Spin qubits | QuTech Academy

Summary

This script chronicles the evolution of semiconductor technology, from the first transistor in 1947 to the development of integrated circuits and quantum bits. It delves into the intricacies of isolating and controlling individual electrons using quantum dots and charge sensing, highlighting the potential for large-scale quantum circuits. Collaborations with industry leaders like Intel are paving the way for standardized production of quantum dot arrays, aiming for thousands of qubits in compact, efficient designs.

Takeaways

• 🔬 The first transistor was demonstrated in 1947, marking a significant milestone in semiconductor technology.
• 📻 The first transistor radio was developed shortly after the invention of the transistor, utilizing multiple transistors along with capacitors and resistors on a circuit.
• 🛠 The initial transistor technology had limitations due to the manual assembly of components, which led to the development of the integrated circuit in 1958.
• 💡 Integrated circuits allowed for the integration of multiple components in a single piece of germanium or silicon, paving the way for complex processors and memory chips.
• 🚀 In the last decade, the same core technology of transistors and integrated circuits has been used to create high-quality quantum bits.
• 🛑 A transistor functions as a switch that controls electron flow using a gate voltage, and by modifying this design, quantum dots can be created to isolate electrons.
• 🔋 Quantum dots are small spaces where electrons are isolated, and at the extreme limit, a single electron can be isolated to represent a quantum bit.
• ⚡ The concept of charging energy, resulting from the Coulomb repulsion between electrons, is crucial for controlling the number of electrons in quantum dots.
• 🔍 Charge sensing is a method used to detect the presence of individual electrons by observing changes in current through neighboring quantum dots.
• 🔄 The development of large-scale quantum circuits requires uniformity and precision, which has led to collaborations with industry-standard clean rooms like Intel.
• 🔗 Cross-bar technology, similar to that used in displays and memory chips, is being explored to wire and control two-dimensional arrays of quantum dots.
• 🌐 The vision for quantum computing includes the integration of local arrays with quantum links to transfer entanglement and classical electronics for efficient signal distribution.

Q & A

What was the significance of the first transistor demonstrated in 1947?

The first transistor in 1947 marked a pivotal moment in the development of semiconductor technology, paving the way for the miniaturization and integration of electronic components, which eventually led to the creation of more complex devices such as transistor radios.

How did the invention of the transistor radio represent a limitation in the early stages of semiconductor technology?

The transistor radio, despite being a remarkable innovation, highlighted the limitations of manually soldering components onto a circuit board. It raised the question of how many components could realistically be assembled in this manner, indicating the need for a more efficient method of integration.

What was the second major advancement in semiconductor technology mentioned in the script?

The second major advancement was the demonstration of the integrated circuit in 1958, which integrated all relevant components monolithically in a single piece of germanium or silicon, leading to the development of powerful processors and memory chips used today.

How are quantum bits related to the technology of transistors and integrated circuits?

Quantum bits, or qubits, are built using the same core technology of transistors and integrated circuits. The development of high-quality qubits in the last decade has been facilitated by the ability to isolate and control individual electrons using the principles that govern transistor operation.

What is a quantum dot and how is it used in the context of qubits?

A quantum dot is a small space where electrons are isolated below a gate, separated from other puddles of electrons. By taking the concept of a transistor and using multiple gates to isolate individual electrons, the spin of these electrons can be used as a quantum bit for quantum computing.

What is the concept of charging energy in the context of quantum dots?

Charging energy is the energy resulting from the Coulomb repulsion between electrons. In the context of quantum dots, the charging energy is the energy required to add one single electron charge to the quantum dot, which can be larger than the thermal energy at low temperatures, enabling the control of individual electron charges.

How is the number of charges on a quantum dot controlled at low temperatures?

At temperatures as low as 4K, the charging energy required to add a single electron charge can be larger than the thermal energy, allowing for precise control of the number of charges on the quantum dot, effectively enabling the isolation of individual electrons.

What is Coulomb blockade and how does it relate to the flow of electrons in a quantum dot?

Coulomb blockade is a phenomenon where the flow of electrons is blocked due to the energy required to add an electron being larger than the energy available from the Fermi energy of the reservoirs. This blockade can be lifted by adjusting the gate voltage to align the electrochemical potential within the biased window between source and drain, allowing individual electrons to move through the quantum dot.

What is charge sensing and how is it used to detect individual electrons in quantum dots?

Charge sensing is a method that involves using a neighboring quantum dot to detect the presence or absence of individual electrons in a primary quantum dot. A single charge in the neighboring dot can shift the energy levels in the primary dot through capacitive coupling, changing the current flow in a measurable way.

How does the script suggest scaling up from simple quantum dot arrays to large-scale quantum circuits?

The script suggests that to scale up, quantum dots need to be made as identical and uniform as possible. It also mentions the need for industry-standard clean rooms for producing high-quality quantum dot arrays and the use of cross-bar technology for wiring in two-dimensional arrays. The integration of classical electronics with qubits and the development of quantum links for transferring quantum information are also discussed.

What is the envisioned size of a single array containing 1024 qubits according to the script?

The script envisions that an array containing approximately 1024 qubits could be integrated into an area no larger than 30 x 30 micrometres, using cross-bar technology for efficient wiring.

Outlines

00:00

📻 The Birth of Transistors and Quantum Bits

This paragraph outlines the historical development of semiconductor technology, highlighting the invention of the transistor in 1947 and its integration into the first transistor radio. It discusses the limitations of manual assembly and the subsequent milestone of the integrated circuit in 1958. The narrative then shifts to recent advancements, describing the creation of high-quality quantum bits利用 transistor and integrated circuit technology. The explanation delves into the construction of a transistor as a switch and the concept of quantum dots for isolating electrons, which are essential for quantum computing. The paragraph also covers the technical aspects of controlling individual electrons through charging energy and Coulomb repulsion, and the method of detecting electron addition via electrochemical potential alignment, known as Coulomb blockade.

05:02

🔍 Charge Sensing and Large-Scale Quantum Circuits

The second paragraph introduces charge sensing as a method for detecting individual electron presence, utilizing the interaction between adjacent quantum dots. It explains how a change in charge in one quantum dot can affect the energy levels in another, leading to measurable current changes. The paragraph further discusses the visualization of this effect through gate voltage space diagrams, illustrating the alignment and shifts of charge transitions. Moving towards practical applications, it emphasizes the need for uniformity in quantum dot arrays for large-scale circuit development, mentioning a collaboration with Intel to produce such arrays using 300 mm technology. It also touches on the challenges of wiring in two-dimensional arrays and the potential use of cross-bar technology, similar to that used in displays and memory chips, to manage a large number of qubits efficiently. The vision for quantum integrated circuits, including quantum links for information transfer and classical electronics for signal distribution, is also presented.

10:03

🚀 The Challenge of Realizing Complex Quantum Circuits

In the concluding paragraph, the speaker acknowledges the immense challenge of creating complex quantum circuits. It reflects on the progress made so far, with all necessary components gradually coming together, and expresses a commitment to the ongoing efforts to realize this advanced technology. This paragraph serves as a motivational note, emphasizing the dedication and hard work required to overcome the obstacles in the path to quantum computing advancements.

Keywords

💡Transistor

A transistor is a fundamental component in modern electronics that acts as a switch to control the flow of current. It was first demonstrated in 1947 and is pivotal to the development of the semiconductor technology discussed in the video. The script mentions the creation of the first transistor radio, highlighting the early applications of this technology.

💡Integrated Circuit

An integrated circuit (IC) is a compact arrangement of electronic components on a single piece of semiconductor material. The script refers to the 1958 demonstration of the first IC, which marked a significant advancement by allowing the integration of multiple components, leading to the complex processors and memory chips used today.

💡Quantum Bit (Qubit)

A quantum bit, or qubit, is the basic unit of quantum information used in quantum computing. The video explains how high-quality qubits can be built using the same core technology of transistors and integrated circuits. The script describes the process of isolating individual electrons to use their spin as a qubit.

💡Quantum Dot

A quantum dot is a small space where electrons are confined and can be manipulated to create qubits. The script explains how quantum dots are formed by using multiple gates to isolate 'puddles' of electrons, which is essential for the creation and control of qubits.

💡Charging Energy

Charging energy is the energy required to add a single electron charge to an electrically neutral object. The video script uses the concept of charging energy to explain how the energy cost of adding electrons to a small capacitance can be greater than thermal energy, enabling the control of individual electron charges.

💡Coulomb Repulsion

Coulomb repulsion is the force that pushes apart two electrically charged particles. In the context of the video, this repulsion is crucial for understanding the charging energy and how it contributes to the isolation of individual electrons within quantum dots.

💡Electrochemical Potential

Electrochemical potential is a measure of the energy required to add an electron to a system. The script describes how adjusting the gate voltage can change the electrochemical potential, which in turn controls the flow of electrons into and out of quantum dots.

💡Coulomb Blockade

Coulomb blockade is a phenomenon where the addition of a single charge to a system significantly changes its electrical properties. The video uses this concept to explain how the current is blocked when the electrochemical potential does not align with the Fermi energy of the reservoirs.

💡Charge Sensing

Charge sensing is a method used to detect the presence or absence of individual electrons in a quantum dot. The script explains how a change in the charge of one quantum dot can affect the current through a neighboring quantum dot, providing a way to measure the presence of individual electrons.

💡Quantum Integrated Circuits

Quantum integrated circuits are envisioned as the next step in scaling up quantum computing, where local arrays of qubits are interconnected using quantum links. The script discusses the idea of creating these circuits to transfer quantum information and entanglement between qubits.

💡Cross-bar Technology

Cross-bar technology is a method of wiring used in displays and memory chips, where a limited number of horizontal and vertical wires can control a larger number of components. The video script suggests using a similar approach to manage the wiring for two-dimensional arrays of quantum dots or qubits.

Highlights

In 1947, the first transistor was demonstrated, marking a pivotal moment in semiconductor technology development.

The first transistor radio was developed shortly after the invention of the transistor, utilizing multiple transistors, capacitors, and resistors in a circuit.

The introduction of the integrated circuit in 1958 represented a significant leap, integrating all components monolithically within a single piece of germanium or silicon.

The core technology of transistors and integrated circuits has been harnessed in the last decade to build high-quality quantum bits.

A transistor operates as a switch, controlling electron flow between contacts using a gate voltage.

Quantum dots are created by isolating small groups of electrons using multiple gates, a technique derived from transistor technology.

Isolating a single electron under a gate to form a quantum bit is now a routine practice in the scientific community.

Charging energy, resulting from Coulomb repulsion between electrons, plays a crucial role in controlling the number of electrons on quantum islands.

At 4K, the charging energy can exceed thermal energy, allowing for precise control over the number of charges on quantum islands.

The current flowing through a quantum dot can indicate the addition of individual electrons, demonstrating the Coulomb blockade effect.

Charge sensing is a method to detect the presence or absence of individual electrons by observing changes in current through adjacent quantum dots.

Crosstalk and discrete shifts in charge transitions are observed when two quantum dots are in proximity, affecting their mutual potential alignment.

Uniformity and identical properties of quantum dots are essential for large-scale circuit development, necessitating industry-standard clean rooms.

Intel collaboration since 2015 has focused on producing high-quality quantum dot arrays using 300 mm technology.

Cross-bar technology, used in displays and memory chips, is being explored for adjusting qubits in two-dimensional arrays.

The vision is to integrate up to 1024 qubits into a single array, using a limited number of wires for control.

Quantum integrated circuits are envisioned, with local arrays interconnected using quantum links for entanglement transfer.

The integration of classical electronics with qubits is considered for efficient signal distribution on the chip.

Realizing complex quantum circuits is a significant challenge, with ongoing efforts to bring all the necessary components together.

Transcripts

When we look back at the development of semi-conducting technology

a great moment was in 1947

- the demonstration of the first transistor.

Shortly after that, people began to put several transistors together on a circuit with capacitors and resistors;

and developed the first transistor radio.

Even though this technology was quite remarkable, it clearly had its limits;

how many components could you really solder together by hand on a board?

So a second momentous step was in 1958

- the first demonstration of an integrated circuit,

where all of the relevant components were integrated monolithically in the same piece

of germanium or silicon,

and that's led to the remarkably powerful and complex processors

that we use in computers and memory chips today.

Now, based on this very same core technology of transistors and integrated circuits,

we have found in the last decade how to build some of the highest-quality quantum bits.

If we look at how a transistor is built, essentially, it is a switch

that controls the flow of electrons between two contacts

using the voltage applied to a single gate.

If we now imagine that we replace the one gate by multiple gates side-by-side separating the contacts,

then with the gates we can locally pull in electrons or push away electrons,

depending on the polarity of the voltage that we apply to the gates.

In this way it is actually possible to isolate small puddles of electrons from the rest of the world.

A quantum dot is the small space where electrons are pulled in below one gate

separated by other puddles of electrons.

As a community, it's now become routine to go to the extreme limit, where below each of these electrodes,

just one single electron is isolated;

and the spin of that electron is going to be our quantum bit.

So how in the world can we isolate individual electrons and control them?

It starts with the notion of the charging energy,

that is the energy that results from the Coulomb repulsion between electrons.

As we add charges to the island, this costs energy, and if you go to very small capacitances,

it turns out that the charging energy, the energy required to add one single electron charge,

can be larger than the thermal energy.

To give some examples, for a small island with a radius of a 100 nanometers,

the charging energy is 3 meV.

To put that number into perspective, the thermal energy at 4K is ten times less than this.

So at 4K it is actually possible to control the number of charges on these islands one-by-one.

How do we know that individual charges are being added to the island?

The standard method is to look at the current that flows between the contacts through the quantum dot

– the central island.

In this schematic, we see a set of lines, called electrochemical potential lines,

that can be pushed up or pulled down by the gate voltage as a ladder,

and each line indicates the energy needed to add the next electron to the island.

What we see in the configuration on the left is that this energy is larger

than the energy of the highest occupied state in the reservoirs,

called the Fermi energy,

so the electrons in the reservoir don’t have enough energy to go into the quantum dot.

Furthermore, the line at the dot below it is lower than the Fermi energy in both reservoirs

so no electron can leave the dot.

In other words, electrons cannot be removed from the island and electrons cannot be added to the island;

current is blocked.

We call this Coulomb blockade.

In the configuration on the right, we have adjusted the gate voltage in such a way that the ladder comes down,

and the electrochemical potential μ(N) lies exactly within the window

between the source and drain Fermi energies or electrochemical potentials.

In this condition, if an electron can move from the source into the island,

then from the island it can move out to the drain.

However, before the first electron leaves, no second electron can enter.

So individual electrons are really being added one by one as they pass through the quantum dot.

But altogether, these many electrons moving through one by one, do produce a measurable current.

The current flows whenever the electrochemical potential lies within this biased window

between source and drain.

We see this beautifully in measurements as sharp peaks in the conductors and current

through the quantum dot for specific gate voltages where we have reached an alignment

as shown in the schematic on the right,

and for the intermediate gate voltages, the current is blocked.

A second important method to measure and detect the presence or absence of individual electrons

is what we call charge sensing.

Basically, if you imagine a single quantum dot and another quantum dot next to it,

as you have seen, the current through the second quantum dot sensitively depends on the gate voltage;

with a small change in gate voltage we can produce a large current

or completely shut off the current.

Now imagine a second quantum dot is placed next to the first.

It turns out that a single charge added to the second quantum dot acts like a small shifting

gate voltage through capacitive coupling,

it shifts the position of the levels in the first quantum dot.

So when a single charge is added to one quantum dot, the current through a neighbouring quantum dot

is changed in a measurable way.

Let’s look at an example of two quantum dots.

In this case we need an additional gate in between the two quantum dots to control their coupling.

By lowering the voltage on this tunnel barrier gate we can tune the electrical potential

in such way that the two electrons are well isolated from each other.

If we draw a gate voltage space where the gate voltage that controls the potential of one quantum dot

is plotted on the horizontal axis,

and the gate voltage that controls the second quantum dot on the vertical axis;

then, if the two quantum dots are uncoupled,

for specific voltages on the first gate electrode electrons are added to the first quantum dot.

These transitions are represented by the vertical lines in the plot.

Similarly, for specific voltages on the second gate electrode, electrons are added

one at a time to the second quantum dot.

These are the horizontal lines.

If we now consider two quantum dots in each other's vicinity, two effects will happen.

The first is that crosstalk.

The voltage applied to the first gate electrode also affects the potential of the second quantum dot,

which sits to its side; and vice versa.

That's why, the lines that were vertical and horizontal before, are now aligned at an angle.

A second effect is that a change in the number of electrons in one quantum dot

changes the alignment of the levels in the second quantum dot,

which is seen in the diagram as discrete shifts in the position of the charge transitions.

So now, how to go from these simple concepts of early demonstrations to large-scale circuits?

First we require all the quantum dots to be as identical and uniform as they can be,

so that it becomes practical to control the properties of many quantum dots, representing the qubits.

To do that, even though university clean rooms have been a great way to get started,

it has become clear that we really need access to industry-standard clean rooms,

that are optimized not for creative science and exploring new ideas, like university clean rooms,

but that are optimized for producing many components that are all as clean and identical as they can be.

With this motivation, in 2015, we started a close collaboration with Intel

that is focused on using 300 mm technology to produce high quality quantum dot arrays.

Now, even if quantum dot arrays are placed along a line, eventually, we do need two-dimensional arrays.

Bringing wires to all the qubits in a two-dimensional array may be very difficult;

however fruitful ideas have been explored to adjust qubits or quantum dots using cross-bar technology

- the same technology that is used today in displays and memory chips.

Where a limited number of wires, that are running horizontally and vertically,

can be used to adjust a much larger number of components,

like pixels on a display or quantum dots in a two-dimensional array.

With this approach, we envision that perhaps something like 1024 qubits can be integrated

into a single array, no larger than 30 x 30 micrometres.

To go beyond that, we envision true quantum integrated circuits, where different local arrays

are interconnected with other local arrays on the same chip using quantum links,

links that can carry quantum information, that can transfer entanglement.

Many ideas for producing such links have been tested and are being tested in the lab today.

In addition, we imagine a layer of classical electronics co-integrated with the qubits

in the same way, to distribute signals on the chip efficiently.

No doubt, it's a phenomenal challenge to realize such complex circuits.

All the pieces are falling in place and we are working hard to make it happen.