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Quantum Computing Hardware

Which element is essential for a superconducting qubit (transmon) to work as a qubit?

  1. AA pn junction diode
  2. BA MOSFET with a floating gate
  3. CA Josephson junction
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Answer: C. A Josephson junction

A Josephson junction consists of two superconductors separated by a thin insulating film about 1–2 nm thick; a superconducting current flows through it without producing a voltage, and it acts as a lossless nonlinear inductor. A transmon is a resonant circuit that combines this junction with a capacitor. In an ordinary LC resonator made of a coil and a capacitor the energy levels are equally spaced, so the transition between |0⟩ and |1⟩ cannot be addressed on its own; the nonlinearity of the junction makes the spacing unequal, so the two lowest levels can be used as a qubit. The junction is usually made of aluminum and aluminum oxide formed by oxidizing its surface.

  • A. A pn junction is a semiconductor device and is not used in the operation of superconducting qubits.
  • B. A floating gate is the structure that stores charge in flash memory.
  • C. Correct. The Josephson junction acts as a lossless nonlinear inductor.
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More Quantum Computing Hardware questions

  1. What is the main reason superconducting qubits are cooled to about 10–20 mK in a dilution refrigerator?
  2. 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.
  3. 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)?
  4. 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?
  5. Among the factors that limit the energy relaxation time T1 of superconducting qubits, which is most closely related to fabrication processes such as thin-film deposition and surface treatment?