Observations Are Not Gentle In The Quantum World

Principle 1: Merely observing or measuring properties alters quantum entities

Most of the physics you learned until now applies to the macroscopic world.

Your car stays at the same spot where you left it.

You can precisely measure how fast to drive your car and when to brake it to avoid a collision.

You also can measure the speed of other cars around you without causing real damage to them.

Unfortunately, as you move towards the microscopic world in the subatomic realm, things start to differ, and sometimes they make no sense at all.

A particle is no longer a definite particle and can exist in multiple states.

More precisely, it can exist in a combination of multiple states simultaneously. This is called Superposition.

Mathematically, the state of a quantum system is represented by a wave function or ψ (psi).

The quantum state is the probability distribution of all possible states (called Eigenstates) of a quantum system.

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What Happens When You Measure The Quantum State?

Consider the macroscopic world.

When you look at (observe) a car, you can measure its fundamental properties (type, company, colour, and more) without altering them.

In technical language — 

The act of observation/ measurement does not significantly disturb a system in the macroscopic world.

This is not true for the quantum world.

The act of measurement changes the quantum system permanently.

Measurement forces a quantum system from superposition into a single state.

In other words, it collapses the wave function of a quantum system.

When you observe a photon/ light particle, you actually change its properties!

Sounds spooky?

You don’t need a sophisticated lab to confirm this. Here’s an easy experiment you could do at your home today that will completely blow your mind.

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Polarizers Are All You Need

Light exists as a wave that oscillates in all directions, perpendicular to its direction of travel.

A Polarizer is an optical filter that only permits the component of the light wave that matches its orientation to pass through.

Bringing a polarizer in front of a beam of light thus blocks all light that is not aligned to its axis.

In other words, a polarizer makes the light linearly polarized in the direction of the polarizer’s axis.

Let’s begin the experiment now.

First, we bring a vertical polarizer before the light beam (unpolarized at the beginning) to get vertically polarized light.

Next, we bring the horizontal polarizer before the vertically polarized light.

You guessed it right: we will get no light at the end.

Now, the interesting part of the experiment.

Something weird happens when we add a third polarizer between the vertical and horizontal ones at a 45° angle.

We see that some light passes through.

This should not be happening.

How did the vertically polarized light suddenly develop a horizontal component?

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Understanding The Wave-Particle Duality

Wave-Particle duality is another interesting property in the quantum realm.

Things behave as both waves and particles, depending on how they are observed or measured.

Light in the above experiment can be represented as particles (photons) instead of waves.

This will make things easy to understand for us.

Let’s start at the beginning of our experiment when the light is unpolarized or all the photons are in no specific polarization state.

When the photons encounter the Vertical polarizer, their wave function collapses into the vertical polarization state represented by ∣V⟩.

Note that the phenomena of photons passing through the polarizer is considered a measurement of those photons. This leads to their wave collapsing from randomly oriented into just the vertical state.

Mathematically, the quantum state of the photons after passing through this polarizer is:

∣ψ​(1)⟩ = ∣V⟩

(The above notation is called the Bra-ket notation or Dirac notation, but we will get to it in one of the next lessons.)

If there were no 45° polarizer, the quantum state of the photons after passing through the horizontal polarizer would be:

∣ψ​(2)⟩ = 0

This is because all of the verticle component or |V> is blocked by the horizontal polarizer.

Let’s see what happens when we introduce a 45° polarizer.

First, we can visualise the vertical state as a combination of two perpendicular axes oriented at 45° and -45° as shown below. (These axes are called Basis.)

When the photons encounter (or are measured by) the 45° polarizer, their wave function collapses into:

• ∣45°⟩ with a 50% probability

• ∣−45°⟩ with a 50% probability

Since the polarizer only allows the ∣45°⟩ orientation to pass, only photons aligned with this continue.

The quantum state of the photon at this point is:

∣ψ(2)​⟩ = ∣45°⟩

Next, the photons encounter the horizontal polarizer.

The quantum state here can be visualised using the horizontal (∣H⟩) and vertical (|V⟩) perpendicular axes or basis as follows.

On being “measured” by the horizontal polarizer, the wave function of the photos collapses into:

• ∣H⟩ with a 50% probability

• ∣V⟩ with a 50% probability

From here on, only the horizontal component can pass through the polarizer, and the final quantum state of the photos is thus:

∣ψ(3)​⟩ = ∣H⟩

This is why we can see some light at this three-polariser experiment's end.

I hope it makes sense.

The takeaway is that observations/ measurements are not gentle in quantum mechanics. They alter the quantum state by collapsing wave functions.

See you in the next lesson.