(PhysicsWorld) “Lattice surgery” has been used to quantum-mechanically entangle fault-tolerant topological qubits – an achievement that could lead to the production of more reliable quantum logic gates. Created by researchers in Austria, Switzerland and Germany, the entanglement technique could prove useful in the development of quantum error correction algorithms and ultimately to achieve scalable, large-scale quantum computation.
A big problem with today’s quantum devices is that errors are quickly introduced to a calculation via interactions with noise, heat and other disturbances from the surrounding environment. In classical computers, errors can be measured and corrected, but in quantum computers the very act of measuring a quantum bit (qubit) of information causes it to collapse. This build-up of errors puts severe limits on the size of a quantum computer and the size of the computation it can achieve.
To create much larger quantum processors, physicists are trying to develop more reliable qubits as well as “quantum error correction” protocols to deal with the inevitable faults that will occur.
“People take a collection of physical qubits, they impose some code structure to create a logical qubit, and then they show that you can do operations on that single logical qubit,” explains Friis. “But in order to obtain a universal gate set that will enable you to do quantum computation on any number of logical qubits, you need entangling operations on two logical qubits.”
In their new work, Friis and colleagues produced two topologically-protected logical qubits, each encoded into the topological surface state of four ions. The ions were held in an ion trap containing ten ions. By tuning the laser frequencies to alter interactions between the ions, the team performed a technique called lattice surgery, stitching the surface states together into one large state.

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