Perhaps its best to start off explaining what a GHZ state is and why it is an accomplishment to create one that uses 50 qubits. Most people are familiar with two entangled qubits known as a Bell pair such that the state of these qubits are linked such that when one changes a state the other one will also change it state even if these entangled qubits may be far away from each other without any connection. Einstein used to call this “Spooky action at a distance”. A GHZ state is an extension of this such that you have more than two qubits entangled to each other such that all the qubits in the GHZ state will change together. In a perfect GHZ state, each of the qubits will be in the superposition of “0” and “1” state and when measured all will collapse together to either the “0” state or the “1” state. The picture above shows the results of Quantinuum’s experiment that show some errors as represented by the bars in the middle of the chart. If there were no errors, 50% of the time the measured results for all the qubits would be in the far left bar (0000….) and the other 50% of the time, the measured results for all the qubits would be in the far right bar (1111…).
Quantinuum has taken their 56 qubit H2-1 processor and created a 50 qubit GHZ state with the help of a [[52,50,2]] error detection code. This code utilized 52 physical qubits to create 50 logical qubits with a code distance of 2 between codewords. Note that this code only provides the capability of detecting error, but not to correct errors on the fly. However, if an error is detected, the quantum processor can repeat the operation until no errors are detected. In the chart above, the orange bars shows the results without using the error detection code while the blue bars show the results using this error detection code. So as we would hope, it does show improved results when the error detection code is turned on.
The Quantinuum results more than doubles results reported last month by Microsoft and Atom Computing who were able to demonstrate 24 entangled logical qubits. Microsoft and Atom used a [[4,2,2]] code which provides error and loss detection. That demonstration was performed on Atom’s 256 qubit neutral atom based processor and used about 48 physical qubits to create 24 logical qubits with a code distance of two.
So, like many things in the competitive quantum computing ecosystem, vendors are vying to set new performance records for many different performance measures. Although this demonstration from Quantinuum sets a new record for the GHZ state in 2024, we fully expect that in coming years others will be able to raise the bar even further with larger and more powerful quantum processors along with more advanced error detection and correction codes.
For more information about Quantinuum’s advancements in logical quantum computation, we refer you to a presentation made at the recent Q2B Silicon Valley conference by David Hayes, Quantinuum’s Director of Computational Theory and Design. You access it here.
December 14, 2024
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