Like logic gates in classical computers, quantum computing applications require quantum gates to change the state of qubits —- enabling them to perform complex calculations.
However, it is only quantum gates with high fidelity that can lead to powerful and reliable quantum computing operations. Fidelity refers to how accurately a quantum gate performs an intended operation.
It’s a measure of how close the actual output is to the ideal output. High fidelity means the gate works as expected with minimal errors. This is very important in quantum computing as even small errors can build up and fail the entire calculations a quantum computer performs.
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 intermediate-scale quantum (NISQ) devices.
Realizing the power of double-transmon coupler
The newly developed gate utilizes a double-transmon coupler (DTC), a component that allows precise control over how two qubits interact. It acts like a bridge that helps qubits effectively communicate and cooperate, improving the accuracy and reliability of quantum computing tasks.
However, until now, DTC was just a theoretical concept. “We report the first experimental realization of the DTC,” the study authors note. The realized DTC device functions like an adjustable connector for qubits.
It is made up of two fixed-frequency transmons (the type of qubits that resist noise caused by charge) which are linked through an extra Josephson junction — a tiny device made of a very thin non-superconducting layer placed between two superconductors.
The Josephson junction allows current to flow through it without resistance under specific quantum mechanical conditions, playing a crucial role in the maintenance of the required qubit states.
This unique design of DTC tackles a key challenge in quantum computing i.e. creating hardware that connects qubits with high accuracy and low error. For instance, when the study authors tested the DTC-based quantum gate, it achieved “gate fidelities of 99.9% for two-qubit gates and 99.98% for single-qubit gates.”
“An average gate fidelity surpassing 99.9%, for example, would enable not only efficient fault-tolerant quantum computing with error correction but also effective mitigation of errors in current noisy intermediate-scale quantum devices,” the study authors added.
NISQs are early-stage quantum computers that have a limited number of qubits (ranging between tens to hundreds only). Their current versions are prone to errors and noise but the new gate 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 major types of errors that cause quantum systems to fail; leakage error and decoherence error.
The former occurs when a qubit changes its state, switching from its intended quantum state to another unwanted state. Decoherence, on the other side, 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 fidelity even in the case of detuned qubits, qubits that are deliberately made to operate on a frequency that is different from their natural frequency, preventing 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,” Yasunobu Nakamura, one of the study authors and director of the RIKEN Center for Quantum Computing, said.
The gate can be used for both existing and future quantum computing applications. Hopefully, it will boost the development of accurate and reliable quantum devices.
The study is published in the journal Physical Review X.
