There’s a Lot of Quantum About These Days
It’s disconcerting to discover how much remains unknown at the quantum level (Image source: pixabay.com)
I just heard that China has developed the world’s first mobile quantum satellite communications station. At first, I didn’t know what to think about this news, but this is largely because I don’t understand “quantum.”
Much of modern physics is based on two incredibly sophisticated and successful theories: general relativity and quantum mechanics. The former works well in describing the macro world we experience in everyday living; the latter is tremendously successful at describing and explaining subatomic phenomena like the quantization of energy, wave-particle duality, and the uncertainty principle.
The problem is that these two theories don’t “play well” together. All this means is that we haven’t yet determined the full picture; nonetheless it’s a little disconcerting to learn that we really don’t have much idea how the universe works. As I wrote in Is Time Truly an Illusion?, the majority of today’s physicists no longer believe in time as an independent, fundamental property or quality of the universe. Instead, the current interpretation is that what we perceive as time is nothing more than an illusion manifested by the human mind, where this illusion is supported by the entropy generated by the functioning of our brains.
Sometimes I wish I was like the character Dr. Sheldon Cooper from The Big Bang Theory. I am, of course talking about his knowledge of theoretical physics, not his annoying quirks (I have enough of those of my own).
As the British science fiction writer, inventor, and futurist Arthur C. Clarke famously said, “Any sufficiently advanced technology is indistinguishable from magic.” To a large extent, this also applies to quantum phenomena, which also look like magic to the uninitiated.
Speaking of which, the Discworld novels, which were gifted to us by the late, great, Terry Pratchett, are set in a disc-shaped land perched on top of four humongous elephants, which are themselves standing on the shell of a ginormous turtle swimming through space. Not surprisingly, the Discworld inhabits a “bubble” of spacetime saturated with magical potential. The point is that the term “quantum” in the Discworld serves much the same function as does the work "magic" here on Earth. As described in The Discworld Companion, “quantum” is a sort of “get-out-of-half-understood-explanation-free card; essentially, an explanation that does not, in fact, explain anything.”
You may think I’m being a tad unfair, but I’m not going to listen to you until you can explain the double-slit (or two-slit) experiment to my satisfaction. Just to refresh ourselves as to how puzzling this is, let’s start by considering an equivalent setup in our macro world (click here to see a larger version of this image).
Macro representation of the double-slit experiment (Image source: Max Maxfield)
Suppose we have a paintball gun firing paintballs at a wall that has a single vertical slit (a). Due to slight variations associated with each shot, many of the balls will splat against the wall, but some will make their way through the slit to create a vertical column of paint splats on a second wall. Now let’s suppose we have two slits in the first wall (b). Not surprisingly, over time, we will end up with two vertical columns of paint splats on the second wall.
Our first two experiments involved particles. Let’s now consider waves. Suppose we have a machine that periodically drops pebbles into a pool of water. Let’s further suppose that these ripples meet a wall with a single slit (c). In this case, a portion of these ripples will pass through the slit to create a new set of ripples on the far side of the first wall. These new ripples will propagate until they come into contact with the second wall, where – if we had an appropriate sensor -- we would detect an area of maximum intensity in line with the first slit.
Finally, let’s repeat our pebbles-in-pool experiment with two slits in the first wall. When the ripples generated by a pebble pass through the two slits, it’s almost like we dropped two pebbles into the water at the slits, thereby generating two new sets of ripples. These two sets of ripples interact with each other – where they are in phase, they boost each other; where they are out of phase, they cancel each other out. The result, as seen at the second wall, is a classic interference pattern.
All of the above makes perfect sense in the macro world, but one’s brain starts to wobble on its gimbals when we create equivalent setups in the quantum world. Let’s suppose we replace our paintball gun with a “photon cannon” that fires a series of photons one at a time. Let’s also suppose that our first wall has two appropriately sized slits.
If we use one sort of detector whose function is to determine through which of the two slits each photon passes, we will end up observing two vertical columns of “photon splats” on the second wall. This would indicate that photons are particles.
However, if we use a different sort of sensor and place these sensors all along the back wall, we will detect a classic interference pattern. Remembering that we are firing the photons one at a time, this would indicate that each photon is passing through both slits simultaneously and then interfering with itself. In turn, this would indicate the photons are waves.
We could just say that photons, which are massless, are tricky little rascals and move on with our lives, but it’s not that easy, because later experiments revealed that the same thing happens with electrons and protons. Even worse (or better, depending on your point of view), the same thing happens with entire atoms and even with multi-atom molecules.
Seriously? A multi-atom molecule can pass through two slits and interfere with itself. Actually, it’s worse than you think, because a 2012 column in LiveScience reported that researchers managed to replicate the double-slit experiment using molecules comprising 114 atoms. More recently, a 2019 article in Scientific American described how an international team of researchers coerced molecules composed of up to 2,000 atoms to perform the double-split experiment. One way to look at this is that they managed to make a 2,000-atom molecule occupy two places at the same time. I don’t know about you, but my mind is well and truly boggled.
We’re not going to talk about quantum computing here – we’ll save that as a topic for another day. At the moment, I’m trying to wrap my brain around quantum entanglement. The idea here is that it’s possible to link, or entangle, two or more subatomic particles together (don’t ask me how). Any measurements of physical properties -- such as position, momentum, spin, and polarization -- performed on entangled particles are found to be perfectly correlated, even when the particles are separated by great distance, such as a base station on the Earth and a satellite in space.
Even weirder is the fact that the communication between the entangled particles occurs instantaneously; that is, faster than the speed of light. When things were still at the theoretical stage, Albert Einstein famously derided entanglement as "spukhafte Fernwirkung" or "spooky action at a distance." Since then, however, this weird and wonderful aspect of the quantum world has been verified in multiple experiments.
One obvious potential use of quantum entanglement is communication; another is cryptography, in which messages can be secured by sharing entangled photons between two sites and using them to establish encryption keys for coding and decoding messages. This is obviously of tremendous interest, because security – or the lack thereof – is now a critical concern.
Are you familiar with New Scientist magazine? Based in London, England, and first published in 1956, which was the year before I decided to grace this planet with my presence, New Scientist is a weekly English-language magazine that covers all aspects of science and technology. I personally think of it as the UK’s equivalent of Scientific American.
In August 2016, New Scientist reported China launches world's first quantum communications satellite, which was called Mozi; in June 2017 we were informed that Chinese satellite beats distance record for quantum entanglement; and, just a few days ago as I pen these words, we heard that China has developed the world’s first mobile quantum satellite station.
This news has triggered multiple thoughts in my mind. For example, the first electronic computers circa the 1940s and 1950s were room-sized behemoths. Now we have more computing power in our wristwatches.
Similarly, the first GPS receivers were the size of large electric toasters. In 1989, the first commercial handheld GPS receiver – the Magellan NAV 1000 – was clunky at best. Its 4-row x 16-character text display could only present alphanumerical information like your current geographical location as latitude and longitude in degrees, minutes, and seconds. Even worse, there weren’t that many GPS satellites around, so the NAV 1000 included a handy-dandy function to inform users when determining their location would be possible at a specific time and place. Believe it or not, these devices initially sold for around US$2,900 each. Now, we have GPS functionality in our smartphones.
When the Mozi satellite launched in 2016, the ground station China used to communicate with it weighed more than 10 tonnes. By comparison, the mobile station announced a couple of days ago weighs around 80 kilograms, which is small enough to be installed on the roof of a car. So, how long will it be before quantum entanglement-enabled communications and cryptology capabilities can be made available in our computers and smartphones?
Unfortunately, this leads me to another thought, which is that China appears to be way out in front of this race, while my impression is that America and its allies are trailing behind. My knee-jerk reaction is that this does not bode well for the future. What say you?