The Science Fiction World of Xueba

In the next few days, Pang Xuelin focused his energy 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 to a quantum computer can be described by a quantum system with finite energy levels.

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

The qubit -Ψ\u003e=α-0\u003e+β-1\u003e can be any combination of -0\u003e state and -1\u003e state, where α and β represent the proportional coefficients in the coherent superposition state, respectively.

Based on the quantum coherence effect, there are infinitely many sets of conditional coefficient values ​​for 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.

Use the energy levels of trapped ions or atoms to build qubits, a so-called ion-type quantum computer.

At present, ion-type quantum computers have not yet been manufactured. Scientists in Sweden and Austria have collaborated to create the basic components of ion-type quantum computers, but it is still some time before the real ion-type quantum computers are manufactured.

The last one is the superconducting quantum computer, which uses superconducting lines, including Cooper pairs and left/right-handed circulation superposition states related to the direction of circulation, to construct qubits.

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

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

The transformation realized by the quantum computer for each superposition component is equivalent to a classical calculation. All these classical calculations are completed at the same time, and are superimposed according to a certain probability amplitude to give the output result of the quantum computer.

Therefore, a quantum computer is essentially a kind of parallel computing. Under parallel conditions, it can solve problems that can only be solved in a classical computer exponential time in polynomial time.

For example, a quantum computer can factor a large 250-digit number into the product of two primes in seconds, a task that would take current computers a million years to complete.

Because of this, there are countless top scholars in the world from the fields of mathematics, physics, chemistry, etc., who have become interested in quantum computers.

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

But so far, so-called quantum computers have remained an expensive toy.

Interspersed with the non-scientific competition of large companies such as Google, IBM, Microsoft, etc. to dominate the industry.

For example, the so-called quantum hegemony announced by Google a few months ago was more due to commercial interests than to achieve that level technically.

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

They are quantum algorithm and physical realization respectively.

Practical quantum algorithms can be divided into three categories. The first category is the periodic problem based on the quantum Fourier transform method represented by the Shor algorithm, which can be further reduced to the problem of Abelian implicit subgroups.

The second type of algorithm is called the Gover algorithm.

Gover algorithm builds the basic framework of a class of problems based on probability amplitude amplification method, including 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 simulations. Recently, there are also algorithms based on quantum random walks, especially continuous-time quantum random walks. It includes the Boolean logic calculation algorithm of the NAND tree proposed by Edward Farley, director of the Center for Theoretical Physics of the Massachusetts Institute of Technology, and Gutman.

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

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

First, qubits with scalable, well-characterized properties.

Second, it is possible to initialize the qubit to some reference state, such as -000...\u003e.

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

Fourth, have a universal set of quantum gates.

Fifth, the ability to measure specific qubits.

In order to realize quantum computing physically, researchers have carried out in-depth research in two directions based on the above requirements.

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

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

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

Including ion trap, cavity quantum electrodynamic system, linear optical system, photonic crystal and photonic crystal bound cold atom system and other realization schemes.

...

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

Then he discovered that it is unlikely to be possible in a short time to manufacture the quantum computer given by the system in reality.

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

To make such a quantum chip, it needs to be based on a quasiparticle with a 1/4 charge, which behaves very differently from those with an odd 1/4 charge. When a particle of one-half charge swaps places with another particle, there is not much overall effect.

In contrast, the exchange of positions of 1/4-charged quasiparticles can weave a "braid" that can retain particle history information, showing "non-Abelian" characteristics.

Although in the real world as early as 2008, Israeli scientists have discovered the existence of such quasi-particles.

However, in order to accurately find the corresponding materials, the manpower and material resources required are basically astronomical.

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

The Majorana fermion is precisely the most critical step to realize quantum topological computing in the true sense.

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

Pang Xuelin muttered to himself.