Error Protected Quantum Bits Entangled: A Milestone in the Development of Fault-Tolerant Quantum Computers
Quantum particles lined up in a lattice form the basis for an error-tolerant quantum processor.
Credit: Uni Innsbruck/Harald Ritsch
Even computers can miscalculate. Already small disturbances change stored information and corrupt results. That is why computers use methods to continuously correct such errors.
In quantum computers, the vulnerability to errors can be reduced by storing quantum information in more than a single quantum particle. These logical quantum bits are less sensitive to errors.
In recent years, theorists have developed many different error correction codes and optimized them for different tasks. “The most promising codes in quantum error correction are those defined on a two-dimensional lattice,” explains Thomas Monz from the Department of Experimental Physics at the University of Innsbruck. “This is due to the fact that the physical structure of current quantum computers can be very well mapped through such lattices.”
With the help of the codes, logical quantum bits can be distributed over several quantum objects. The quantum physicists from Innsbruck have now succeeded for the first time in entangling two quantum bits coded in this way. The entanglement of two quantum bits is an important resource of quantum computers, giving them a performance advantage over classical computers.
A kind of quantum sewing machine
For their experiment, the physicists use an ion-trap quantum computer with ten ions. Into these ions the logical quantum bits are encoded. Using a technique that scientists refer to as ‘lattice surgery’, two logical qubits encoded on a lattice can be ‘stitched together’. “A new larger qubit is created from the qubits stitched together in this way,” explains Alexander Erhard from the Innsbruck team. In turn, a large logical qubit can be separated into two individual logical qubits by lattice surgery.
In contrast to the standard operations between two logical qubits, lattice surgery only requires operations along the boundary of the encoded qubits, not on their entire surface. “This reduces the number of operations required to create entanglement between two encoded qubits,” explains the theoretical physicists Nicolai Friis and Hendrik Poulsen Nautrup.