by Tim James
If you do these teleportations enough times you could theoretically transfer every particle property one by one to another particle on a satellite. And if the particle on the satellite becomes identical to the one you started with on Earth, you have effectively teleported it.
DID I MENTION QUANTUM MECHANICS TRAVELS IN TIME?
Kind of. The experiment in question is called the ‘delayed choice quantum eraser’ and it belongs in the forbidden section of Hogwarts library for corrupting the minds of innocent young physicists.
Create an entangled pair of particles and send one of them (Alice) off to a simple double-slit apparatus with a screen on the other side. Then, separately, send Bob off towards a particle detector that will collapse his wavefunction if switched on, but leave him in a superposition if switched off.
Because Bob is entangled with Alice, what happens to him at the detector will immediately affect what happens to her at the double slit. If the detector is switched on when Bob approaches, he will collapse, forcing Alice to do likewise and she will go through only one of the slits as a classic particle. If the detector is off, however, Bob will carry on as a probability wavefunction and Alice will go through both slits at the same time equally uncollapsed, until she hits the screen somewhere in one of the zebra stripes.
Do this a few hundred times and you will end up with a perfect match. If the Bob detector is switched on 15 per cent of the time, 15 per cent of the Alice particles hit the screen in a classical way, while the other 85 per cent end up ‘going zebra’.
If you put the double slit next to where Alice is generated by the entangler (a word which is starting to sound like a pretty awesome X-Men villain) Alice will pass through it and be forced into either particle or wave behaviour, depending on whether Bob is detected or not. But what if Bob’s detector is a kilometre away?
When Alice gets to the double slit, Bob has not yet reached the detector and discovered if it is switched on or off. If Bob is detected, she has to go through the double slit as a particle, but if Bob is not detected, she has to go through as a wave. But Bob has not decided so Alice does not know if the detector is switched on. What does she do?
We have delayed Alice’s choice of whether to collapse or not until after she has to make it, so the effect is being forced to happen prior to the cause. Surely a machine like this is madness? Well, in 1999 Yoon-Ho Kim built one. And Alice got it right every time.5 How in the name of Niels Bohr’s football-chewing ghost is that possible?
If the experimenters set the detector to measure Bob on 42 per cent of the experiments, 42 per cent of the Alice particles go through as particles. If the Bob detector was on 89 per cent of the time, 89 per cent of the Alice particles go through. No matter how often the detector was on, Alice somehow made the right call.
It is as if Alice sees the future and knows what Bob is going to communicate through their entanglement link. Entanglement information can apparently travel faster than instantaneous. So, can we send messages to the past from the future?
Say we fired three pairs of entangled particles through our experiment and watched the Alice electrons. We create a Bob pathway that is so long that Bob does not reach it for twenty-four hours. One of the scientists agrees that in twenty-four hours she will go to the Bob detector and switch it off and on in a specific pattern to describe the weather. ON-OFF-ON means the weather is sunny whereas OFF-ON-OFF means the weather is wet.
The Alice particle will hit the screen in a corresponding way, telling us which order the experimenter is going to set up the Bob detector in the future. We would have successfully sent a message backwards in time.
Sadly, there is a catch once more. We cannot tell by looking at an individual Alice particle whether it went through both slits or one. Each particle hits the screen in a random location, which could be the result of either classical or quantum behaviour. We only observe the stripy quantum effect when we look at thousands of Alice/Bob pairs and compare the percentages.
This means the time-travel effect is only observed at the end of the experiment and not during. We only see quantum effects if we erase our knowledge of what Alice is doing on each individual attempt. Hence the name: delayed choice quantum eraser.
This phenomenon, whatever it is, is happening inside a metaphorically closed bo… chicken and we only see it after the fact. To be honest, quantum mechanics is a flirtatious tease.
THE WORLD SHOULD NOT MAKE SENSE
In the Copenhagen interpretation, quantum effects are said to dominate up to a point called the ‘Heisenberg cut’, after which classical physics takes over. Anything smaller than the Heisenberg cut will obey Schrödinger’s equation and anything larger will obey Newton’s. This is physics speak for ‘we have no idea what is going on’.
The problem is that, thanks to entanglement, the Heisenberg cut cannot really exist. If you start applying quantum mechanics to a single particle you can easily apply them to two, three, four or however many you want. Entanglements can join any number of particles together so it should be possible to describe Schrödinger wavefunctions for whole people, populations and planets.
We should be seeing quantum craziness all the time, but we obviously do not. This was one of the big five questions we met earlier and is actually one of the ones with which we have made decent progress in recent years.
Consider a measurement on an entangled particle which determines a spin state e.g. up-spin. Since its entangled partner particle instantly becomes down-spin, the two no longer have a bond. Entanglement is broken since they are not governed by the same wavefunction and we can describe them as independent eigenstates.
Now, consider the act of measurement itself. What happens when we measure a particle is that the particle becomes entangled with a particle in our detector, severing connections with any particle to which it was previously entangled.
Measuring one particle of an entangled pair is swapping one entanglement for another, which means measuring something is ultimately the same as entangling with it. Even if we are measuring a single particle.
If a lone particle is in a superposition of up/down spin prior to measurement, effectively it is entangled with itself. Its two possible outcomes are linked via the same wavefunction and when we take a measurement we are breaking that self-entanglement.
SCHRÖDINGER’S CAT IS SAVED/KILLED
If we flip a coin we think of there being two outcomes: heads or tails. But there is technically a third possibility. The coin could land on its side, spinning halfway between heads and tails.
Now imagine slowly bringing your finger towards it as it spins. As soon as you touch it, it will collapse to one side or the other, ending the precarious dance. The coin represents a particle with two possible states, the spinning represents a superposition and our finger stands for a measurement apparatus collapsing the wavefunction.
So far, so Copenhagen. Except the analogy is flawed. The finger that touches the coin is not a different type of object. It is a detector made of particles, bound by the same quantum laws as the coin. So, instead of a finger, we should visualise the detector as a new coin being spun across the table towards the one we are interested in. Both the detector and the particle are in a superposition and when they meet, they slam together and both collapse to an eigenstate.
But if we get the speeds of rotation just right, the coins could theoretically mesh and remain in a combined superposition as spinning dance partners – an entangled pair. For real coins this does not happen very often but for particles it happens easily, provided their wavefunctions are aligned.
Imagine how difficult it would be to get a hundred coins spinning together though, twirling in one great big entanglement. Even if you did get it to happen it would be a fragile arrangement. A single rogue coin in the mix or an external coin entering the system could collapse everything. The more particles you have interacting, therefore, the harder it is to get them in superposition.
There is not a specific number of particles that suddenly
makes a system classical. It is just that classical objects have so many particles it is almost impossible to get them all in phase at the same time. The classical world arises because superpositions are unstable but there’s no law against saying we could not put a big object into superposition.
A cat might contain trillions of trillions of particles and if you get each to entangle in perfect synchrony, the superpositions could apply to the whole cat. You are unlikely to see that happening, however, because a single air molecule touching the cat would crash the whole thing.
Schrödinger argued that a cat could not be alive and dead simultaneously because it was metaphysically impossible. He was right, but for the wrong reasons. The radioactive particle in the box can be in superposition, but as soon as it entangles with something bigger it becomes less and less likely to hold on to the superposition state.
If we did somehow manage to get every particle inside a cat perfectly coherent and entangled though, the superposition of dead/alive would decohere as soon as it interacted with the box itself. The only way to keep the cat alive and dead at the same time would be to isolate it from its surroundings. Which is what the physicist Aaron O’Connell did in August 2014.
BIG QUANTUM
The ‘cat’ in O’Connell’s experiment was actually a diving-board-shaped piece of metal sixty thousandths of a metre wide, roughly the size of a human hair. The board was suspended above a miniature swimming pool, placed in a box and cooled to a few degrees above absolute zero in order to stop particles inside interacting with each other out of phase. Any random vibration in the material will collapse everything but if all the particles are cold, the object acts as one big particle.
The board was then wired to a circuit outside the box measuring the amount of current flowing through, thus allowing O’Connell to detect how it was behaving without opening the box directly. Once it was in this cold state, O’Connell switched the machine on and sucked all the air out to prevent entanglements forming. Quantum magic ensued.
The diving board began to vibrate gently and vigorously at the same time. The particles were simultaneously moving a lot and a little, meaning every few nanoseconds the atoms were in two places at once, near the equilibrium position and displaced a long way from it. O’Connell had built the world’s first quantum machine.
In May 2018 Michael Vanner scaled things up by building a quantum drum that could be both struck and not struck at the same time. In his experiment a 1.7 mm membrane (about the width of a grain of sand) was placed in the pathway of photons, which were given a choice to hit it or not. In superposition, both will happen, meaning the drum will vibrate as it absorbs momentum from the photons and also remain untouched as photons choose the less rhythmic pathway.
The vibrations are too small to be seen by the human eye – only a few photons are actually hitting the drum per second – but Vanner’s sensitive apparatus was able to detect photons going along both paths, meaning the drum vibrated and stayed still. Vanner’s experiment is even more remarkable because it works at room temperature.
The biggest quantum phenomenon observed, however, was done the year before, by accident. In 2017 a team of researchers led by David Lidzey were experimenting on Chlorobaculum tepidum bacteria specimens, shining laser light on them inside a mirrored box in an attempt to affect electrons in their photosynthetic cells.
What they did not realise, and was pointed out the following year by Chiara Marletto,6 was that photons inside the laser light were entangling with the bacterial electrons, putting the bacteria into a superposition with the light beams. This was possible because the box was so flooded with light that there was nothing else for the bacteria to interact with, meaning their entanglement links to the lasers could survive for a reasonable amount of time. Quantum phenomena can apparently apply to living things.
We are alive at a truly amazing time in physics history and what is about to happen is unprecedented. Quantum mechanics has always been assumed to be confined to the world of the very small, the microscopic, the world of Marvel’s Ant-man. But in the last few years, we have started applying quantum rules to objects of the everyday macroscopic world. The age of big quantum has begun.
CHAPTER TEN Quantum Mechanics Proves I am Batman
MORE THAN ONE WAY TO SKIN SCHRÖDINGER’S CAT
When trying to grapple with understanding quantum mechanics, the big question is always: can we do better than the Copenhagen shrug of ‘it just happens’?
The majority of physics textbooks teach the Copenhagen interpretation because Bohr was very much the big cheese and for decades his way of doing things was the only way. But nowadays, the Copenhagen interpretation is no longer the only game in town.
Obviously we have to abandon classical ideas sooner or later, and any interpretation of quantum mechanics is going to involve some serious weirdness, but since ye saintly days of Copenhagen, a smorgasbord of alternative approaches has been developed.
It is hard to know which interpretations of quantum mechanics to include in a book such as this and in truth they could fill a library. But I decided to follow my gut and discuss three quantum perspectives that were invented by sci-fi enthusiasts, expanding on work of earlier physics giants.
FORGET WHAT I SAID EARLIER
In 1927 Louis de Broglie gave a lecture at the fifth Solvay conference in Brussels. These conferences were physics get-togethers organised by the Belgian industrialist Ernest Solvay who had made millions in the 1860s inventing an industrial method to manufacture sodium carbonate (a key ingredient in glass making).
Solvay envisioned his conferences as summer camp for scientists. Get the world’s smartest people together in one building for a month and let them thrash out the big topics. Lectures would be given, debates would be organised and if conclusions were not reached, pointy sticks would be distributed.
The first conference in 1911 was on Planck and Einstein’s quantum theory. The second (1913) was on the structure of matter, the third (1921) was about atoms and light, the fourth (1924) was on electricity and the fifth was dedicated to the Copenhagen interpretation and whether it should reign forevermore. In attendance at this legendary event were people such as Schrödinger, Heisenberg, Sommerfeld, de Broglie, Bohr, Born, Planck, Curie, Einstein and numerous others.
There is an iconic photograph of all the scientists in attendance sitting on bleachers like the nerdiest yearbook photo ever taken. Madame Curie is the only woman, Schrödinger is the only one wearing a bow tie and the chemist Paul Debye is the only one sporting a Charlie Chaplin-style moustache, a style which went out of fashion the following decade (for pretty obvious reasons).
At this conference, the softly spoken and affable Louis de Broglie put forward what he felt was a workable alternative to Copenhagen. He decided he had made a mistake introducing wave–particle duality and that electrons and photons were particles only. They did not have wave character, but were instead surrounded by some background substance that did have waves in it. The particles were pushed around by these invisible ‘guide waves’ and thus would appear to move in wave-like trajectories.
According to legend, as he outlined his idea he was heckled loudly by the brash Wolfgang Pauli who had a reputation for interrupting speakers if he did not feel their lecture was up to scratch. Pauli was a superb physicist in his own right (he developed the theory of entanglement we spent the last two chapters explaining) but he was an intimidating presence and de Broglie was basically a nice guy.
De Broglie took Pauli’s interruptions with dignity, admitting there were flaws in his hypothesis, but after the lecture was over people were more interested in Pauli’s questions than de Broglie’s answers and the guide-wave idea was subsequently forgotten.
It was not until 1952 that it was dredged up by David Bohm, a nuclear physicist who had discovered his love of science via sci-fi magazines as a boy1 and worked on the Manhattan project during the Second World War.
Throughout most of his adulthood, Bohm was a Copenhagen sup
porter but after being cajoled by Einstein, he began to feel it required too many leaps of faith and turned to de Broglie’s guide-wave theory instead.
It even seemed to have some decent experimental evidence going for it. If you send a water wave towards a double slit the same way Thomas Young did, you obviously get an interference pattern, but if you put a tiny object such as a droplet of oil onto the surface of these waves, it will ride them like a boat in a storm, carried towards one of the zebra stripes at the far end. It is still a particle but its final destination is determined by the guiding wave.
The challenge he faced was that this system would mean that the electron should follow the same contours each time, but in a real quantum experiment the electron can show up in any of the zebra stripes seemingly at random. To get around this, Bohm proposed that when each electron is launched from an emitter, it has hidden variables inside it – tiny variations in energy we cannot detect, but which lead to different paths taken at the double slit.
Mathematically, the Bohmian view of quantum mechanics has an extra layer of complexity because you have to talk about the values of these guide waves (called quantum potentials) and that requires more equations on top of Schrödinger’s. But in the plus column, it does answer why a particle’s properties appear to get decided on detection – properties such as location and spin come from the guide wave instead of the particle. That is why it looks as if particles do not have properties sometimes, they genuinely do not – the guide wave has them.
BOHM, SWEET BOHM
According to the de Broglie–Bohm view of things, quantum behaviour is not really random because you could, in theory, explain the double-slit results with classical physics. In 2010, Yves Couder created an experiment that claimed to show precisely that.
