In March of this year, Google announced the release of Bristlecone, a new quantum calculator with 72 qubits, achieving a low error rate of 1%, which is equal to the quantum computer with 9 qubits. Google engineers involved in this work expressed their optimism that Quantum hegemony could be achieved within a few months if everything worked well.
Google’s goal is to build a quantum computer that can be used to solve real-world problems. Its strategy is to use a system that is compatible with universal error correction quantum computers to explore recent applications. Today, Google Blog published an article introducing two recent quantum computing studies published in Science and Nature Physics, detailing Google’s quantum hegemony blueprint.
The theoretical basis for the actual demonstration of quantum hegemony: the "hello world" program of quantum computers
Quantum computing combines the two largest technological revolutions of the past half century—information technology and quantum mechanics. If we use the rules of quantum mechanics instead of binary logic, some tricky computational tasks can become feasible. In the pursuit of general-purpose quantum computers, an important goal is to determine the minimum computational tasks that are too difficult for today's classic computers. This intersection is known as the boundary of quantum supremacy, which is a key step toward more powerful and useful computers.
Recently, Google published a paper entitled "Characterizing Quantum Supremacy in Near-Term Devices" in Nature Physics, which proposed the theoretical basis for practical demonstration of quantum hegemony in short-term devices. This paper proposes the task of sampling bit-strings from the output of a random quantum circuit, which can be thought of as a "hello world" program for quantum computers.
As a result of the controversy, the output of the stochastic chaotic system (associative butterfly effect) will be more difficult to predict as the running time becomes longer. If you create a random, chaotic quantum-bit system and test how long it takes for a classic system to simulate it, then it's a good time to measure when a quantum computer can go beyond classical computers. It can be said that this is the most powerful theoretical proposal to prove the exponential separation between the computing power of classical computers and quantum computers.
In order to determine where the boundaries of quantum hegemony are, sampling random quantum circuits has quickly become an exciting area of ​​research. On the one hand, classical algorithms for improving analog quantum circuits aim to increase the size of quantum circuits needed to establish quantum advantages. This forces experimental quantum devices with a sufficient number of qubits and a sufficiently low error rate to implement sufficiently deep circuits (ie, a sufficient number of gate layers in the circuit) to achieve supremacy.
On the other hand, we now have a better understanding of how the specific choice of quantum gates used to construct random quantum circuits will affect the cost of simulations, leading to the recent improvement of benchmarks for quantum hegemony, in some The scenario is more expensive than the original scenario simulation.
Benchmark can be downloaded here: https://github.com/sboixo/GRCS
Sampling from random quantum circuits is a good calibration benchmark for quantum computers, which we call cross-entropy benchmarks. A successful random circuit quantum hegemony experiment will prove the basic building blocks of large-scale fault-tolerant quantum computers. In addition, quantum physics has not yet tested such highly complex quantum states.
Figure: The space-time volume of a quantum circuit calculation. The computational cost of quantum simulation increases with the size of the quantum circuit and generally increases exponentially with the number of quanta and circuit depth. For asymmetric meshes of qubits, the calculation of spatiotemporal volume grows with depth slower than that of symmetric meshes and may result in the circuit being more easily simulated.
Demonstrate quantum hegemony blueprint with superconducting quantum bits
In another paper published in Science, "A blueprint for demonstrating quantum supremacy with superconducting qubits", Google elaborated a blueprint for quantum hegemony, and for the first time, an experiment proved that Principle verified version.
In the paper, we discussed two key elements of quantum hegemony: exponential complexity and accurate calculations. We first run the algorithm on a part of the running device, ranging from 5 to 9 qubits. We find that the cost of classical simulation increases exponentially with the increase in the number of qubits. These results are intended to provide clear examples of the exponential capabilities of these devices.
Equipment: 9-bit array. This is an optical micrograph of the device. The gray area is aluminum, and the black area is where aluminum is etched to define the features. Other colors are used to distinguish between readout circuitry, qubits, couplers, and control lines.
Next, we use the cross-entropy benchmark to compare our results with the results of a normal computer and show that our calculations are very accurate. In fact, the error rate is sufficiently low that quantum hegemony can be implemented with a larger quantum processor.
In addition to quantum hegemony, quantum platforms should provide definitive applications. In our paper, we apply our algorithm to the computational problems in quantum statistical mechanics. These problems use complex multi-qubit gates compared to surface code corrections designed for digital quantum processors. The two-qubit gates are wrong, it is the opposite. We have demonstrated that our equipment can be used to study the basic properties of materials, such as the slight differences between metals and insulators. By extending these results to next-generation devices with about 50 qubits, we hope to answer scientific questions that exceed the capabilities of any other computing platform.
Figure: Two gmon superconducting qubits and their tunable couplers developed by Charles Neill and Pedram Roushan
These two papers put forward a realistic proposal for the recent quantum hegemony, and for the first time proved a proof of principle version. Google said it will continue to reduce the error rate and increase the number of quantum bits in quantum processors to reach the quantum hegemony boundary and develop quantum algorithms for short-term practical applications.
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