This year, two of the biggest science prizes in the world, the Nobel Prizes and the Breakthrough Prizes, went to researchers in the field of quantum science. Alain Aspect, John F Clauser and Anton Zeilinger shared the Nobel Prize in Physics for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science, while Charles H Bennett, Gilles Brassard, David Deutsch and Peter Shor, won the Breakthrough Prize in Fundamental Physics for pioneering the field of quantum information.
Jayanth Vyasanakere and Murthy OVSN, physicists from Azim Premji University, combined their knowledge and expertise to break it all down for us.
“It’s funny because though neither of us has actually worked in quantum computation, Jayanth’s area—superconductors—is where many of the results in quantum computation have so far come from, whereas my area—semiconductors—is one of the promising candidates for scaling up quantum computers,” pointed out Murthy, as we settled into a fascinating discussion.
Let’s start with quantum mechanics. Why did the need for it arise?
Murthy: Newton, and the people around at the time, gave us a very nice theory that could explain the motion of objects in the classical world, that is the world that we can see around us, and even extending to planets, stars and galaxies. For example, “what happens if you drop a ball” can be explained using classical theory.
But people noticed that atomic or subatomic particles did not follow the same kind of laws. There were similar issues with light as well. So, we had particles that behaved like waves, and light, which physics tells us is a wave, was behaving like particles. We needed a theory that could explain all of this consistently, and quantum mechanics does this.
It offers a description that works for particles, for light, and even for things like “spin”— which is not actually spin the way we think of it. Nobody knows what exactly we mean by the “spin” of the electron, but we are able to work with it [thanks to quantum mechanics].
People noticed that atomic or subatomic particles did not follow the same kind of laws (of classical mechanics). There were similar issues with light as well. So, we had particles that behaved like waves, and light, which physics tells us is a wave, was behaving like particles. We needed a theory that could explain all of this consistently, and quantum mechanics does this.
What does quantum mechanics actually tell us?
Jayanth: In classical mechanics, the aim is to get the position of the particle as a function of time. If you are able to specify the position, you’re done. Whereas in quantum mechanics, we can get the wave function at any given time; you’re not specifying that this particle will be found here. Already, some kind of ambiguity has come in the way we are describing the world.
So what can I do with this wave function? I can talk about the probability of finding the particle at this location, or that location. We cannot describe the precise location of the particle.
So quantum mechanics doesn’t give us an exact answer about the position of a particle. Is that a weakness of the theory?
Jayanth: That was the question. Is it our ignorance? This is what the ‘realist school of thought’ believed. According to this, the fact that we are only able to give a probabilistic or statistical description of the location of a particle is only our ignorance. There is a more detailed theory of mechanics yet to be uncovered, which would probably tell us precisely where the particle would be found. The realists said that quantum mechanics is some kind of approximate science.
But there was another position called the ‘orthodox position’ or the Copenhagen interpretation. According to this, it’s not our ignorance — quantum mechanics is doing whatever can be done best. It’s just that the position of the particle never existed before we actually tried to make a measurement. The act of measurement forced the particle to take a stand.
This was the debate. Einstein led the realist position, whereas Niels Bohr and others led the orthodox position.
An article headline regarding the Einstein – Podolsky – Rosen (EPR) paradox paper, in the 4 May 1935 issue of The New York Times:
It’s important to note that it’s not that Einstein disbelieved quantum mechanics, it’s just that he took a position saying there is a more complete theory yet to be unearthed.
However, the majority, I would say, took a third stand — the agnostic view. These people said ‘okay, while the rest of you discuss the conceptual foundations of quantum mechanics, we will go ahead with the calculations’.
They said that there was no point in debating whether the particle really had a position before the measurement. Because how do we verify if something was there before the measurement? All you can do is measure and see what is there now, not what was there before. So these people believed that this question can never be settled.
Can you elaborate on the realist vs orthodox schools of thought?
Jayanth: Let me use an example motivated by Einstein, Podolsky and Rosen’s 1935 argument. Consider the decay of a neutral pion (a subatomic particle). It decays into an electron and a positron. The conservation of angular momentum tells us that if the electron has spin “up”, the positron has to have spin “down”, and vice-versa. The question is which among these two combinations will actually occur.
Quantum mechanics predicts that the state of the system is “up down minus down up”, or non-committal. This means that whether this particle has spin-up or spin-down is undecided. All that can be said is that if you’re going to make a measurement of the electron and it turns out to be spin-down, then the positron has to have spin-up.
Suppose you make a measurement on the electron and you find that it has spin-down, a realist would say that the electron always had spin-down. That’s why the measurement showed so. Whereas, the orthodox position would say that there was no spin assigned to the electron until the time of measurement.
It was the measurement that forced the electron to take this spin-down state. And the positron becomes spin-up. Conversely, if the electron happened to be spin-up, the positron will immediately be forced to take the spin-down state.
Einstein said that for the orthodox position to be true, something that is happening here has to influence something at some distance away, and this influence happens instantaneously. This influence at a distance is problematic… spooky. This came to be called “spooky action at a distance.” Because we have something that is travelling at a speed faster than light — and this is against relativity, which by 1935 was a well-established theory.
It’s not that Einstein disbelieved quantum mechanics, it’s just that he took a position saying there is a more complete theory yet to be unearthed.
It’s not that Einstein disbelieved quantum mechanics, it’s just that he took a position saying there is a more complete theory yet to be unearthed.
How did the quantum debate eventually get settled?
Jayanth: In 1964, John Bell made a breakthrough. He assumed that there was a more complete theory as Einstein suggested. This complete theory involved some “hidden variables”, that were yet to be understood. Based on this, Bell came up with some inequalities.
Mathematically, inequality is something that is less than something or bounded by something. Bell showed that if there did exist a more complete theory than quantum mechanics (i.e., the realist position), then this set of inequalities would need to be satisfied.
Bell’s inequality was independent of the nature of the hidden variable, so this eliminated the agnostic position. Remember the agnostic position said that the question of what existed before measuring could never be answered, but Bell’s inequality showed that it could.
The realist versus orthodox debate could be settled through experimentation, after all. If the experiment obeyed Bell’s inequalities, the realists would be correct (and also, quantum mechanics would require modifications). If it did not, then the orthodox school would be correct.
Here is an illustration depicting the difference between the realist position (hidden variable) and the orthodox position (quantum mechanics):
There is a machine that throws out balls of opposite colours (or spin) in opposite directions. When Bob catches a ball and sees that it is black (or spin-up), he immediately knows that Alice has caught a white one (spin-down).
In a theory that uses hidden variables (realist), the balls had always contained hidden information about what colour (spin) to show. However, quantum mechanics (orthodox) says that the balls were grey (no spin) until someone looked at them, when one randomly turned white (spin-down) and the other black (spin-up).
Bell inequalities show that there are experiments that can differentiate between these cases. Such experiments have proven that quantum mechanics’ description is correct.
So who did the experiments and what was the verdict?
Jayanth: John Clauser, and later Alain Aspect, designed some fantastic experiments — not with spin, but with photons. They measured the polarisation of photons and their correlations and showed that Bell’s inequalities were violated. This means that there is no (local) hidden variable. This proved that the orthodox position was indeed correct.
But if spooky action is real, does that mean that relativity is being violated?
Jayanth: Einstein had earlier objected that if something happening here instantaneously impacts something at a distance, then a violation of relativity was taking place. However, it turns out that this is not a violation.
The main intent of relativity is that no causal signal can travel faster than light. A causal signal is something that actually carries information. In this case, whether the electron is spin-up or spin-down is completely probabilistic, and we cannot force the state of the positron by “deciding” what the state of the electron should be. That’s why spooky action is not actually a violation of relativity.
How do we go from quantum mechanics to teleportation?
Murthy: This state of ‘up down minus down up’, where the two states of — say, a positron and electron — are correlated to each other, is called entanglement. This phenomenon of entanglement was utilised to come up with the scheme of teleporting.
This means that a state in one place can be transferred to another place. Technologically, these teleported states are useful — for example, you can have very strong encryption or cryptographic schemes that cannot be broken. This is not like the Star Wars kind of teleportation, but the teleportation of quantum states.
Can you break down for us the discoveries that won Alain Aspect, John F Clauser, and Anton Zeilinger their Nobel Prize in Physics in 2022?
Jayanth: The foundations of this quantum mechanics are still being debated. And this Nobel, to some extent, is about people who have tried to settle the debate.
Clauser was the one who first came up with experiments to check Bell’s inequalities. He decisively showed that Bell’s inequalities were violated. Aspect was the one who removed certain loopholes in these experiments.
To be precise, the orientation of the detectors was quasi-randomised, so that the orientation didn’t influence the state of the particles, to begin with. Zeilinger won the prize for his experiments on quantum teleportation.
Quantum algorithms use features of quantum mechanics like a superposition of states, entanglement and measurement to solve problems that conventional computers cannot.
Not just computers of today, but even futuristic conventional classical computers, because the classical computers use the digital bit, which is either zero or one. But quantum mechanics uses a “qubit”, which can take a continuum of values [between two states].
There is a lot of hype about quantum computers. What is special about them?
Murthy: Current classical computers are really fast. They can outperform humans in things like chess, or mathematical problems. They can solve these problems very fast. But these are things that we also can do. There are still mathematical problems that are not within the reach of classical computers.
To give you an example, let’s take this lore of the chess board. When the King offered the inventor of chess to name his award, the inventor asked him for grain. How much grain? He asked the king to place a single grain on the first square of the chess board, and then double it for each subsequent square until all 64 squares were filled. The king happily said yes, but soon realised that this would drain all the grain! This is kind of representational of the kind of speed-up that quantum computers can give us.
Quantum algorithms use features of quantum mechanics like a superposition of states, entanglement and measurement to solve problems that conventional computers cannot. Not just computers of today, but even futuristic conventional classical computers because the classical computers use the digital bit, which is either zero or one. But quantum mechanics uses a “qubit”, which can take a continuum of values [between two states].
A qubit is a quantity that you can encode using quantum mechanics, and encode in particles that obey quantum mechanics. You can design algorithms to do these very clever things.
If a chessboard were to have wheat placed upon each square such that one grain were placed on the first square, two on the second, four on the third, and so on, the total number of grains would be eighteen quintillions, four hundred forty-six quadrillion, seven hundred nine million, five hundred fifty-one thousand, six hundred and fifteen, which is over 2,000 times the annual world production of wheat.
How long do we have to wait to get ourselves a quantum computer?
Murthy: You’re not going to have a quantum computer in your pocket anytime soon! That’s out of the question. One of the major challenges in quantum computation is that of scaling. Right now, we are working with a few qubits. The most that I know of is only six, but we need to scale them to numbers like 2048 qubits. It’s not easy to maintain and monitor such large coherent quantum states.
There have been developments in error correction to overcome noise issues, but we are still some way off from having a quantum computer in our hands. Some, in fact, even say that it may not be possible to maintain such a superposition for a long time or monitor so many variables. They are highly sceptical of this whole quantum technology.
Can you break down for us the work of Charles H Bennett, Gilles Brassard, David Deutsch, and Peter Shor, who won the Breakthrough Prize in Fundamental Physics this year for pioneering the field of quantum information?
Murthy: It’s interesting that while the Nobel Prize committee gave away prizes honouring the work that tried to settle the conceptual foundations of quantum mechanics, the Breakthrough Prizes were awarded for the technological revolution that we hope to see using quantum mechanics.
For instance, Peter Shor developed a fantastic factorisation algorithm that uses entanglement that uses measurement trickery to factorise large numbers in highly sped-up times. This kind of factorization can break the cryptography that we depend on for everyday banking and e‑commerce.
On the other side, the very fact that you cannot copy a quantum state can be utilised to have a cryptographic protocol that will be stronger than any conventional protocol that you can think of. Conventional bits, like digital data, can be copied endlessly. Quantum mechanical states cannot be copied.
In 1984, Bennett and Brassard came up with one of the first quantum mechanical protocols for cryptography — incidentally, that protocol was announced at a conference here in Bengaluru!
So, while Bennett and Brassard strengthened cryptography quantum mechanically, Peter Shor demonstrated that classical cryptography can be broken easily.
David Deutsch, the other Breakthrough Prize winner, won it for his description of what could be called a universal quantum computer. Now, that is a mathematical formalism to describe quantum computers, in a similar way that we think of Alan Turing with his universal computing ideas.
Turing came up with a mathematical idealisation of what a computer should be, and [classical] computers of today or even tomorrow would be dependent on those ideas. So, Deutsch’s ideas on the universal quantum computer are similar to that.
Watch Jayanth and Murthy discuss quantum mechanics and the prize-winning discoveries:
- Winners of the 2023 Breakthrough Prizes in Life Sciences, Mathematics, and Fundamental Physics announced. (2022, September 23). Breakthrough Prize. https://breakthroughprize.org/News/73
- How entanglement has become a powerful tool. (2022, December 21). The Nobel Prize. https://www.nobelprize.org/prizes/physics/2022/popular-information/
- Einstein and the EPR Paradox. (2005, November). American Physical Society (APS) Advancing Physics. https://www.aps.org/publications/apsnews/200511/history.cfm
- Brubaker, B. (2021, July 20). How Bell’s theorem proved ‘spooky action at a distance’ is real. Quanta. https://www.quantamagazine.org/how-bells-theorem-proved-spooky-action-at-a-distance-is-real-20210720/
- O’Connell, C. (2019, July 5). Quantum computing for the qubit curious. Cosmos. https://cosmosmagazine.com/science/quantum-computing-for-the-qubit-curious/