IBM announced that it has reached 512 quantum volume. Why is the QV of superconductivity not comparable to that of ion traps？

Jay Gambetta, head of IBM Quantum, who just attended the 28th Solvay Conference, announced a good news on Twitter, IBM's new gate architecture (Falcon R10) provides higher fidelity and low crosstalk, circuit quality Better, so achieved 512 quantum volume (QV), IBM's second upgrade in two months. 

The Photon Box summarizes the competition between IBM and Honeywell on quantum volume indicators since January 2020. Last month, IBM announced that it had reached 256QV for the first time, but at the same time Honeywell was already a record 4096.

Many people may have questions. IBM's qubits are already 127. Why is the quantum volume not comparable to the Honeywell ion trap quantum computer with only 12 qubits?

Here we have to talk about the definition of quantum volume. Its calculation formula is as follows:

Among them, VQ is the quantum volume, and for an n-qubit processor, m≤n, d(m) is the number of qubits in the largest square circuit. That is to say, quantum volume is related to the number of qubits a processor has, and it is also related to the depth of the largest square circuit that a quantum processor can operate reliably.

random square circuit

Take IBM's 512QV processor as an example. From the pictures released by Gambetta, this processor has at least 31 qubits, but the number of qubits in the largest square circuit is only 9, which is calculated as 512QV according to the formula.

Why is this happening? Because quantum volume is a comprehensive indicator, its influencing factors include the number of qubits, fidelity, connectivity, and circuit compilation efficiency. The difference between superconducting and ion trap platforms is mainly in connectivity. As shown in the figure below, one qubit of the superconducting quantum chip on the left is only connected to the adjacent four qubits. Now IBM adopts a hexagonal structure with one qubit. At most, it is connected to three adjacent qubits, and the qubits of the ion trap quantum chip on the right are connected to each other in pairs.

In ion trap processors, almost all qubits can be used as effective qubits in quantum volume tests, as recently Honeywell's 4096QV, but since its processors have only 12 qubits, the QV has reached its limit unless Keep adding qubits -- that's IBM's strength.

IBM can upgrade quickly even with a huge disadvantage in connectivity, especially with the new architecture (as to why IBM is doing this, read "Understanding IBM's New Topology for Quantum Processors: To Reduce Errors" Compromises made”), just to prove that IBM is better in other areas, as Gambetta said, higher fidelity and low crosstalk.

If Honeywell or Quantinuum can't make a breakthrough in the number of qubits, it is very likely that the quantum volume will be overtaken by IBM's quantum computer in the future.