Laboratory flasks are used for explanation during the announcement of the winners of the 2023 Nobel Prize in chemistry at Royal Swedish Academy of Sciences in Stockholm on October 4, 2023.
| Photo Credit: AFP

The story so far: Alexei I. Ekimov, Louis E. Brus, and Moungi G. Bawendi have been awarded the 2023 Nobel Prize for chemistry “for the discovery and synthesis of quantum dots”.

What is a quantum dot?

A quantum dot is a really small assembly of atoms (just a few thousand) around a few nanometres wide. The ‘quantum’ in its name comes from the fact that the electrons in these atoms have very little space to move around, so the crystal as a whole displays the quirky effects of quantum mechanics — effects that otherwise would be hard to ‘see’ without more sophisticated instruments. Quantum dots have also been called ‘artificial atoms’ because the dot as a whole behaves like an atom in some circumstances.

Why are they of interest?

There are two broad types of materials: atomic and bulk. Atomic of course refers to individual atoms and their specific properties. Bulk refers to large assemblies of atoms and molecules. Quantum dots lie somewhere in between and behave in ways that neither atoms nor bulk materials do. One particular behaviour distinguishes them: the properties of a quantum dot change based on how big it is. Just by tweaking its size, scientists can change, say, the quantum dot’s melting point or how readily it participates in a chemical reaction.

Editorial | Inspiring colours: On the 2023 Chemistry Nobel

When light is shined on a quantum dot, it absorbs and then re-emits it at a different frequency. Smaller dots emit bluer light and larger dots, redder light. This happens because light shone on the dot energises some electrons to jump from one energy level to a higher one, before jumping back down and releasing the energy at a different frequency. So, quantum dots can be easily adapted for a variety of applications including surgical oncology, advanced electronics, and quantum computing.

What did the Nobel laureates do?

For centuries, people have been creating coloured glass by tinting it with a small amount of some compound. How much of the compound, or dopant, is added and how the glass is prepared changed which colour the glass finally had. By the late 1970s, scientists had developed techniques to deposit very thin films on other surfaces and observe quantum effects in the films. But they didn’t have a material per se — an object wholly reigned by quantum effects. In the early 1980s, Alexei Ekimov, from the erstwhile Soviet Union, and his colleagues changed this. They added different amounts of copper chloride to a glass before heating it to different temperatures for different durations, tracking the dopants’ structure and properties. They found that the glass’s colour changed depending on the size of the copper chloride nanocrystals (which depended on the preparation process) — a telltale sign that the crystals were quantum dots. In 1983, a group led by Louis Brus in the U.S. succeeded in making quantum dots in a liquid — rather than trapped within glass, as in Dr. Ekimov’s work. Both Dr. Brus and Dr. Ekimov further studied quantum dots, working out a mathematical description of their behaviour and how it related to their structure. But both of them lacked one thing: a simple way to make quantum dots with just the right properties.

A team led by Moungi Bawendi at the Massachusetts Institute of Technology achieved this in 1993, with the hot-injection method. A reagent is injected into a carefully chosen solvent (with a high boiling point) until it is saturated, and heated until the growth temperature, that is, when the reagent’s atoms clump together to form nanocrystals in the solution. Larger crystals form if the solution is heated for longer. Their birth within a liquid makes their surfaces smooth. Finally, crystals of the desired size can simply be filtered out. This method accelerated the adoption of quantum dots in a variety of technologies.

What are quantum dots’ applications?

An array of quantum dots can be a TV screen by receiving electric signals and emitting light of different colours. Scientists can control the path of a chemical reaction by placing some quantum dots in the mix and making them release electrons by shining light on them. If one of the energy levels an electron jumps between in a quantum-dot atom is the conduction band, the dot can operate like a semiconductor. Also, solar cells made with quantum dots are expected to have a thermodynamic efficiency as high as 66%. A quantum dot can also highlight a tumour that a surgeon needs to remove, hasten chemical reactions that extract hydrogen from water, and as a multiplexer in telecommunications.

Paris:

Quantum dots are tiny crystals that scientists can tune to different colours, giving an extra-vivid pop to next-generation TV screens or illuminating tumours inside bodies so surgeons can hunt them down. Three scientists won the Nobel Chemistry Prize on Wednesday for their work turning an idea first theorised in the 1930s into a reality that now has pride of place in living rooms across the world.

What are quantum dots?

Quantum dots are semiconducting particles just one-thousandth the width of a human hair. In 1937, the physicist Herbert Froehlich predicted that once particles were small enough — so-called nanoparticles — they would come under the strange spell of quantum mechanics.

To explain this quantum phenomenon, American Chemical Society president Judith Giordan said to “think of it like a little box”.

When a particle is shrunk down small enough, the electron is “going to whack into the sides of the box,” she told AFP.

In a larger box, the electrons would whack the sides less often, meaning they have less energy.

For quantum dots, the larger boxes emit red light, while the smaller ones show up blue.

This means that by controlling the size of the particle, scientists can make their crystals red, blue and everything in between.

Leah Frenette, an expert on quantum dots at Imperial College London, told AFP that working with the nanomaterial was like “watching rainbows all day”. 

But it would be 40 years after Froehlich’s prediction that anyone was able to actually observe this phenomenon.

In the early 1980s, Russian-born physicist Alexei Ekimov — one of Wednesday’s new laureates — melted coloured glass and X-rayed the results. 

He noticed that the smaller particles were more blue, also recognising that this was a quantum effect.

But being glass, the material was not easy to manipulate — and being published in a Soviet scientific journal meant few noticed.

At around the same time in the United States, another new laureate Louis Brus — oblivious of Ekimov’s work — became the first to discover this colourful quantum effect in a liquid solution.

“For a long time, nobody thought you could ever actually make such small particles, yet this year’s laureates succeeded,” Nobel Committee member Johan Aqvist said.

“However, for quantum dots to become really useful, you needed to be able to make them in solution with exquisite control of their size and surface.”

The third new Nobel winner, French-born Moungi Bawendi, found a way to do just this in his lab at the Massachusetts Institute of Technology in 1993.

By precisely controlling the temperature of a liquid mixture of particles called colloids, Bawendi was able to grow nanocrystals to the exact size he wanted, paving the way for mass production.

What are quantum dots used in?

The most common everyday use of quantum dots is probably in “QLED” televisions.

Cyril Aymonier, head of France’s Institute of Condensed Matter Chemistry, told AFP that the nanocrystals “improve the resolution of the screen and preserve the quality of the colour for longer”.

Doctors also use their bright fluorescence to highlight organs or tumours in the bodies of patients.

Frenette said she is working on diagnostic tests which would use the dots as “little beacons” for diseases in medical samples. 

One problem is that most quantum dots are made using cadmium, a toxic heavy metal.

Both Aymonier and Frenette said they are working on quantum dots that are not toxic.

What’s quantum dots’ future use?

In the future, quantum dots could have the potential to double the efficiency of solar cells, Giordan said.

Their strange quantum powers could produce twice as many electrons as existing technology, she explained.

“That’s amazing because we are coming closer to the limit of current solar materials,” she added.

Were quantum dots used in the past?

While quantum dots are considered on the cutting edge of science, people have probably been using them for centuries without knowing it.

The reds and yellows in stained glass windows as far as back as the 10th-century show that artists of the time unwittingly took advantage of techniques that resulted in quantum dots, according to scientists.

(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)

Left to right: Moungi Bawendi, Louis Brus and Alexei Ekimov.
| Photo Credit: Niklas Elmehed/Nobel Prize Outreach

The 2023 Nobel Prize for chemistry has been awarded to Alexei Ekimov, Louis Brus, and Moungi Bawendi for their work on quantum dots – very small crystals with peculiar properties that have found application in a variety of fields, from new-age LED screens to quantum computers.

Earlier in the day, reports emerged that the Nobel Committee had inadvertently revealed the names of the winners in an email, accessed by the Swedish press, in an unusual break from a tradition in which the identity of the laureates remains a closely guarded secret until the announcement.

Quantum dots are crystals just a few nanometres wide, holding only a few thousand atoms. To compare, a single grain of sand can hold around a sextillion atoms. The electrons in the dot’s atoms are very close to each other, with little wiggle room. At this nanoscale, the effects of quantum mechanics are more apparent.

When some light is shined on a quantum dot, it will absorb and re-emit it at a different frequency, or colour – just like some atoms. Uniquely, the colour depends on the size of the dot: the smaller the dot, the bluer the colour of the re-emitted light. This relationship between size and colour is the result of electrons in the atoms jumping from a lower to a higher energy level, before jumping back. The gap between these levels depends on the size of the dot.

In the early 1980s, Dr. Ekimov and Dr. Brus (separately) synthesised the first quantum dots in glass and a liquid, respectively, proving the existence of such crystals and confirming their ability to fluoresce light of different colours based on their size. But they both had a problem: they couldn’t consistently synthesise high-quality dots.

In 1993, Dr. Bawendi and his team had the answer. They injected small dollops of a compound into a specific solvent until it was saturated, and heated the solution. The compound soon began to coalesce into nanocrystals in the liquid, with the solvent itself giving them a smooth shape. Larger crystals formed when the solution was heated for longer.

“Quantum dots are … bringing the greatest benefit to humankind, and we have just begun to explore their potential,” the Royal Swedish Academy of Sciences said in a statement. “Researchers believe that in the future, quantum dots can contribute to flexible electronics, miniscule sensors, slimmer solar cells, and perhaps encrypted quantum communication.”

Information technology (IT) has become essential to communication, banking, business, health, education, entertainment, and many other walks of our lives. Its prevalence makes us wonder if society can survive without it. IT relies on gadgets that store and process vast amounts of information at humanly impossible speeds.

Gate in computing

A bit is the smallest piece of information storage (it is a portmanteau of binary digit). Often, a large number of bits is required to convey meaningful information. With the advent of modern semiconductor technology, we routinely speak of household computers having a few terabytes (8 trillion bits) of information storage. One terabyte can store 500 hours of high-definition video content.

In a computer, a bit is a physical system with two easily discernible configurations, or states – e.g. high and low voltage. These physical bits are useful to represent and process expressions that involve 0s and 1s: for instance, low voltage can represent 0 and high voltage can represent 1.

A gate is a circuit that changes the states of bits in a predictable way. The speed at which these gates work determines how fast a computer functions.

The quantum gate

Modern computers use semiconductor transistors to build circuits that function as bits. A semiconductor chip hosts more than 100 million transistors on 1 sq. mm. Imagine how small an individual transistor is and how close it is to adjacent transistors. As transistors become smaller, they become more susceptible to quantum effects. This is not desirable as the existing technology will then become unreliable for computational tasks. So there is a limit to how many transistors a computer can have.

Moore’s law, announced in 1965, states that computing power increases tenfold every five years. This law no longer holds as we have already slowed to a two-fold increase every five years. But this doesn’t have to mean we are nearing the end of computing development: the quantum revolution is coming.

The most basic unit of a quantum computer is a quantum bit, or qubit. Like in a conventional computer, it is a physical object that has two states. For example, the spin of a particle can point along two different directions, so the particle can function as a qubit. Or it can be a superconducting circuit that mimics an atom, and its two states can be a ground state, where it has lower energy, and a higher ‘excited’ state.

A quantum gate is a physical process or circuit that changes the state of a qubit or a collection of qubits.

In the quantum-computing context, if particles or superconducting qubits are the physical qubits, the gate is often an electromagnetic pulse.

Interlude: Superposition

A fundamental limitation of conventional computing architecture is that each bit can exist in only one of the two states, 0 or 1. But according to quantum physics, a qubit can also be in a superposition of its two states at the same time.

Imagine you’re walking in the northeast direction. It is equivalent to moving partly along the north and the rest along the east. Your northeast movement is a superposition of walking along the north and along the east. So by combining different distances along the two directions, you can realise some movement in any direction between the two.

The basis states of the qubit are similar to the north and east directions. A qubit in a superposition has some contributions from each basis state. Different superpositions correspond to different amounts of contributions.

If a qubit is in a superposition, then measuring the qubit will cause it to collapse to one of the two states (i.e. either north or east). However, we can only predict the probability that it will collapse to one state. Quantum computers use this to their advantage.

For example, to perform one calculation that requires 16 different inputs, a classical computer requires a total of four bits and sixteen computations. But with four qubits in superposition, a quantum computer could generate answers corresponding to all 16 inputs in a single computation.

Superposition is one of the main factors responsible for speeding up a quantum computer.

But while superposition provides enormous advantages, it is a fragile effect. It deteriorates when qubits interact with their environment. Identifying ways to sidestep or overcome this fragility is an active area of research today.

What gates do

In quantum computers, quantum gates act on qubits to process information. For example, a quantum NOT gate changes the state of a qubit from 0 to 1 and vice versa. The effect of the NOT gate on a superposition is again a superposition, resulting from the action of the NOT gate on each basis state in the initial superposition.

Notably, this feature is common to all quantum gates: the effect of a quantum gate on a superposition is the superposition of the effects of the quantum gate on the basis states contributing to the initial superposition.

So as the quantum NOT gate inter-converts the states 0 and 1, its action is to swap the contributions of the basis states in the superposition.

The Hadamard gate is a type of gate that acts on a single qubit: it generates a superposition of the basis states.

The controlled-NOT, or CNOT, gate acts on two qubits: a control qubit and a target qubit. The control qubit is unaffected by the CNOT gate. The target qubit flips from 0 to 1 or 1 to 0 if the control qubit state is 1.

CNOT plus a few other gates (that act on single qubits) can perform all possible logical operations on binary information encoded on qubits. That is, they can be combined to form quantum circuits capable of processing information.

Research on reliable quantum computers and suitable quantum algorithms is happening in many institutes, universities, and research labs worldwide. Large-scale, reliable quantum computers will benefit industries ranging from drug design to safe communications.

S. Srinivasan is a professor of physics at Krea University.