Over the last few episodes (check here), we have been exploring the fascinating world of quantum sensing and precision measurement. We've seen how quantum physics allows us to detect incredibly small signals and make measurements that would otherwise be impossible.

In this episode, we will discover another remarkable application of quantum precision: scanning the human brain.

Today the story begins with Biology and our brain! At first glance, biology and quantum physics may seem like completely different worlds.

But our bodies are full of physics.

Every thought, movement, memory, or sensation involves countless neurons communicating with each other through tiny electrical impulses. These electrical currents are incredibly small, but they are constantly flowing throughout the brain and nervous system.

The detection and measurement of these signals are crucial for understanding the functioning of the human brain and for diagnosing abnormalities that may lead to neurological diseases.

The challenge is that these signals are difficult to detect from outside the body. Fortunately, physics gives us a clue.

Many people think electricity and magnetism are separate phenomena.

In reality, they are deeply connected. Whenever an electric current flows, it creates a magnetic field around it. This relationship is one of the foundations of electromagnetism, described by Maxwell's equations in the 19th century.

So instead of trying to directly measure the tiny electrical currents inside the brain, we can detect the equally tiny magnetic fields they produce.

The problem?

These magnetic fields are extremely weak—far weaker than the magnetic noise coming from the environment around us.

To detect them, we need an extraordinarily sensitive sensor. This is where quantum technology enters the story.

Welcome to SQUIDs.

And no, we are not talking about the Netflix series or the sea creature!

SQUID stands for Superconducting Quantum Interference Device.

Despite the intimidating name, the idea behind it is surprisingly simple. SQUIDs are among the most sensitive magnetic field detectors ever built. They can measure magnetic fields that are billions of times weaker than the Earth's magnetic field.

So, how do SQUIDs measure tiny magnetic fields with ultra-high precision?

We have encountered superconductivity before (#5). In ordinary wires, electrons constantly collide with atoms as they move through the material. These collisions cause electrical resistance, converting some of the electrical energy into heat. This is why wires can become warm and why a portion of the energy is inevitably lost.

However, when certain materials are cooled below a critical temperature, something remarkable happens: their electrical resistance suddenly drops to zero. Current can then flow without any energy loss, allowing the material to conduct electricity with extraordinary efficiency. In this state, the material is called a superconductor.

It is important to note that a superconductor is not necessarily a different material. Rather, it is the same material exhibiting a different behavior at extremely low temperatures. The dramatic change arises from a unique quantum state that emerges when the material is cooled sufficiently.

This unique quantum state provides the foundation for SQUID technology.

The second ingredient is the Josephson Junction.

A Josephson Junction consists of two superconductors separated by an extremely thin insulating barrier. Classically, current should not be able to cross the barrier.

But quantum mechanics has other ideas. Thanks to quantum tunneling (read #5), Cooper pairs can pass through the barrier even though classical physics says they shouldn't.

This phenomenon creates a current that is exquisitely sensitive to magnetic fields.

That sensitivity is exactly what makes SQUIDs possible.

A typical SQUID consists of a superconducting loop containing two Josephson Junctions.

When a magnetic field passes through the loop, it affects the quantum mechanical wavefunctions of the superconducting electrons traveling around the loop.

The two paths around the loop interfere with one another, much like light waves interfering in the famous double-slit experiment. This phenomenon is called quantum interference.

Even an incredibly tiny change in magnetic field alters the interference pattern, producing a measurable change in the electrical properties of the device.

In other words:

Magnetic field → Quantum interference → Measurable electrical signal

This allows SQUIDs to detect magnetic fields so small that they would be invisible to almost any other sensor.

Now we can return to our original question. How do we use SQUIDs to study the brain?

As neurons fire, tiny electrical currents flow through neural networks. These currents generate equally tiny magnetic fields that extend outside the skull.

Arrays of SQUID sensors can be placed around a person's head to measure these magnetic fields.

The technique is known as Magnetoencephalography (MEG).

Unlike many imaging methods that focus on the structure of the brain, MEG allows scientists to observe brain activity in real time, tracking neural signals on millisecond timescales.

Researchers use MEG to study:

All of this becomes possible because SQUIDs can detect magnetic fields that are unimaginably small.

Brain imaging is only one application.

SQUIDs are also used in:

Anywhere scientists need to measure tiny magnetic fields, SQUIDs are often among the best tools available.

What makes SQUIDs so fascinating is how many areas of science come together in a single device.

We start with biology and the electrical activity of neurons. We use the connection between electricity and magnetism. We exploit superconductivity, a macroscopic quantum phenomenon. We harness quantum tunneling through Josephson Junctions. And finally, we use quantum interference to detect magnetic fields so small that they would otherwise be lost in the noise.

The result is a machine capable of "listening" to the magnetic whispers of the human brain.