Searching for "MOSS" in the real world (Part 2): Five basic requirements for quantum computers

Produced by: Science Popularization China

Author: Luan Chunyang (Department of Physics, Tsinghua University)

Producer: China Science Expo

In this year's hit science fiction movie "The Wandering Earth 2", the most powerful quantum computer "MOSS" demonstrated its unparalleled computing power. By exploring the past and present of the most powerful quantum computer "MOSS", we can understand that the creation inspiration of "MOSS" comes from the progress of real science, and also incorporates the wonderful imagination of the film creation team about quantum computers in the future world.

MOSS in The Wandering Earth

(Photo source: Stills from The Wandering Earth)

Quantum computers are the cutting-edge development direction in the field of science and technology today. They can use quantum bits (qubits) in a 1/0 entangled state to perform parallel computing, thereby obtaining exponentially growing super computing power and have broad application prospects. Since the 1990s, scientists have been working hard to develop quantum computers and have made a series of progress. Today, quantum computers have been applied in fields such as quantum chemistry and quantum optimization, bringing revolutionary changes.

(Photo source: Veer Gallery)

That is to say, if there are two distinguishable energy states (energy levels) in a physical system, and the energy levels can achieve probabilistic transitions under external drive, then these two energy levels can be encoded into 1 and 0 states respectively, thereby realizing 1/0 entangled state encoding and participating in the parallel operation of quantum computers as quantum bits. Quantum computers can be divided into different types according to the different physical carriers of the 1/0 entangled state encoding.

Until now, there are many mainstream candidate schemes that are expected to realize universal quantum computers, but in fact, no matter what type of quantum computer is used, it must meet some specific basic requirements. Next, we will sort out the five basic requirements for building a quantum computer:

Need quantum bits that can encode 0/1 superposition states

The computing devices we are currently exposed to in our lives are all classical computers, and classical computers use classical bits to perform binary operations. A classical bit can only be in one of the states 1 or 0, just like a coin has only two states, the front and the back, and all the operations of classical computers are based on the conversion and combination between the 1 state and the 0 state.

The qubit is the basic computing unit in a quantum computer. A single qubit can be in multiple possible states at the same time. The most basic quantum states are 1 and 0, but a single qubit can also be in a superposition of 1 and 0 at the same time.

For example, for a quantum bit that is in a superposition of state 1 and state 0 with equal probability, before being measured by the outside world, this quantum bit has a 50% probability of being in state 1 and a 50% probability of being in state 0. In other words, the quantum bit can be in state 1 with any probability P and in state 0 with probability Q, and the sum of probabilities P and Q is always 100%.

(Photo source: Veer Gallery)

The number of qubits is one of the important indicators to measure the computing power of a quantum computer. The more qubits there are, the stronger the computing power of the quantum computer is, and the more complex problems it can handle. For example, a quantum computer with 50 qubits can factorize a very large integer of 2048 bits in a few hours, while a classical computer would take thousands of years to solve the same problem.

This calculation result is of great significance in the field of cryptography and information security, because most of the encryption algorithms today are based on the time complexity of factoring very large integers, and the emergence of quantum computers may threaten traditional encryption methods.

Currently, many new encryption algorithms have been proposed, and one of the most widely studied alternatives is an encryption method based on quantum mechanics, called quantum key distribution (QKD). The encryption scheme of quantum key distribution takes advantage of the non-replicability and non-locality of quantum states, so that the two communicating parties can establish a highly secure communication channel without being eavesdropped.

A universal set of quantum logic gate operations is needed

In classical computers, arbitrarily complex logic gate operations can be decomposed into basic Boolean operations, including AND, OR, and NOT gates.

Among them, the AND operation means that the output is 1 only when both inputs are 1, otherwise the output is 0.

The OR operation means that if one of the two inputs is 1, the output is 1, otherwise the output is 0.

The NOT operation negates an input and outputs its opposite value.

Therefore, these basic logic gate operations can be used in combination to build more complex logic circuits and realize the complex computing requirements of classical computers.

Similar to the logic gate operation in classical computing, any complex operation in a quantum computer can also be decomposed into a combination of several basic logic gates, and these basic logic gates can be used to build complex quantum algorithms and quantum circuits. Therefore, these basic logic gates are called universal quantum logic gates.

Vector set of three basic logic gate symbols

(Photo source: Veer Gallery)

Generally speaking, a set of universal quantum logic gates usually includes single-qubit gates and multi-qubit gates, which can perform different types of transformations and interactions on qubits. Among them, the single-qubit gate is a gate operation that acts on a single qubit to change the state of a single qubit. The multi-qubit gate can act on multiple qubits, thereby realizing entanglement operations between qubits, etc.

By performing a series of compound operations on single-qubit gates and multi-qubit gates, a quantum computer can implement any specific quantum algorithm, thereby achieving efficient computing tasks in quantum computing. Therefore, a set of universal quantum logic gate operations is of great significance and value, and it also provides the basis and support for the design and implementation of quantum algorithms.

Able to prepare quantum states with high quality and initialize

In order to execute specific operating algorithms in quantum computing, the quantum bits in the quantum computer need to be prepared in a specific quantum state under the drive of external conditions to complete the initialization. For example, in experiments, the quantum bits can be directly prepared to the 1 state or the 0 state, or the superposition state of the 1 state and the 0 state with equal probability. In this way, the quantum bits prepared to a specific quantum state have completed the initialization operation in the calculation.

However, the quantum state of quantum bits is very susceptible to external interference and errors. Therefore, in order to ensure the accuracy and reliability of quantum computing results, it is necessary to be able to prepare and initialize the quantum state with high quality.

In experiments, high-quality quantum state preparation and initialization require the use of some special operating methods. For example, by using laser cooling technology, almost static charged ions can be prepared and accurately adjusted to the energy level state required for quantum computing, thereby achieving high-quality preparation and initialization.

(Image source: Phys.org)

Therefore, high-quality quantum state preparation and initialization are critical to the success of quantum computing, and can reduce errors and interference in quantum computing, thereby ensuring the overall computing performance of the quantum computer.

A sufficiently large ratio of coherence time to logic gate operation time is required

We can imagine a quantum bit as a rotating ball. In the absence of external interference, the ball can keep rotating and remain in a stable 1/0 entangled state. However, the quantum state of the quantum bit is very susceptible to external interference, so the ball will gradually lose its rotation state and destroy the 1/0 entangled state. The length of time that the quantum bit maintains this entangled state is the "coherence time".

(Photo source: Veer Gallery)

During this period of time, the quantum state of the qubit will remain stably in the 1/0 entangled state, which is the key foundation of quantum computing. The length of the coherence time depends on the external environment in which the qubit is located and the physical structure of the qubit. Usually, the coherence time of a qubit is only a few milliseconds or less.

Therefore, in order to perform enough logic gate operations on quantum bits within this limited coherence time, the ratio of the coherence time to the logic gate operation time needs to be large enough.

In practical applications, people will take various measures to extend the coherence time of quantum bits, such as lowering the temperature of the experimental environment, adopting optimized quantum bit structure and design, using quantum error correction technology, etc. to improve the accuracy and reliability of quantum computing.

Finally, the state of the quantum bit can be detected with high quality

At the end of a quantum computer's calculation, we need to perform high-quality detection of the state of the quantum bit, because the accuracy and reliability of the final result of quantum computing depends on the accurate reading of the quantum bit. At the same time, high-quality quantum bit detection technology is also the basis for applications such as quantum communication and quantum key distribution (QKD).

Generally speaking, the state of a quantum bit is read through a detector because the detector can detect the state of the quantum bit, such as its spin, position, energy, etc.

In addition, high-quality quantum bit detection not only needs to have the characteristics of high precision and high speed, but also needs to be applicable to different types of quantum bits. At present, a series of detection technologies suitable for different types of quantum bits have been developed, such as those based on superconducting quantum interferometers and single-photon detectors.

(Photo source: Veer Gallery)

These high-quality detection methods can not only help us accurately read the state of qubits, but also detect even smaller quantum effects. Therefore, high-quality detection of the state of qubits is crucial to the success of quantum computing.

In summary, realizing a truly practical quantum computer is currently a huge challenge, which requires meeting the five basic requirements for realizing a quantum computer and overcoming countless experimental and technical difficulties.

Conclusion

Although there are still many challenges in the development of quantum computing technology, scientists are constantly exploring, researching and developing various quantum computing solutions. Currently, many quantum computing solutions have been proposed, such as ion trap systems, superconducting quantum systems, photon systems, neutral atoms, quantum dots, diamond NV color centers, and topological quantum systems.

(Photo source: Veer Gallery)

Although each candidate has its own unique advantages and disadvantages, they are still catching up on the track to quantum computing. We have reason to believe that with the continuous development and progress of technology, in the near future, we will see the advent of truly practical quantum computers, which will bring unprecedented productivity progress to the development of human society.

Editor: Sun Chenyu