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Physicist Jim Al-Khalili charts the full history and implications of quantum mechanics in this hour-long discourse at the Royal Institution, tracing the framework from its pioneers in the 1920s through to the technologies now being built on its foundations. He argues that quantum mechanics, despite remaining philosophically unresolved and deeply counterintuitive, stands as the most experimentally validated framework in science. The talk moves chronologically from the early breakthroughs of Planck, Bohr, and Heisenberg through the mid-century refinements, then pivots to explain what makes quantum mechanics so strange—superposition, tunneling, and entanglement—before turning to its practical applications, past and coming.
Al-Khalili gives sustained attention to entanglement and the historical puzzle it posed. He walks through the EPR paradox, Einstein's famous objection to "spooky action at a distance," and how Bell's theorem provided a testable prediction. He describes the experiments by Clauser, Aspect, and Zeilinger that proved Einstein wrong, confirming that entanglement is not merely incomplete knowledge but a real feature of nature. From there, the scope widens considerably. He traces how the first quantum revolution produced lasers, semiconductors, computers, GPS, and MRI machines—essentially all of modern electronics. The second quantum revolution, now underway, encompasses quantum clocks of extraordinary precision, quantum sensing that can detect single neurons firing, ghost imaging that sees objects indirectly, and quantum computing, though he tempers expectations there, noting that practical machines remain one to two decades away. Throughout, he emphasizes that entanglement may be so fundamental that space and time themselves could emerge from it.
Quantum mechanics, despite its profound counterintuitivity and 100-year history, has evolved from a theoretical framework into practical technologies that are revolutionizing computing, imaging, sensing, and timekeeping through exploitation of quantum phenomena like entanglement, superposition, and tunneling.
- Quantum mechanics transformed from an abstract theory of the atomic world into the foundation of modern electronics, GPS, and medical imaging
- A second quantum revolution is now emerging with quantum computing, quantum imaging, quantum sensing, and quantum communication leveraging quantum entanglement and superposition
- Quantum effects once thought merely weird philosophical puzzles are now engineered into practical devices that outperform classical approaches
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The sun shines because of quantum tunneling: hydrogen nuclei (protons) can quantum tunnel together as the first step toward nuclear fusion into helium, enabling thermonuclear fusion that produces the sun's energy.
“the sun shines because of quantum tunneling. Hydrogen uh nuclei protons can quantum tunnel together in the first step towards nuclear fusion to make helium”
In quantum mechanics, what you observe depends on what you measure: if you probe a photon's wavelength it behaves as a wave, and if you measure its position it behaves as a particle, and until measured it is neither but a mixture of both.
“in quantum mechanics, the lesson is that what you get depends on what you measure. If you want to see it behaving like a wave, sure enough, it'll oblige and behave like a wave.”
The EPR paradox argues that since measuring one of two entangled photons instantly determines a property of the distant photon without touching it, that second photon must have possessed those properties from the start, implying quantum mechanics is incomplete rather than that spooky instantaneous communication occurs.
“they they could know or could have known both the position and momentum of photon 2 without interacting with it. How could it possibly have that? It must have had those properties from the start”
Quantum entanglement means two photons are described by a single shared quantum state, so neither has decided whether it is a particle or wave; forcing one photon to make up its mind instantly affects what the other is doing regardless of distance.
“Quantum mechanics says photon one and photon 2, neither of them have decided whether they're a particle or a wave... they are combined together. We say they're quantum entangled. They're described by the same quantum state.”
Experimental tests of Bell's inequality by Clauser (early 1970s), Aspect (early 1980s) and Zeilinger consistently yielded correlation values greater than two, proving Einstein wrong and confirming that quantum entanglement is real; the three won the 2022 Nobel Prize in Physics for this work.
“all three of those men had to wait a long time 2022 they won the Nobel Prize for tests of Bell's inequality what do they find quantum mechanics is right you get a number more than two Einstein couldn't be right”
The first quantum revolution produced essentially all of modern technology—lasers, diodes, LEDs, integrated circuits, computers, GPS, the internet, smartphones, MRI machines, and electron microscopes—because understanding the quantum world gave us semiconductors and chips.
“the first quantum revolution involved developing many devices that relied on these ideas in quantum mechanics. The laser is is a famous one. But of course we have the diode, LEDs, integrated circuits, the computer, GPS, internet... smartphones.”
Quantum mechanics is not a theory but rather a mathematical framework for understanding the workings of the microcosm—the subatomic world of atoms and beyond—equivalent to how classical Newtonian mechanics is a framework for understanding everyday forces, momentum, and energy.
“Quantum mechanics isn't a theory. Quantum mechanics is a framework for un a mathematical framework for understanding the workings of the microcosm, the subatomic world, the world of atoms and and and and beyond. In the same way that classical Newtonian mechanics isn't a theory again that's a framework for understanding the workings of our everyday world, forces and momentum and energy and so on.”
Bell's theorem (1964), formalized by the CHSH inequality, provided a testable formula: if nature is local (as Einstein believed), a correlation quantity between entangled particles cannot exceed two; a measured value greater than two proves non-local instantaneous entanglement.
“if uh if if Einstein's right and nature is local then there's a formula that says there's only so much that... those two particles can have in common... This quantity here cannot be greater than two.”
The Copenhagen interpretation holds that the atomic world cannot be pictured or visualized; quantum mechanics only gives predictions about the results of measurements, and one cannot speak about what an atom is doing when not being observed.
“the atomic world wasn't something that we could picture we couldn't talk about an atom as a miniature solar system... What quantum mechanics could give you, they argued, was predictions about the results of measurements”
Quantum entanglement is so prevalent that it is probably the most important feature of physical reality, and some current fundamental theories suggest entanglement may be the source code of reality, with even time and space themselves possibly emerging from it.
“current theories in very the fundamentals of physics are even suggesting that quantum entanglement may be the the the source code base of reality. Even time and space themselves may emerge from quantum entanglement”
Optical lattice clocks use lasers to pump energy into atoms that emit at optical (higher) frequencies rather than microwave, yielding far more accurate timekeeping—so accurate they would lose less than a second over the entire age of the universe.
“these atomic clocks are so accurate they lose something like less than a second over the age of the entire universe.”
Ghost imaging exploits quantum entanglement so that one can obtain an image of an object using entangled photons where the photon that interacts with the object is not the one that reaches the camera—allowing you to see something while not directly looking at it.
“ghost imaging is this wonderful idea where you can look at something while you're actually not looking at it. You send light to probe it and then light in another direction goes to a camera and in the camera is the image of the thing that you weren't looking at.”
Quantum sensing using atom interferometry with entangled atoms can measure Earth's gravity extremely accurately, including at sea or underground, and a brain-scanner version worn like a cycling helmet is sensitive enough to detect the magnetic field of a single firing neuron—offering an alternative to confining MRI scanners.
“a brain scanner... using lasers and atoms and quantum entanglement that is so sensitive they can pick up the weakest of magnetic fields even the magnetic fields created by the firing of a single neuron in the brain”
Quantum tunneling is real and not merely theoretical—the sun shines because hydrogen nuclei (protons) can quantum tunnel together, initiating the first step toward nuclear fusion that creates helium and the thermonuclear fusion energy of the sun.
“Um quantum tunneling is real and not just because the guys won a Nobel Prize for it uh this year. Um the reason we are here today is because we get life we we life gets energy uh light and warmth from the sun and the sun shines because of quantum tunneling. Hydrogen uh nuclei protons can quantum tunnel together in the first step towards nuclear fusion to make helium and that's what creates a thermonuclear fusion the energy of the sun.”
Four physicists—Clauser, Horn, Shimony, and Holt—formulated Bell's inequality (called the CHSH inequality) as a neat mathematical inequality that experimentalists could test to determine once and for all whether Einstein or Bohr was correct.
“he said this formula it was it was formed in the in in in as an inequality. Uh four other physicists Clauser Horn Shimon Halt called the CHSH inequality put it in a nice neat mathematical uh formulation that allowed experimentalists to go away and check it once and for all. Can we shut Einstein up?”
The 1927 Solvay Conference in Brussels is often regarded as the most intelligent picture ever taken and marked the point when the full quantum mechanics framework had evolved from old quantum theory, attributed to both Heisenberg and Schrödinger.
“Quantum mechanics culminated in this famous conference, the fifth Solve conference uh on physics held in Brussels in 1927. Very often this photograph is regarded as the most intelligent picture ever taken.”
In the summer of 1925, Werner Heisenberg, while recovering from hay fever on the treeless island of Helgoland in the North Sea, developed the mathematical framework that became quantum mechanics and fundamentally changed our view of the universe.
“100 years ago. Um, in the summer of 1925, our view of the universe changed forever. A young German scientist, Vera Heisenberg, was famously recovering from a bout of hay fever on the treeless island of Helgoland in the North Sea, barren island. Um, and while there, while recuperating, he uh came up with the mathematical framework that today we call quantum mechanics.”
Ghost imaging is a quantum imaging technique where you can see something without directly looking at it—you send light to probe an object and light in a different direction goes to a camera that captures an image of the unprobed object through quantum entanglement.
“So ghost imaging is this wonderful idea where you can look at something while you're actually not looking at it. You send light to probe it and then light in another direction goes to a camera and in the camera is the image of the thing that you weren't looking at. It's I know I mean it's it's crackers, right? But it relies on this idea of quantum entanglement. The fact that you know like the two photons that go back to back, they're still entangled. They're still talking to each other.”
Marie Curie is the only woman in the 1927 Solvay Conference photograph and is the only person in the photograph who won two Nobel prizes, while none of the other physicists present achieved that distinction.
“Mari Cury, the only woman in the whole photo photograph. However, she has something over the rest of them. She won two Nobel prizes. None of them managed that.”
Einstein and Niels Bohr engaged in a famous debate at the 1927 Solvay Conference about the nature of reality, and simplified versions of history suggest that Bohr won the argument, establishing the Copenhagen interpretation.
“at that conference and often the way history is taught uh it would be suggested that quantum mechanics was complete. It was finished. They'd sorted it all out. Now let's go and use it to understand physics and chemistry and and build the modern world by this time. Famously the 1927 uh Solve conference was also a time when um the two giants of the field namely Einstein and Neils Boore had their famous debate about the nature of reality and um simplified versions of history would say well look bore won the argument.”
John Clauser first performed the Bell's inequality test at Berkeley in the early 1970s, followed by Alan Aspect and collaborators at Orsay, France in the early 1980s, and then many other experiments, particularly by Anton Zeilinger, an Austrian physicist.
“Well, that experiment first was first done by uh John Clauser uh at Berkeley in the early 1970s. uh then repeated in the early 80s by Alan Aspect and his collaborators in Orsay in France and then uh many other experiments particularly by an Austrian physicist Anton Zylinger”
In the 1950s, physicists like Eugene Wigner and John Wheeler considered whether quantum mechanics had fully answered the foundational questions, and they were sympathetic to maverick scientists like David Bohm and Hugh Everett who developed alternative theories to explain quantum phenomena without accepting the Copenhagen view.
“There were still very powerful physicists in the 1950s people like um Eugene Vner and John Wheeler who were at least prepared to think about is is is it completely answered? Is it is are we happy that we understand what's going on in the quantum realm? They were certainly more sympathetic to young scientists who dared, and I use that term sort of quite literally, they dared to argue against the Copenhagen school. People like David Bow and Hugh Everett. They came up with alternative theories that would explain the EPR paper, that would explain the cat in the box uh thought experiment.”
Decoherence as a concept was developed mainly by Dieter Zurek up to about a half-century ago, and it is now a very well-developed idea in physics that solves puzzles like Schrödinger's cat paradox.
“Decoherence is something that was developed mainly by Daz and VC Zoric uh um up to half a century ago now. Um and uh it's a very well-developed idea in in in physics. It solves things like the Schroinger's cat paradox.”
GPS works because navigation satellites carry atomic clocks that measure time extremely accurately, which is necessary to measure distance from the satellites precisely enough to triangulate a phone's location.
“those satellites that are sending radio signals to your phone have atomic clocks on board. And those atomic clocks are needed to measure time very very accurately in order for us to measure the distance from those satellites very accurately because that's the only way you can triangulate where you are.”
Quantum electrodynamics (QED), developed by Feynman, Schwinger and Tomonaga in the 1940s, applies quantum mechanics to all matter and electromagnetic radiation and is often regarded as the most accurate theory in all of science, describing most phenomena except gravity.
“Quantum electronamics is often regarded as as as the most accurate theory in all of science. Very very powerful.”
Quantum mechanics is not a theory but a mathematical framework for understanding the microcosm, analogous to how classical Newtonian mechanics is a framework for the everyday world rather than a theory.
“Quantum mechanics isn't a theory. Quantum mechanics is a framework for un a mathematical framework for understanding the workings of the microcosm”
Quantum tunneling allows a particle given insufficient energy to surmount an energy barrier to nonetheless disappear and reappear on the other side with a certain probability, behaving like a ghost passing through a solid wall, because particles are waves and waves can leak through the barrier.
“There's a certain probability that it gets halfway up, disappears, reappears on the other, and rolls down like a ghost phantom walking through a solid wall.”
Quantum mechanics explains the structure of the periodic table by governing the rules for how electrons arrange themselves around atomic nuclei, which in turn determine the physical and chemical properties of the elements.
“Quantum mechanics explains the rules for how electrons arrange themselves around atomic nuclei and give us the properties of the elements. Without quantum mechanics, you wouldn't understand why and how the periodic table looks the way it does.”
Decoherence is the process by which the everyday world disturbs and destroys delicate quantum entanglement; for example, detecting whether one photon is a particle or wave destroys the entanglement, and this concept (developed largely by Zeh and Zurek) resolves paradoxes like Schrödinger's cat.
“decoherence is when our everyday world messes with disturbs quantum entangle quantum entanglement... if you measure it... you destroy the entanglement”
Einstein was unhappy with Boris Podolski leaking the EPR paper to the New York Times and publicizing it in this manner, not because he disagreed with the paper's content but because Einstein felt the work was not actually his own.
“But in fact, Einstein didn't write that paper. It was Boris Podsolski who'd written the paper. And Einstein was unhappy to say the least that Boris Podolski then leaks it to the New York Times as well. So he didn't like the idea that it was being publicized in this way, not because he didn't agree uh with with the sentiments of the paper, but it wasn't really his his work.”
Entanglement-based imaging cameras are being used to detect tiny details in breast cancer tumor biopsies, picking up far more detail than infrared cameras alone because the visible-light photon provides much sharper images while the entangled infrared photon probes the object.
“This is now being used to uh pick up um tiny details in in biopsies of tumors in breast cancer. They can pick up far more detail than you could just with infrared cameras for example because visible light cameras are much more precise, much sharper images.”
A second quantum revolution, sometimes called Quantum 2.0, is now underway based on emerging quantum technologies, though the speaker notes that 'Quantum 2.0' terminology is misleading since there is no 'Quantum 3.0'.
“But we now have a second quantum revolution. Sometimes it's called quantum 2.0 Z, but that suggests there's a 3.0 and there's like it's like, you know, it's like Windows something. It's not. It's it's the second quantum revolution.”
A four-qubit quantum processor uses quantum entanglement to link the qubits together, with information fed into the system, quantum operations performed, and results extracted—representing a small-scale demonstration of quantum computing.
“this is a example uh provided by a researcher at the quantum center for the uh quantum computing N Baker. Um this is a a 4 cubit quantum processor. These blue squares are the cubits. They're quantum entangled. They're linked together and you have to feed in information. They do their job and then they send the information out.”
Despite corporate claims from Google, IBM and others that quantum computers are imminent, realistically practical quantum computers are one to two decades away because of challenges in scaling up qubits, error correction against decoherence, uncertainty over the physical platform, and the scarcity of quantum algorithms.
“you'll hear the big um corporations working at Google and IBM and others saying you know we've we've done it. We've cracked it... Realistically we're talking about one or two decades away before quantum computers actually arrive.”
Current fundamental physics theories suggest that quantum entanglement may be the source code or base of reality, with even time and space themselves possibly emerging from quantum entanglement.
“current theories in very the fundamentals of physics are even suggesting that quantum entanglement may be the the the source code base of reality. Even time and space themselves may emerge from quantum entanglement is one suggestion.”
Quantum computing relies on superposition so that a quantum bit (qubit) can be both 0 and 1 at the same time rather than a binary 0 or 1, and could eventually aid drug discovery, battery and solar panel design, finance, logistics, and simulation of subatomic systems.
“it relies on this idea that uh of superposition that rather than having a logic uh gate that's 0 or one binary digits, now we have zero and one at the same time.”
Quantum timing, or quantum clocks, are the first technology of the second quantum revolution, relying on the discrete energy levels of electrons in atoms to generate microwave radiation of extremely precise frequency.
“Quantum timing is is is the is the the first one. Uh by which I mean quantum clocks. The atomic clock. Now the original atomic clocks uh they rely on an an early feature of the the quantum world namely this idea that uh uh electrons and atoms can't have any old energy. They can't orbit around in any distance from from the the nucleus. Uh they can they have to have discrete energies. uh and that is used in order to uh uh generate uh in the early atomic clocks um uh light well microwave radiation coming from um certain type of atom that you can measure the wavelength of very very accurately.”
Quantum sensing is another technology already with us today that has many applications, including measuring Earth's gravity very accurately, both at sea and underground.
“quantum sensing I is another uh technology that is already with us today. Uh it has lots of many applications. For example, you can use it to d to measure earth's gravity very very accurately. Um you can even measure it out at sea with these devices are very versatile or down underground.”
In 2022, Clauser, Aspect, and Zeilinger won the Nobel Prize in Physics for tests of Bell's inequality, confirming that quantum mechanics is right and Einstein could not be right—quantum entanglement is real.
“and in fact all three of those men had to wait a long time 2022 they won the Nobel Prize for tests of Bell's inequality what do they find quantum mechanics is right you get a number more than two Einstein couldn't be right quantum entanglement, however weird you might think it is, is real.”
Many of the quantum technologies being developed rely on quantum entanglement, making quantum entanglement not just philosophically weird but practically important for developing devices that will benefit humanity.
“but more importantly, many of the technologies I'm going to mention in a moment rely on quantum entanglement and use quantum entanglement. So, it's not just weird, it's, you know, however weird you might think it is, it's being used. We're developing devices that are going to benefit humanity because quantum mechanics is real.”
Physicists have built increasingly large particle accelerators to smash matter together at ever-higher energies, breaking it into tinier constituents and eventually arriving at the Standard Model, which describes the building blocks of matter as far as currently known.
“uh we've carried out we've we've built larger and larger particle accelerators to smash matter together at ever higher energies to to break it up into tinier and tinier constituents and we arrive at this the standard model which I mentioned earlier which tells us the building blocks of of matter as far as we know so far”
The modern periodic table of elements is classified according to the physical and chemical properties of elements, and this classification is explained by quantum mechanics—specifically by the rules governing how electrons arrange themselves around atomic nuclei.
“A modern periodic table of the elements as you would see on any school wall in the world...the reason why the periodic table is classified in this way according to uh the physical and chemical properties of the elements is because of quantum mechanics. Quantum mechanics explains the rules for how electrons arrange themselves around atomic nuclei and give us the properties of the elements.”
Without understanding quantum mechanics, we would not have electron microscopes, photomultipliers, or MRI machines—medical and scientific instruments whose operation depends fundamentally on quantum mechanical principles.
“let alone things like the the electron microscope the the uh photo multiplier which is a device that many of you will not have even heard of but is is is used in so many other instruments um MRI machines they're quantum it's not just sticking a patient in a strong magnetic field it works because of quantum mechanics”
Modern electronics as a whole depends on understanding semiconductors and their quantum mechanical properties, which enabled computer chips and all digital technology.
“the whole of modern electronics is really thanks to our understanding of the quantum world because it told us gave us understanding of semiconductors and semiconductors understanding of chips and so on and we have computers.”
Atomic clocks are the basis of global navigation systems (GPS), enabling smartphones and services like Google Maps by providing satellites with extremely accurate time measurements, which are necessary to triangulate position.
“These sorts of atomic clocks are the basis of uh uh global navigation system GPS systems. The reason uh your smartphone, the reason you can use Google Earth and find out or or or or Google Maps to find out where you are because those satellites that are sending radio signals to your phone have atomic clocks on board. And those atomic clocks are needed to measure time very very accurately in order for us to measure the distance from those uh satellites very accurately because that's the only way you can triangulate where you are.”
Quantum computing relies on the idea that rather than logic gates being 0 or 1 (binary digits), quantum gates can be 0 and 1 simultaneously, enabling quantum computers to explore multiple computational pathways in parallel.
“Quantum computing of course is is the famous one. Um and it relies on this idea that uh of superposition that rather than having a logic uh gate that's 0 or one binary digits, now we have zero and one at the same time.”
Superconducting quantum computers use superconducting wires that lose electrical resistance when cooled to near absolute zero, requiring complex cryogenic systems (like Google's famous chandelier-style cryostat) to cool the qubits.
“Well there are lots of uh um candidates. One of them is is is is using what are called superconducting uh cubits. So this is a famous chandelier kryostat developed by Google. Um essentially what you're looking at is basically the wiring that cools down the the the real thing that's doing the quantum computation down to near absolute zero.”
Trapped ion quantum computers use ions (atoms that have gained or lost electrons and are thus charged) rather than neutral atoms, with the advantage that electromagnetic fields can hold them in place more easily than using magnetic fields.
“And then there are others like the uh like trapped iron quantum computers. So they're like neutral atoms, but an ion, remember, is an atom that's gained or or lost electrons, so it's charged. The advantage there is that you can use electromagnetic fields to hold them in place.”
A quantum brain scanner using lasers, atoms, and quantum entanglement can detect the weakest magnetic fields, including those created by the firing of a single neuron, enabling brain monitoring without the need for conventional MRI scanners.
“on the right is a is a is a brain scanner. So like a cycling helmet invol using lasers and atoms and quantum entanglement that is so sensitive they can pick up the weakest of magnetic fields even the magnetic fields created by the firing of a single neuron in the brain”
In the EPR paradox scenario, two photons are produced with equal and opposite momentum and energy; if one measures the wavelength of photon one (a wave property), one instantly knows the wavelength of photon two without measuring it, but if one measures the position of photon one (a particle property), one instantly knows the position of photon two.
“imagine you have uh uh some device that produces two particles of light, two photons, and it sends them out back to back equal and uh opposite momenta, equal energies...What if we stick some instrument onto photon one to measure its wavelength?...you know photon 2 has the same energy therefore the same wavelength you instantly know without measuring anything to do with photon 2 you know what its wavelength is. It's behaving like a wave. However, we could instead have used a different instrument that measures the position of photon one. That is its particle property. And if you know where photon one is, photon 2 will also be behaving like a particle uh in the sense that they they have equal and opposite momentum.”
Einstein objected to quantum mechanics by questioning whether unobserved objects truly lack properties, asking if the moon is not there when he is not looking and whether observation creates reality for quantum particles.
“Einstein famously said, "If everything ultimately is is is made of uh quantum particles, then is the moon not there when I'm not looking? Do I only bring it into existence when I observe it as you're telling me happens when I observe atoms?"”
The original quantum theory seed began in 1900 when Max Planck suggested that radiation given off by warm bodies is not continuous but discrete—coming in lumps or separate drips rather than as a smooth stream.
“the original quantum theory the original seed of the idea began in the year 1900 thanks to another German physicist Max plank. Plank suggested that radiation given off by warm bodies isn't continuous like a the stream of water from a tap but if you look carefully enough it's discreet. It comes in lumps like separate drips.”
Einstein extended Planck's idea by proposing that all electromagnetic radiation—light in all its wavelengths—can be understood as quantized particles called photons.
“A few years later Einstein extended that idea and said that actually all electromagnetic radiation light um in all its wavelengths ultimately can also be thought of quanticles of light which we now call photons.”
Niels Bohr extended quantum ideas to describe the energy of electrons as they arrange themselves around an atomic nucleus, proposing that electron energy comes in discrete levels rather than allowing electrons to have any energy they want.
“Neils Boore himself, the Danish physicist uh extended the idea to describe uh the energy of atoms or more correctly the energy of electrons as they arranged themselves around the atomic nucleus...The idea of things are quantized. The energy in of the electrons around the atom also comes in discrete levels discrete values. You could they could electrons couldn't have any energy they wanted.”
Rutherford's discovery revealed that atoms consist mostly of empty space with a tiny dense nucleus in the middle and electrons around the outside, resembling a miniature solar system—a picture that was soon discovered to be incorrect.
“Ernest Rutherford had uh given us the very first inkling of what an atom looks like. Mostly empty space, a tiny uh dense nucleus in the middle with electrons around the outside. Uh that was a picture of the atom that looked like a miniature solar system which we soon discovered wasn't really quite correct.”
Both Werner Heisenberg and Erwin Schrödinger developed distinct mathematical approaches to quantum mechanics, but these approaches were soon discovered to be equivalent, and modern quantum mechanics teaching uses a mixture of both.
“Heisenberg and Schrodinger both developed mathematical approaches to understand the atomic world. Uh, but rather different in in in their uh their approach. But it was soon discovered that they were actually equivalent. And today we when we talk about quantum mechanics, when we teach quantum mechanics to students, we use a mixture of both.”
Throughout the 20th century, quantum mechanics evolved from the old quantum theory of 1900 to fully-fledged quantum mechanics by the 1920s, then advanced to quantum field theory by the 1940s through work by Feynman, Schwinger, and Tomonaga on quantum electrodynamics.
“if you think about the the uh the progression of quantum mechanics throughout the 20th century, we start with the old quantum theory of 1900. Uh by the 1920s, we have fullyfledged quantum mechanics. By the late 20s, people like Paul Dak were already developing um advances on quantum mechanics, saying well the other big theory in physics is relativity theory...people like Paul Dak started to connect that with quantum mechanics uh and and and we get the the early ideas in what's called quantum field theory that develops by the 1940s people like the great American physicists Richard Fineman together with Schwinger and Tommanaga they develop what's called quantum electronamics”
Despite being a revolutionary scientist himself, Einstein remained reluctant to embrace all the predictions and implications of quantum mechanics, publishing the famous EPR paper in the mid-1930s with Boris Podolski and Nathan Rosen to outline his issues with quantum mechanics.
“Einstein, for example, still hadn't quite given up the battle. He despite being a revolutionary scientist himself coming up with his two theories of relativity uh he was reluctant to embrace all the predictions uh and and implications of quantum mechanics. In the mid 1930s he uh published a very famous paper together with two colleagues Boris Podolski and Nathan Rosen.”
Erwin Schrödinger, another physicist who sided with Einstein, wrote a three-paper set of articles in Nature the same year as the EPR paper, in which he first described his famous cat in the box—the cat that is dead and alive at the same time until observed.
“another physicist who sided actually with Einstein was Schroinger, Win Schroinger, the Austrian physicist. Um, that same year, not long after the EPR paper comes out, he wrote a a three paper um set of articles in in Nature. This is where he first describes his famous cat in the box. The cat that's dead and alive at the same time until you open the box to look.”
Einstein later wrote to physicist Max Born complaining about what quantum mechanics says is happening in the EPR paper, and in this letter he first uses the term 'spooky action at a distance' to describe how photon two and photon one could be instantaneously communicating despite being far apart.
“a few years later um Einstein writes a letter to another physicist Max Bourne uh and uh he complains about what quantum mechanics says is going on in his EPR paper uh and and he that's where he first uses the term spooky action at a distance. It's perfect German as far as I'm concerned. Um because how can photon 2 and photon one be instantaneously communicating with each other?”
John Bell was an Irish physicist who famously disliked the Copenhagen interpretation and was reluctant to confront Niels Bohr about it when they were caught in a lift together at some point, but later in the 1960s developed a groundbreaking theorem.
“another maverick outsider in the 1960s was the Irish physicist John Bell. Now, John Bell famously didn't like the Copenhagen view. He he he talks of a time when he was uh uh uh caught in a lift with the great Neils Boore. And he says, "I I just didn't have the courage to to to confront him about the Copenhagen view and say I wasn't quite happy with it."”
Bell's theorem states that if Einstein is right and nature is local (meaning interactions can only occur between nearby things at the speed of light or slower), then there is a formula—an inequality—that constrains the maximum correlation that two particles can have based on their shared origin.
“He said if Einstein's right, if nature is local, now local means you can only interact with things uh close enough to you and if it's far away then you need some time before you can influence it. Time the fastest you can get to it is by sending a light signal...he said so if uh if if Einstein's right and nature is local then there's a formula that says there's only so much that like for example in the EPR photons there's only so much those two particles can h have in common given that they were both at a single origin”
If experimental measurements show values greater than two in the Bell inequality test, the photons must be communicating after they've separated, indicating a non-local connection—quantum entanglement.
“So if they measure these things in an experiment and you get a number more than two, they must be communicating after they've left there must be some what's called non-local connection between them. Instantaneous quantum entanglement.”
Entanglement cameras are being developed for practical medical applications, including picking up tiny details in biopsies of breast cancer tumors far better than classical infrared cameras, providing sharper images because visible light cameras have higher resolution than infrared.
“It's not wacky science fiction. This is now being used to uh pick up um tiny details in in biopsies of tumors in breast cancer. They can pick up far more detail than you could just with infrared cameras for example because visible light cameras are much more precise, much sharper images.”
Researchers at Imperial College and the University of Birmingham have developed demonstration entanglement cameras that were shown at this summer's Royal Society Summer Exhibition.
“researchers at the Imperial College and University of Birmingham who working on these uh these ideas uh are showing it off at the this year's this summer's summer exhibition at the Royal Society just up the road from here.”
There are multiple competing physical platforms for building quantum computers—superconducting qubits, photonic qubits, neutral-atom qubits cooled and manipulated by lasers, and trapped-ion qubits held by electromagnetic fields—and it is not yet known which will prove the most efficient and powerful.
“We don't know which of these quantum uh computers is going to be the one that ends up being the most efficient, the most powerful, the one that's going to give us quantum computing in in the years to come.”
Quantum computers have not yet been built at an advanced level comparable to quantum imaging and sensing technologies, representing a less mature frontier of quantum engineering.
“Have we built quantum computers yet? Well, we're getting there. Not quite as advanced as the imaging and the sensing technologies.”
Building quantum computers requires solving the fundamental question of which physical system to construct qubits from, with multiple competing approaches including superconducting qubits, photonic qubits, neutral atom qubits, and trapped ion qubits.
“So how do you build one? Well there are lots of uh um candidates. One of them is is is is using what are called superconducting uh cubits...There are other um candidates quantum computers using light using photons called photonic cubits. Uh there are uh neutral atom cubits...And then there are others like the uh like trapped iron quantum computers.”
Gravity is the last remaining major phenomenon in nature that has not been brought under the framework of quantum mechanics, and reconciling it is the central challenge of fundamental physics.
“the big challenge in in fundamental physics is how do you bring gravity in? That's the last remaining big phenomenon in in nature that hasn't come under the spell of quantum mechanics.”
The Copenhagen interpretation, associated with Niels Bohr and developed by his group of young geniuses including Heisenberg and Pauli, argued that the atomic world cannot be pictured intuitively and that quantum mechanics can only provide predictions about measurement results, not descriptions of reality itself.
“his view together with his uh uh uh uh group of young geniuses Heisenberg and Powley created what we today call the Copenhagen picture which is a view of reality they argued for was forced upon us by the mathematics by the theory uh and by experimental evidence which suggested that the atomic world wasn't something that we could picture we couldn't talk about an atom as a miniature solar system with electrons buzzing around nucleus. You you you couldn't even imagine what an atom was like. It was maths. It was abstract. What quantum mechanics could give you, they argued, was predictions about the results of measurements if you were to do, if you were to look”
Quantum mechanics successfully explained many experimental results that had been unexplained until that point, demonstrating the correctness of the quantum framework despite the philosophical mysteries at its heart.
“And quantum mechanics explained so many experimental results which until then people hadn't really hadn't been able to figure out. It turned out quantum mechanics got it right.”
Quantum electrodynamics is often regarded as the most accurate theory in all of science, capable of describing essentially all phenomena apart from gravity that we see around us today through its account of how matter and electromagnetic radiation interact.
“Quantum electronamics is often regarded as as as the most accurate theory in all of science. Very very powerful. Pretty much most of phenomena apart from gravity that that we see around us today, we can describe using quantum electron dynamics.”
The primary unresolved challenge in fundamental physics is bringing gravity under the spell of quantum mechanics, as gravity remains the last remaining major phenomenon in nature that has not been unified with quantum field theory.
“And today and certainly for the last few decades, the big challenge in in fundamental physics is how do you bring gravity in? That's the last remaining big phenomenon in in nature that hasn't come under the spell of quantum mechanics.”
Superposition and interference are properties of waves, as demonstrated by Thomas Young with water waves and light waves, but in the quantum world, particles can also exhibit superposition—existing in multiple states or locations simultaneously—and these states can interfere with each other.
“superp position and interference. Now superp position interference are properties of waves...When we say waves superimpose on each other, we can we mean this sort of thing. drop two pebbles in in water and the waves, the concentric circles that move outwards will overlap...Well, in the quantum world, particles also can can can have this interference effect. Um, but it's a superposition not of waves. You can have superp positions of particles for example being in having two energies at the same time or being in two locations in space at the same time.”
Quantum tunneling is the phenomenon where a particle with insufficient energy to classically pass over an energy barrier (like a ball not kicked hard enough to reach a hilltop) can appear on the other side with some probability, as if passing through a wall like a ghost.
“So quantum tunneling is another um uh aspect of of the quantum world that's strange. Um, the example that I like to give is, and many people like to give, is if you have a ball and a hill, you have to kick the ball up the hill hard enough if you want to get it to the top and over to the other side. Uh, if you don't kick it hard enough, it's going to go halfway up the hill and then come back down again. In the quantum world, replace ball with atom or electron or whatever and replace hill with some energy barrier. You can give that atom, say, enough energy to get halfway up the hill and it won't roll back down again. There's a certain probability that it gets halfway up, disappears, reappears on the other, and rolls down like a ghost phantom walking through a solid wall.”
Quantum entanglement appears to be far more weird and counterintuitive than quantum tunneling or quantum superposition, being so outlandish that for many years it seemed like an extremely strange aspect of quantum mechanics.
“And then there's quantum entanglement. Remember the EPR, spooky action, existence. Quantum entanglement will seem to be um for many years this really extremely outlandish counterintuitive aspect of quantum mechanics. It's far far weirder than than quantum tunneling or quantum superposition interference.”
Decoherence is quantum entanglement's 'annoying little brother'—when the everyday world disturbs or measures quantum entanglement (such as by detecting which photon property is being displayed), the entanglement is destroyed and the system is forced to make a definite choice.
“And entanglement comes along with something else called decoherence. That's its annoying little brother. Uh decoherence is when our everyday world messes with disturbs quantum entangle quantum entanglement you know the two photons back to back uh that's a very delicate quantum state if you measure it you open Schroinger's box for example or you you you detect whether photon one is a particle or a wave you destroy the entanglement you force the other particle to do what it uh uh what it's told what it should do to complement the first on.”
The speaker personally met John Bell at an American Physical Society meeting in Baltimore in 1989 while finishing his PhD, and had the courage that Bell lacked—to challenge someone promoting a quantum mechanics idea the speaker thought was nonsense.
“Well, many years later in in the late 80s, I was called to lift with John Bell. Um, this was at a American Physical Society meeting in Baltimore in the 1989. I was finishing my PhD and uh looking for posttock positions in America and I'd just come out of a a talk on quantum mechanics given by someone who clearly was promoting some some new idea which I thought was nonsense. uh and I got on the lift with John Bell and unlike John Bell he didn't have the courage to confront the great Neil spore I had the courage because I agreed with John Bell”
The speaker published a paper last year with PhD student Lance and colleague Paul Stevenson studying the atomic nucleus using a classical computer pretending to be a quantum computer to test whether quantum computing could provide advantages for solving important theoretical physics problems.
“uh just last year I published a paper with a my PhD student uh uh Lance and and my colleague Paul Stevenson where we're trying to study the atomic nucleus uh using so although there's no quantum computer yet we can use a classical computer pretending to be a quantum computer uh uh to see if that gives it an advantage in in solving some of these uh important problems in theoretical physics.”
Today, quantum entanglement is known to be so prevalent that it is probably the most important feature of physical reality, yet quantum entanglement is still not taught seriously to physics undergraduates, which the speaker thinks is a mistake.
“Today we know that entanglement is so prevalent that it's it's it's it's probably the most important feature of physical reality. We still don't teach quantum entanglement seriously to undergraduates in in in in physics at university and I think we should because it's it's it is so important”
Einstein did not actually write the EPR paper—Boris Podolsky wrote it and leaked it to the New York Times, which displeased Einstein not because he disagreed with its content but because of the publicity and that it was not really his work.
“Einstein didn't write that paper. It was Boris Podsolski who'd written the paper. And Einstein was unhappy to say the least that Boris Podolski then leaks it to the New York Times as well.”
The complexity of quantum computing hardware integration is illustrated by the difficulty of connecting non-quantum electronics (like connecting a skybox to a TV), suggesting that mastering the lasers, mirrors, and wiring of quantum systems is daunting even for experienced physicists.
“It's not easy. I mean, look look at the mess. That's I can't even connect my skybox to my TV with the wiring at the back. I don't know how it's lasers and mirrors and wires and that this is why I became a theoretical physicist because that that that would that would give me absolute nightmares.”
Max Planck initiated quantum theory in 1900 by proposing that radiation from warm bodies is not continuous but discrete, coming in lumps or quanta like separate drips rather than a continuous stream.
“Plank suggested that radiation given off by warm bodies isn't continuous like a the stream of water from a tap but if you look carefully enough it's discreet. It comes in lumps like separate drips.”
Superconductivity, where materials cooled to near absolute zero lose their electrical resistance so current can flow without hindrance, is one of the quantum phenomena underpinning superconducting qubit quantum computers.
“materials cooled down to near absolute zero lose their chem their um electrical resistance in their wiring and so there's no reason so they so the current can flow through without any hindrance”
Heisenberg developed the mathematical framework now called quantum mechanics in the summer of 1925 while recuperating from hay fever on the island of Helgoland.
“A young German scientist, Vera Heisenberg, was famously recovering from a bout of hay fever on the treeless island of Helgoland in the North Sea... he uh came up with the mathematical framework that today we call quantum mechanics.”
Marie Curie was the only woman in the 1927 Solvay conference photograph and is distinguished by having won two Nobel Prizes, which none of the other attendees managed.
“Mari Cury, the only woman in the whole photo photograph. However, she has something over the rest of them. She won two Nobel prizes. None of them managed that.”
The future is coming very clearly and very fast, with quantum technologies emerging rapidly from theoretical proposals to practical demonstrations.
“but the the future is is very close. It's coming to us very clearly uh very fast.”
Niels Bohr said that if you are not astonished by quantum mechanics then you clearly haven't understood it.
“the famous quote by the great uh Danish physicist Neils Boore who says if you're not astonished by quantum mechanics then clearly you haven't understood it.”
Niels Bohr did not initially hold the most prominent position at the 1927 Solvay Conference (he was in the second row, not among the 'grandees' at the front), but the 'young guns' like Heisenberg and Pauli sat near the front, showing the generational shift in quantum physics.
“Neils Boore at this point is only in the second row. He's not even one of the grandees yet. But then you get the young guns Heisenberg and Powley and Schroinger.”
The clip from his 2013 Royal Institution lecture on the two-slit experiment, where he challenged anyone with a common-sense explanation to contact him for a Nobel Prize recommendation, has been viewed several million times and still generates one or two emails a week from people claiming to have solved it.
“That clip has been viewed several million times. I to this day 12 years later I get one or two emails a week from people saying I have solved the two experiment.”
The speaker previously gave a talk on quantum biology at a previous Friday evening discourse at the Royal Institution, which he will not repeat in this lecture.
“Some of these devices are with us today and uh I want to end by thanking I did give a talk on quantum biology uh as a as at a previous discourse. So I will I will I won't have to say anything uh about that now.”
The speaker thanks the Institute of Physics (IOP) for giving him the honor of delivering this lecture, which culminates the UK contribution to the International Year of Quantum Science and Technology.
“But I want to particularly I would like to thank the IOP for giving me the the the honor of of of uh giving this this lecture culmination of uh the UK contribution to international year of quantum science and technology.”
Quantum communication and quantum encryption are future technologies that will enable secure quantum internet linking computers together, though the speaker chooses not to elaborate on them in detail.
“I want to very quickly then end um with uh uh talking about quantum uh uh communication uh quantum encryption. I don't want to say too much about these are things in the future. Uh the quantum internet would link computers together.”