The word “quantum” has grown increasingly common. It does not seem so surprising anymore to find in Google search results like “quantum agriculture”, “quantum touch”, and “quantum ESP.” It could be that, in the grand scheme of things this does not matter much, except that I think it should.
I am a Filipino quantum physicist, and I am neither farmer, healer, nor psychic. I do not advocate that physicists be given exclusive rights to the use of the word “quantum,” but I do not think attaching the word “quantum” to make something seem scientific is good practice either. At the very least, I worry that this can turn off the interest of young would-be-(quantum)-physicists (and I am writing this piece especially for them).
Quantum physics is a young subject. Its birth is generally attributed to the German physicist Max Planck who, in 1900, proposed that light is released and absorbed in discrete units, called “quanta”. For example, the quantum of light is called a “photon” (another controversial word, but for another time!).
Quantum physics (or quantum mechanics, to highlight the difference with the mechanics of Isaac Newton) started out as a mathematical description of the physical world at very small scales. Quantum mechanics is a successful theory. It allows us to understand systems as simple as the hydrogen atom, or systems as complex as your computer’s microprocessor.
Quantum mechanics is also a baffling theory. It suggests some peculiar phenomena that have excited and distressed many great minds, even including Albert Einstein.
Perhaps the phenomenon most responsible for the excitement and distress is what we call “entanglement.” Let me illustrate with two thought experiments:
How ordinary objects behave
Imagine a red ball and a blue cube, each of which is placed in a closed box. In Manila, one box is given to Maria and the other box is given to Jose. Maria then goes to Baguio, and Jose goes to Cebu.
They know that the object in their box can either be a red ball or a blue cube, but they cannot open their boxes until they reach their destinations. In Baguio, Maria opens her box and she finds a red ball. The very instant that she sees the red ball, without any communication with Jose, she can predict that Jose has the blue cube.
Similarly, the very instant that Jose sees the blue cube, without any communication with Maria, he can be sure that Maria has the red ball. If Maria finds a red ball, Jose finds a blue cube, and vice versa. If Jose finds the red ball, Maria finds the blue cube, and vice versa. One event implies the other, not surprisingly so. We need not invoke any magical power possessed by either Maria or Jose to explain their instantaneous knowledge of what is inside the other person’s box.
To state the obvious: they can know what the other one has because they know that the box contains either a red ball or a blue cube. More importantly, these are ordinary objects: the red ball is a red ball and the blue cube is a blue cube regardless of whether Maria and Jose were looking inside the box. In this sense, the red ball being a red ball is independent of the blue cube being a blue cube, and vice versa.
Now you’re probably thinking, what a boring thought experiment!
The strangeness of quantum objects
Let us now repeat the experiment, but replace the objects inside the box with “entangled” quantum objects.
Quantum physicists like to talk about the “state,” rather than objects. For ordinary objects, the state is really just a list of its properties such as color and shape. In the standard interpretation of quantum physics, objects do not have a state in the same sense as do ordinary objects. Strange as it seems, the state of quantum objects is not determined until an observation is made.
It is like having a magical object in each of the boxes that Maria and Jose carry, a magical object that is both a red ball and a blue cube at the same time. Whether the object is a red ball or a blue cube is only determined when the box is opened. If we are to perform the same experiment, the result is the same: if Maria finds a red ball, Jose finds a blue cube, and vice versa. One event implies the other, as before.
However, this situation is fundamentally (and very strangely) different. Apparently, the observation of a red ball in one location depends on the observation of a blue cube in another location. The properties of one object can no longer be separated from the other, hence the term “entangled”, and the phenomenon we call “entanglement.”
Our state of affairs
Naturally, Einstein was not happy with quantum mechanics. In his own words: “I cannot seriously believe in the quantum theory because it cannot be reconciled with the idea that physics should represent a reality in time and space, free from spooky actions at a distance.”
He had the luxury to debate with his friends then (the physicists Niels Bohr from Denmark and Pascual Jordan from Germany, for example) because the discussions remained largely in the realm of thought. The first experiments that convincingly showed the existence of entangled states were in the early 1980s, long after the death of the early pioneers of quantum mechanics.
Whether we like it or not, entanglement is experimentally established. We can find objects that exhibit these bizarre correlations; we generate them routinely in the laboratory. They overturn our dearly held notions of reality and space and time.
The peculiarities have inspired a counterculture. Some go all the way as to explain ESP with quantum theory (and as an experimentalist, I have serious doubts!). Some use quantum theory as they invoke a world-mind, collective consciousness and other similar ideas. All these are understandably tempting, considering that Einstein himself has hailed entanglement as “spooky”. Quantum theory opens a lot of questions that keep many people (including me!) occupied, and amused.
Fortunately, the peculiarities also promise to bring about important technologies, some of which are already being used.
Most advances have been made in the area of quantum communication. Here, the security of the information transferred between the sender and receiver is guaranteed by quantum physics, by making the presence of a malicious eavesdropper inevitably detectable by the communicating parties. Most notably, the technology has been used for bank transfers (in Vienna, 2004), Swiss elections (in Geneva, since 2007) and the FIFA World Cup video feeds (Durban, 2010).
It is these two facets: fundamental and philosophical on one hand, and yet technologically game-changing on the other, that has drawn me to quantum physics. It is my mighty hope that these are the facets that will come to mind when we encounter the word “quantum.”
Jacquiline Romero obtained her undergraduate and masters physics degrees from the University of the Philippines. She is currently a physicist at the University of Glasgow, where she obtained her Ph.D. and does quantum entanglement experiments using light. The author thanks Edward Carlo Samson for suggesting the Sean Carroll post, and Beatriz Torre for filtering through “quantum” Google search results.