Like logic gates in classical computers, quantum computing applications require quantum gates to change the state of qubits, allowing them to perform complex calculations.
However, it is only high-reliability quantum gates that can lead to powerful and reliable quantum computing operations. Reliability refers to how accurately a quantum gate performs an intended operation.
It is a measure of how close actual output is to ideal output. High fidelity means the port works as expected, with minimal errors. This is very important in quantum computers, because even small errors can accumulate and the entire calculations that a quantum computer performs fail.
A new study from researchers at Japan’s RIKEN Center for Quantum Computing and Toshiba reveals a high-fidelity quantum gate that promises to significantly improve the performance of existing noisy medium-range quantum devices (NISQ).
Realizing the power of a double-transmon clutch
The newly developed gate uses a double-transmon coupler (DTC), a component that enables precise control over the interaction between two qubits. It acts as a bridge that helps qubits communicate and collaborate effectively, improving the accuracy and reliability of quantum computing tasks.
However, until now, DTC has only been a theoretical concept. “We report the first experimental realization of the DTC,” the study authors note. The realized DTC device functions as an adjustable connector for qubits.
It consists of two fixed-frequency transmons (the type of qubits that are resistant to noise caused by charge) connected via an additional Josephson junction: a small device made of a very thin, non-superconducting layer placed between two superconductors are placed.
The Josephson junction allows current to flow through it without resistance under specific quantum mechanical conditions, and plays a crucial role in maintaining the required qubit states.
This unique design from DTC addresses a key challenge in quantum computing, namely creating hardware that connects qubits with high accuracy and low errors. For example, when the study authors tested the DTC-based quantum gate, they achieved “gate reliability of 99.9% for two-qubit gates and 99.98% for single-qubit gates.”
“For example, an average gate fidelity of more than 99.9% would enable not only efficient fault-tolerant quantum computers with error correction, but also effective mitigation of errors in today’s noisy intermediate-scale quantum devices,” the study authors said.
NISQs are early-stage quantum computers that have a limited number of qubits (ranging from just tens to hundreds). Their current versions are prone to errors and noise, but the new port will hopefully overcome these challenges.
High fidelity for even detuned qubits
What makes the DTC-based gate special is its ability to manage the two main types of errors that cause quantum systems to fail; leakage error and decoherence error.
The first occurs when a qubit changes state, switching from its intended quantum state to another unwanted state. Decoherence, on the other hand, is observed when a qubit loses its properties such as superposition, coherence or entanglement under the influence of its environment.
The newly developed gate maintains a balanced state and achieves high reliability even in the case of detuned qubits, qubits that are deliberately made to operate at a frequency different from their natural frequency, thus avoiding interference with other qubits around them.
“This device’s ability to perform effectively with highly detuned qubits makes it a versatile and competitive building block for various quantum computing architectures,” said Yasunobu Nakamura, one of the study authors and director of the RIKEN Center for Quantum Computing.
The gate can be used for both existing and future quantum computing applications. Hopefully it will spur the development of precise and reliable quantum devices.
The research has been published in the journal Physical Assessment X.