare by stopping the light that has been polarised by reflection from horizontal surfaces. 

Spin Polarisation

Particles also have a polarisation property associated with the spin of the particle. Spin polarisation is the degree to which the particle's spin is aligned to a defined direction.  Spin polarisation can be induced by influencing the particle with a magnetic field.

So, our quantum systems may be characterised by properties such as position, momentum, spin and polarisation, among many others.

Entangled properties between electrons and photons  

The decay of a spin 0 Higgs boson to two spin ½ fermions is just one example of entangled properties. 

Oddly, there are situations where the spin of an electron can be entangled with the polarisation of a photon, a phenomenon that may hold the key to quantum computers. 

It’s interesting to note that an electron’s dynamics are defined by Schrödinger's quantum wave equation, while the dynamics of a photon are described by Maxwell’s classical field equations.  They share some mathematical similarities but describe fundamentally different things.

Separation of Entangled Quantum Particles

In classical physics, the evolution of state is a local issue.  Two classical particles separated by a vast distance cannot affect each other except by transmitting information at the speed of light from one to the other. 

With quantum particles, entangled at birth, regardless of their ultimate separation in space, measurements performed on one particle immediately determine what correlations will be observed if the second particle is measured in a corresponding way.

How do you create entanglement? 

Some recipes for creating quantum entanglement allowing one to perform experiments on the entangled particles, follow.

1 - Cascade method

As far back as the 1980s, it was found that if you excite calcium atoms such that an electron from the ground state is shot up into a high-level, excited state, it does not fall back to the ground state directly.  Instead, the electron passes through an intermediary level and then down to the ground state nano-seconds later. 

The process of an electron dropping from one energy state to another, via an intermediate level, produces two photons that are emitted in random directions, but each photon is emitted in the opposite direction to the other.  Through conservation of angular momentum, their polarisation states are entangled. 

2 - Second generation method

This method is experimentally very difficult to achieve.  It involves shooting two entangled photons, produced in method 1, at two sets of atoms.  Each of the two photons is absorbed by one of the target atoms and the two atoms involved may become entangled.  Unfortunately, the entangled state of the two atoms doesn’t last very long.

3 - Accidental entanglement 

This method becomes much more complicated.  Photons are emitted from two ions (atoms with more protons than electrons) that are separated in space when their quantum states are re-populated from local, free electrons.  These photons are then channelled together and the process of bringing them together creates entanglement swapping.  This entanglement of the photons percolates back to the two ions that produced them in the first place.

4 - Entanglement by interaction 

In this case, a laser is used to excite an electron in an atom to a very high level above the ground state. When this happens and two atoms are close together, the excited atom affects the second atom’s energy levels such that the laser that produced the excited state in the first atom has no effect on the second atom. 

The energy gap in the second atom has been increased by the effect of the excitation in the first atom due to their close proximity.  So, only one of the two atoms has been excited. Trying to excite the second atom actually creates entanglement between the two adjacent atoms. This is known as the Rydberg blockade mechanism.

If you want more details about these methods, which are extremely complex, then type 'cascade method of entanglement' into Google and hunt out the FORBES.COM link, which will explain all the methods in more detail.

In reality, the above methods for creating entanglement are all very difficult to achieve and require great ingenuity on the part of the experimenters.  The descriptions presented here are very simplistic summaries of the complex processes involved.  Nevertheless, the basis for large-scale, fault tolerant quantum computing centres around the fourth method, which may or may not yield the first truly usable quantum computer, although prototypes are already in existence and in regular use.

Quantum entanglement at a distance

Spooky action can occur between particles irrespective of their distance apart.  Now this is really getting down to the nitty-gritty of quantum mechanics.  How can this be? 

Remember that all quantum objects are defined by wave functions.  These wave functions describe the probability of finding the object anywhere in space.  That’s the clue. 

For entanglement we are talking about a single wave function that defines the components of the entangled system in terms of a superposition of possible states across space. Obviously, the probabilities of finding an object at the limits of the universe are infinitesimally small, but it means, in theory, the entangled particles continue to be described by a single quantum state regardless of the distance separating them.

So, to summarise, the recipe for experimenting with entanglement at a distance is;

1) Create an entangled pair of quantum entities.

2) Separate them by a vast distance.

3) Observe one (single wave function of the entangled entities collapses).

4) Depending on what state you measure for the close-by entity, you know what state the one farther away must be in, or vice versa.

The distance record is being broken frequently. Suffice it to say that currently, at the time of writing this article, the distance record is now a separation of over a thousand kilometres between entangled particles involving satellite experiments.

Entanglement at a distance is best achieved when a photon is entangled with an ion, described earlier.

One method is to send the photon down a long fibre cable and check its quantum state on arrival at the other end.  This then tells you what the quantum state of the ion must be back at the source.

To learn more, type 'quantum entanglement at a distance' into Google and hunt for the link that tells you a new record has been achieved!  Einstein would have been appalled!

Uses of entanglement

So, apart from quantum computers, to what sort of applications can entanglement be put?

1 - Ultra-precise timekeeping

Atomic clocks monitor the specific EM frequency produced when electrons drop between energy levels. The quantum logic clock at the National Institute of Standards and Technology only loses or gains one second in a staggering 3.7 billion years! The strontium clock, also being developed by the institute, will be accurate for 5 billion years! These clocks use a collection of atoms to measure time, detecting and correcting for minute differences between neighbouring atoms. 

Cramming more atoms into the space used creates greater precision.  By entangling the atoms, it is expected that local differences will almost vanish.

2 - Producing uncrackable codes

Traditionally, to secure data being transmitted, the transmitter uses a key to encrypt data before transmission.  The data is then pretty much unfathomable if someone manages to intercept it.  To decode the message, you need the decryption key. 

So, the transmission is safe, until someone steals the decryption key.  Worst still, if the hacker knows the encryption key, then false information can be transmitted.

The problem of hacking into transmission data streams when the hacker possesses the required decryption key, can be solved by using quantum key distribution.  In this case, information about the decryption key is sent using entangled photons that have been randomly polarised.  The recipient then decodes the photons that provide the key. 

Data can be transmitted over standard communication channels and any hacker that intercepts the message will have to decode the quantum key. 'Reading' the randomly polarised photons that constitute the quantum key will change the state of the entangled atoms at the other end of the transmission line and both ends of the transmission path can then be alerted to the security breach.

The technology still has a way to go to get the distances greater than a few tens of miles.

3 - Improved Microscopes

At Hokkaido university an entanglement-enhanced microscope has been developed. The microscope fires two beams of photons at the target and measures the interference pattern produced by the reflected beams. 

The pattern changes depending on how rough the surface being explored is.  Using entangled photons in the beams yields more information about the target.

You can get more information about these subjects if you type in the titles used above into Google and search out the required references.  As this area of research is on-going, things may well change in the near future.

What happens to all the other possibilities in entanglement?

If we go back to the decay of the Higgs boson for a second, the single wave function describing the entangled particles contains a range of possible measurement outcomes.

When a measurement is made, only one outcome is observed.

In the Higgs decay, there are two spin possibilities and three colour charge possibilities, but only one result of each type will manifest itself. So, what happens to the other possibilities? Perhaps the theory of the multiverse has an answer.

All possibilities in the multiverse

It is a matter of probability which state in a superposition of different outcomes becomes the dominant outcome to an observer, resulting in a game of chance, much to Einstein’s disgust.  He is reputed to have said 'God does not play dice with the universe'.

We now move into the realms of pure speculation: just suppose that, in fact, all the outcomes actually occur when a wave function collapses.  Theoretically, this could happen if each outcome is played out in a separate universe. 

This sounds quite far-fetched, but a theory that has gained a great deal of momentum over a period of decades, is that of the multiverse: the existence of countless universes in which all outcomes to a single quantum event can be played out.

Different Realities

One interpretation that has attracted considerable attention over the decades is the Many-Worlds Interpretation of quantum mechanics. In this theory, all possible outcomes continue to exist in separate branches of reality.

Suppose that for every event of a particular type, there are 2 possible results.  One result is played out in our universe and the second in universe 2. Quite a concept!

It does, however, explain the quantum world rather well.

In the modern era, Erwin Schrödinger, the physicist who developed the wave equation used throughout quantum physics, stated in 1952 that his equation defined different histories, which were not alternatives, but things that happened simultaneously. He pointed out that this idea was hard to believe, but he, himself, did believe it. This is considered to be the first indication of a possible multiverse, used in the modern sense. 

However, the modern Many-Worlds Interpretation was proposed by Hugh Everett in 1957. It suggests that all possible outcomes of a quantum event continue to exist in separate branches of reality rather than a single outcome being selected by wave-function collapse.

No Evidence exists for a Multiverse

Currently, there is no evidence that there is a multiverse, but a specualtive case has been put forward that if it does exist, then other universes must have had an effect on the cosmic egg, which was our universe at the beginning of time.

It is possible that evidence does exist in the cosmic microwave background and there is a project to see if there is any validity in this.

To read more about the multiverse and the work that is being done to see if evidence can be found for its existence, have a read of the blog article 'What created the big bang'? Published on 6^th April 2026.

The multiverse is not an established scientific fact. It is one possible interpretation of quantum mechanics that attempts to explain what happens to the alternative outcomes represented in a quantum wave function. Other interpretations of quantum mechanics offer different explanations.

Final Takeaway

Quantum entanglement remains one of the deepest and most puzzling features of quantum mechanics, continuing to challenge our understanding of reality itself.

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