Random changes in qubits can lead to significant calculation disruptions, making error prevention a critical challenge for quantum engineers.
Securing Data Through Logical Qubits
To mitigate these errors, researchers are integrating multiple physical qubits into a single logical qubit and employing continuous error correction techniques. This approach enhances the longevity of quantum information, ensuring more stable storage. However, executing a quantum algorithm requires the active manipulation of qubits through quantum gates, which are fundamental to quantum computing.
Performing these operations without introducing new errors has proven to be considerably more complex than simply maintaining qubits in a stable state.
An Innovative Approach to Error Correction
A research team led by Professor Andreas Wallraff from D-PHYS has unveiled a method that directly addresses this challenge. Collaborating with experts from the Paul Scherrer Institute (PSI) and theorists from RWTH Aachen University and Forschungszentrum Jülich, they demonstrated how to conduct quantum operations on superconducting logical qubits while simultaneously correcting errors. Their groundbreaking findings were published in Nature Physics.
This achievement represents a significant milestone toward fault-tolerant quantum computing, enabling calculations to proceed without being disrupted by persistent errors.
The Uniqueness of Quantum Error Correction
Unlike classical computers, which rely on duplicating information for error correction, quantum systems present a unique challenge. "With qubits, the complexity increases," notes Dr. Ilya Besedin, a postdoctoral researcher involved in the study. Quantum information cannot be copied; it must be distributed across entangled qubits, and quantum systems also face phase flip errors that classical computing does not.
Utilizing Surface Codes for Error Correction
One prevalent method in this field is the use of surface codes, where a single qubit's information is spread across multiple physical data qubits. Error detection is achieved through repeated measurements of stabilizers that work alongside data qubits to form the logical qubit.
These stabilizers are monitored using additional qubits linked to the data qubits, allowing for the detection of bit flips or phase flips. Z-type stabilizers identify changes in bit value, while X-type stabilizers monitor phase changes, ensuring the data qubits can safely maintain the corrected quantum state.
The Complexity of Logical Operations
Applying logical operations, such as a controlled-NOT gate between logical qubits, adds another layer of complexity, as errors can occur during the operation itself and need to be corrected.
"Executing a logical operation in a fault-tolerant manner would be straightforward if we could freely move our qubits and connect them as needed," explains Kerschbaum. However, in superconducting quantum processors, qubits are stationary, restricting interaction to neighboring qubits.
Employing Lattice Surgery
To navigate these limitations, the team adopted a technique known as lattice surgery. In their experiment, they began with a single logical qubit encoded across seventeen physical qubits arranged in a square pattern. Stabilizers were measured every 1.66 microseconds to correct both bit and phase flips.
At a crucial point, three data qubits at the center of the square were measured, effectively splitting the surface code into two halves while pausing measurements of X-type stabilizers.
The outcome of this operation resulted in two entangled logical qubits. During the splitting process, bit flip errors were continuously corrected, and post-splitting, error correction resumed independently for each half. While this operation alone does not create a controlled-NOT gate, it can be integrated with further splitting and merging steps to achieve that goal.
A Milestone for Superconducting Qubits
"One could argue that the lattice surgery operation is foundational, from which all others can be derived," Besedin asserts. He adds, "This is, to our knowledge, the first instance of lattice surgery applied to superconducting qubits, and while we have more work ahead--such as requiring 41 physical qubits for stable splitting against phase flips--this demonstration is a crucial step toward realizing the ambitious vision of constructing functional quantum computers with thousands of qubits.
