Bringing quantum computing a step closer

A technique that exploits the environment of a quantum system to improve the control of quantum bits, or qubits, could be a significant step towards making quantum computers more accurate and useful.

In quantum computing, information is encoded in qubits — mechanical systems that are the quantum analogue of the bits used in today’s digital computers. In practice, these quantum systems are often open, meaning that they interact with their immediate environments, which could be random radiation, quantum gas, or spins of other surrounding particles. This unavoidable type of interaction is one of the main obstacles to building a practical quantum computer.

While such an interaction is generally considered to diminish the ability to manipulate and control the state of quantum systems, it can be used to enhance so-called coherent control. Coherent control is the primary tool for controlling the state of single or multi-qubit systems and can be achieved using external forces like laser pulses. Using the environment as a resource to enhance coherent control is called ‘incoherent control’.

Now, Oleg Morzhin and Alexander Pechen from the Steklov Mathematical Institute of the Russian Academy of Sciences in Moscow have developed a numerical model of how incoherent control can enhance the ability of coherent control tools to manipulate the state of a single qubit.

“While coherent control is a powerful and widely used tool for manipulating and controlling quantum systems, it has certain limitations in the environment,” explained Pechen. “Particularly, for the creation of quantum gates, the building blocks of the quantum circuits used in quantum computers.”

By combining three different tools — coherent and incoherent control, the Uhlmann-Jozsa fidelity, which measures the ‘closeness’ of two quantum states, and stochastic optimization methods, the researchers were able to test fundamental limitations on the manipulation and control of quantum systems.

According to Pechen, these tools have previously been used separately for various problems in quantum control, but this is the first time they have been combined to build a new framework for constructing quantum controls.

“The approach allowed us to exclude very complicated control functions quickly and to find optimal controls that would allow us to achieve the desired manipulation and control of the qubit,” explained Morzhin.

The work lays the foundation for developing a range of technologies, including quantum computers, quantum communications, and quantum metrology and sensing devices.

“We now plan to extend our approach from a single qubit to multi-qubit systems driven by coherent and incoherent controls, which is a much bigger challenge,” said Pechen.