## 1.2 Quantum interference: the failure of probability theory

Modern mathematical probability theory is based on three axioms, proposed by Andrey Nikolaevich Kolmogorov (1903–1987) in his monograph with the impressive German title *Grundbegriffe der Wahrscheinlichkeitsrechnung* (“Foundations of Probability Theory”).
The **Kolmogorov axioms** are simple and intuitive:^{9}

- Once you identify all elementary outcomes, or events, you may then assign probabilities to them.
- Probability is a number between
0 and1 , and an event which is certain has probability1 . - Last but not least, the probability of any event can be calculated using a deceptively simple rule — the
**additivity axiom**:*Whenever an event can occur in several mutually exclusive ways, the probability for the event is the sum of the probabilities for each way considered separately.*

Obvious, isn’t it? So obvious, in fact, that probability theory was accepted as a mathematical framework theory, a language that can be used to describe actual physical phenomena. Physics should be able to identify elementary events and assign numerical probabilities to them. Once this is done we may revert to mathematical formalism of probability theory. The Kolmogorov axioms will take care of the mathematical consistency and will guide us whenever there is a need to calculate probabilities of more complex events. This is a very sensible approach, apart from the fact that it does not always work! Today, we know that probability theory, as ubiquitous as it is, fails to describe many common quantum phenomena. In order to see the need for quantum theory let us consider a simple experiment in which probability theory fails to give the right predictions.

### 1.2.1 The double slit experiment

In a double slit experiment, a particle emitted from a source

The particle emitted from a source

Following the “quantum rules”, first we add the amplitudes and then we square the absolute value of the sum to get the probability.
Thus, the particle will reach the detector with probability
*modified* by the **interference term** **relative phase** **destructive** interference) or positive (**constructive** interference), leading to either suppression or enhancement of the total probability

The algebra is simple; our focus is on the physical interpretation.
Firstly, note that the important quantity here is the *relative* phase *either* through the upper or the lower slit, because it has travelled through *both*.
In the same way, quantum computers follow, in some tangible way, *all* computational paths simultaneously, producing answers that depend on *all* these alternative calculations.
Weird, but this is how it is!

Secondly, what has happened to the additivity axiom in probability theory?
What was wrong with it?
One problem is the assumption that the processes of taking the upper or the lower slit are mutually exclusive; in reality, as we have just mentioned, the two transitions *both occur*, simultaneously.
However, we cannot learn this from probability theory, nor from any other *a priori* mathematical construct.^{10}

There is no fundamental reason why Nature should conform to the additivity axiom.

We find out how nature works by making intelligent guesses, running experiments, checking what happens and formulating physical theories.
If our guess disagrees with experiments then it is wrong, so we try another intelligent guess, and another, etc.
Right now, quantum theory is the best guess we have: it offers good explanations and predictions that have not been falsified by any of the existing experiments.
This said, rest assured that one day quantum theory *will* be falsified, and then we will have to start guessing all over again.

I always found it an interesting coincidence that the two basic ingredients of modern quantum theory, namely probability and complex numbers, were discovered by the same person, an extraordinary man of many talents: a gambling scholar by the name of Girolamo Cardano (1501–1576).↩︎

According to the philosopher Karl Popper (1902–1994) a theory is genuinely scientific only if it is possible, in principle, to establish that it is false. Genuinely scientific theories are never finally confirmed because no matter how many confirming observations have been made observations that are inconsistent with the empirical predictions of the theory are always possible.↩︎