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Quantum Computing Hardware

Which is the most appropriate reason why the performance of quantum computers cannot be compared by their number of qubits alone?

  1. AThe size of circuit that can be run correctly varies greatly with the gate and measurement error rates, the coherence times and how the qubits are connected
  2. BThe number of qubits changes every time it is measured, so it cannot be stated as a fixed value
  3. CThe more qubits a device has, the smaller the error rate of each qubit always becomes
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Answer: A. The size of circuit that can be run correctly varies greatly with the gate and measurement error rates, the coherence times and how the qubits are connected

Even with many qubits, if the gate error rates are high or distant qubits cannot be connected directly, errors pile up before a deep circuit can be completed and no meaningful result is obtained. Metrics that combine the number of qubits with error rates and other factors have therefore been proposed, such as quantum volume, written 2ⁿ, where n is the largest width of a “square” circuit (equal width and depth) that the device runs correctly. For future error-corrected machines, the number of usable logical qubits and their error rates matter more than the number of physical qubits. When reading the figures in product announcements and news, it is important to check conditions such as error rates together with the qubit count.

  • A. Correct. Even with many qubits, deep circuits cannot be run if there are many errors.
  • B. The number of qubits built into a device is fixed. The problem is that not all of them can necessarily be used without errors.
  • C. Adding qubits makes it harder, not easier, to keep error rates low, because of crowded wiring and frequencies, crosstalk and so on.
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More Quantum Computing Hardware questions

  1. Which element is essential for a superconducting qubit (transmon) to work as a qubit?
  2. What is the main reason superconducting qubits are cooled to about 10–20 mK in a dilution refrigerator?
  3. For a superconducting qubit with a frequency of 5 GHz, which is closest to the energy difference hf between |0⟩ and |1⟩ expressed as a temperature, hf/k_B? Use Planck's constant h = 6.63×10⁻³⁴ J·s and Boltzmann's constant k_B = 1.38×10⁻²³ J/K.
  4. A qubit with a frequency of 5 GHz (hf/k_B ≈ 0.24 K) is in thermal equilibrium at an effective temperature of 50 mK. Which is closest to the ratio of the probabilities of being in |1⟩ and in |0⟩, exp(−hf/k_B T)?
  5. In a transmon, the frequency of the |0⟩→|1⟩ transition is 5.0 GHz and the anharmonicity (the |1⟩→|2⟩ frequency minus the |0⟩→|1⟩ frequency) is −200 MHz. What is the frequency of the |1⟩→|2⟩ transition?