Virtual Two-Qubit Gates: A Step Forward in Quantum Computing Efficiency

Researchers from the University of Tokyo, Osaka University, NTT Corporation, and RIKEN Center for Quantum Computing have successfully demonstrated a virtual two-qubit gate, a fundamental building block of quantum circuits. This development could lead to more efficient quantum algorithms and applications. The team also developed a method to mitigate measurement errors, improving the performance of quantum devices. However, further research is needed to fully evaluate the performance of virtual two-qubit gates. The team plans to explore techniques for simulating large quantum circuits with smaller quantum devices, potentially enhancing the capabilities of noisy intermediate scale quantum (NISQ) devices.

What is a Virtual Two-Qubit Gate and Why is it Important?

Quantum computing is a rapidly advancing field that promises to revolutionize the way we process information. However, current quantum devices, known as noisy intermediate scale quantum (NISQ) devices, are still far from being fully functional quantum computers. They have limited coherence times, low scalability, and non-negligible noises. Despite these limitations, they are proving to be valuable testbeds for many promising quantum algorithms and applications.

One of the key challenges in quantum computing is the implementation of two-qubit gates, which are fundamental building blocks of quantum circuits. A recent study by a team of researchers from the University of Tokyo, Osaka University, NTT Corporation, and RIKEN Center for Quantum Computing has demonstrated a practical approach to implement virtual two-qubit gates with high fidelities. These virtual gates are useful for simulating quantum circuits using fewer qubits and implementing two-qubit gates on a distant pair of qubits.

The researchers experimentally demonstrated a virtual two-qubit gate and characterized it using quantum process tomography (QPT). The virtual two-qubit gate decomposes an actual two-qubit gate into single-qubit unitary gates and projection gates in quantum circuits for expectation-value estimation. The deterministic sampling scheme reduces the number of experimental circuit evaluations required for decomposing a virtual two-qubit gate.

How Does a Virtual Two-Qubit Gate Work?

The virtual two-qubit gate allows us to simulate a two-qubit gate from a quasi-probability decomposition of local single-qubit operations in the quantum circuits used for the expectation-value estimation for observables. The virtual two-qubit gate scheme has been experimentally utilized on a distant pair of superconducting qubits to reduce the number of SWAP operations required and thus reducing the two-qubit errors.

The virtual two-qubit gate requires the implementation of projection gates, which are non-unitary. The researchers implemented the projection gates through mid-circuit measurements. However, this limits the fidelity of the virtual two-qubit gate since measurement errors are typically higher than single-qubit gate errors. To overcome this, the researchers formulated the quantum error mitigation for mid-circuit measurements and applied it to improve the average gate fidelity of the virtual two-qubit gate.

What is the Significance of this Research?

The successful demonstration of a virtual two-qubit gate is a significant step forward in the field of quantum computing. It provides a practical approach to implement two-qubit gates with high fidelities, which are crucial for the operation of quantum circuits. This could potentially lead to the development of more efficient quantum algorithms and applications.

Moreover, the researchers’ approach to mitigating measurement errors could be instrumental in improving the performance of quantum devices. By reducing the number of experimental circuit evaluations required for decomposing a virtual two-qubit gate, the researchers have also made the process more efficient.

What are the Future Directions of this Research?

The researchers have demonstrated the potential of virtual two-qubit gates in quantum computing. However, there is still much work to be done. The characterization of the virtual two-qubit gate was not performed, thereby limiting the ability to evaluate its quality. Future research will need to focus on this aspect to fully understand the potential and limitations of virtual two-qubit gates.

Furthermore, the researchers plan to explore other techniques for simulating large quantum circuits with smaller quantum devices. These techniques could further enhance the capabilities of NISQ devices and bring us one step closer to realizing a scalable, fault-tolerant quantum computer.

Conclusion

The research conducted by the team from the University of Tokyo, Osaka University, NTT Corporation, and RIKEN Center for Quantum Computing represents a significant advancement in the field of quantum computing. The successful demonstration of a virtual two-qubit gate and the development of a method to mitigate measurement errors could pave the way for more efficient and reliable quantum computing systems. However, further research is needed to fully characterize and evaluate the performance of virtual two-qubit gates.