In the next few days, Pang Xuelin's main focus was on the study of quantum computers.

The so-called quantum computer is a kind of physical device that stores quantum information and realizes quantum computing according to the laws of quantum mechanics.

In general, the input of a quantum computer can be described by a quantum system with finite energy levels.

For example, a two-level system is called a qubit.

QUBIT|Ψ>=α|0>+β|1> CAN BE ANY COMBINATION OF |0> AND |1> STATES, WHERE α AND β REPRESENT THE PROPORTIONAL COEFFICIENTS IN COHERENT SUPERPOSITION STATES, RESPECTIVELY.

Based on the quantum coherence effect, there are infinitely many sets of conditional coefficients of α^2+β^2=1, so the information represented by qubits can be greatly enriched.

According to the composition of qubits, quantum computers can be divided into the following types.

Use the polarization of photons to build qubits, so-called optical quantum computers.

In 2017, the world's first optical quantum computer was born at the University of Science and Technology of China.

Qubits are built using the energy levels of trapped ions or atoms, the so-called ionic quantum computer.

At present, ionic quantum computers have not yet been built, and scientists in Sweden and Austria have collaborated to create the basic elements of ionic quantum computers, but there is still some time before a real ionic quantum computer can be built.

Finally, superconducting quantum computers are used to construct qubits using superconducting circuits, including Cooper pairs and superpositions of left/right-handed circulation states related to the direction of the circulation.

At present, IBM, Google, Microsoft and other companies are fiercely competing in this field.

Quantum superposition and quantum coherence are the most essential characteristics of quantum computers.

The transformation of each superimposed component by the quantum computer is equivalent to a kind of classical computation, all of which are completed at the same time and superimposed by a certain probability amplitude to give the output of the quantum computer.

Therefore, a quantum computer is essentially a parallel computation that is capable of solving problems in polynomial time under parallel conditions that can only be solved in exponential time by classical computers.

For example, a quantum computer is able to decompose a large 250-bit number into the product of two prime numbers in a matter of seconds, a task that would take a million years for current computers to complete.

Because of this, countless top scholars in the world from mathematics, physics, chemistry and other fields have become interested in quantum computers.

At the same time, it has also aroused the interest of government departments and the business community.

But so far, so-called quantum computers have been just expensive toys.

In the middle, there is non-scientific competition from large companies such as Google, IBM, Microsoft, etc., in order to dominate the industry.

For example, the so-called quantum supremacy announced by Google a few months ago is more motivated by commercial interests than by the fact that it has actually reached that level technically.

At present, there are two main branches in the field of quantum computer research.

They are quantum algorithms and physical implementations.

Practical quantum algorithms can be divided into three categories, the first type is the problem of finding periodicity based on the quantum Fourier transform method represented by the Shor algorithm, and the problem can be further attributed to the problem of the Abelian hidden subgroup.

The second type of algorithm is called the Gover algorithm.

The Gover algorithm constructs the basic framework of a class of problems based on the probability amplitude amplification method, including the improved Gover algorithm, collision problem, quantum genetic algorithm, quantum simulated annealing algorithm, quantum neural network, etc.

The third category belongs to algorithms for simulating or solving quantum physics problems, including Feynman's original idea of using quantum computers to accelerate quantum physics simulation, and recently there are algorithms based on quantum random walks, especially continuous-time quantum random walks, including the Boolean logic calculation algorithm for NAND trees proposed by Edward Farry, director of the Center for Theoretical Physics at MIT, and Gutman.

The physical implementation of quantum computers is much more difficult than that of quantum algorithms.

First of all, the physical system of a quantum computer must meet the following requirements.

First, qubits with scalability and good characteristics.

Second, it is possible to initialize the qubits to a certain reference state, such as |000...>.

Third, it must have a sufficiently long coherence time, much longer than the operation time to complete a quantum gate.

Fourth, it has a common set of quantum gates.

Fifth, it is possible to measure specific qubits.

In order to be able to realize quantum computing in physics, researchers have conducted in-depth research in two major directions based on the above requirements.

The first is a quantum computer based on solid-state electromagnetic circuits.

This scheme includes spin system, superconducting system, quantum dot system, nuclear magnetic resonance system and other different schemes.

The second type is a quantum computer based on a quantum optical system.

It includes the implementation schemes of ion traps, cavity quantum electrodynamic systems, linear optical systems, photonic crystals and photonic crystal bound cold atom systems.

……

It took half a month for Pang Xuelin to read through all the 100 papers and the technical manuals of quantum computers given by the system, and had a basic understanding of quantum computers.

Then he found that it was unlikely that the quantum computer he wanted to give the system would be built in reality.

Because the quantum computer given by the system belongs to the topological quantum computer, the number of qubits in the quantum chip is as high as 10 million, and the computing power is several orders of magnitude higher than that of all computers in the world combined.

To make such a quantum chip, it would be based on a quasiparticle with a 1/4 charge, which behaves very differently from those with an odd-odd charge, and when an electron, a photon, or a particle with an odd-charged charge swaps places with another particle, it does not have much overall effect.

In contrast, the position exchange of 1/4 charge quasiparticles can weave a "braid" that retains the historical information of the particle, showing "non-abelian" properties.

Although in the real world, as early as 2008, scientists in Israel have discovered the existence of this quasiparticle.

But in order to find the corresponding materials accurately, the manpower and material resources that need to be invested are basically astronomical.

However, although there is no way to use the quantum chip of this quantum computer, through this technical manual, Pang Xuelin has found a way to use the proximity effect of graphene materials and conventional superconductors to construct Majorana fermions.

Majorana fermions are precisely the most critical step in realizing quantum topological computing in the true sense.

"Perhaps, the quantum supremacy that Google said can be achieved in my hands."

Pang Xuelin muttered to himself.

()