I do understand qubits entanglement, and their spins but I do not understand that at circuit level how can we entangle 2 qubits? Is it related to 2 qubits gates?

  • $\begingroup$ By definition, yes, two-qubit gates are required in order to create entanglement. Are you trying to ask physically how we make two-qubit gates? $\endgroup$
    – DanielSank
    Oct 24, 2020 at 20:32
  • $\begingroup$ yes physically? @DanielSank $\endgroup$
    – mevis
    Oct 24, 2020 at 21:00

1 Answer 1


As discussed in the paper by Vool and Devoret, there is a relatively simple way to write the Hamiltonian for an electrical circuit. For example, the Hamiltonian of a transmon qubit (a Josephson junction in parallel with a capacitor) is $$ H = \frac{Q^2}{2C} - E_J \, \cos(2 \pi \Phi / \Phi_0)$$ where $Q$ is the charge on the capacitor, $C$ is the capacitance, $\Phi$ is the flux in the junction, $\Phi_0 = h / (2e)$ is the flux quantum, and $E_J$ is the junction energy scale. Note that the flux and charge have the same commutator as position and momentum, $[\Phi, Q] = i \hbar$.

Now suppose we have two transmons with capacitances $C_a$ and $C_b$ and junction energies $E_{J,a}$ and $E_{J,b}$. If we connect them through a capacitor with capacitance $C_g \ll C_a, C_b$, then the Hamiltonian is approximately$^{[1]}$ \begin{align} H &= \frac{Q_a^2}{2 C_a} - E_{J,a} \cos(2 \pi \Phi_a / \Phi_0) \\ &+\frac{Q_b^2}{2 C_b} - E_{J, b} \cos(2 \pi \Phi_b / \Phi_0) \\ &\underbrace{+ \frac{Q_a Q_b}{C_a C_b / C_g}}_\text{coupling} \, . \end{align} The final term couples the two transmons together and can therefore create entnaglement. If we restrict the analysis to only the two lowest levels of each transmon, the coupling term can be rewritten as $$g \left( \sigma_{y,a} \otimes \sigma_{y, b} \right)$$ where $$g/\hbar = \frac{1}{2} \frac{C_g}{\sqrt{(C_a + C_g)(C_b + C_g)}} \sqrt{\omega_a \omega_b}$$ is called the "coupling strength" and $\omega_{a,b}$ refers to the resonance frequencies of the transmons. Detailed calculations showing these relationships with more attention to details can be found in my theory GitHub repository.

[1]: The form of the Hamiltonian is exactly as written, but the precise definitions of $C_a$, $C_b$, and $C_g$ change slightly in the coupled system.

  • $\begingroup$ Just idea, is it possible to have the coupling capacitor $C_g$ with variable capacity, like a varicap, so that we can change coupling strength? It seems to me that something similar has to be used in quantum annealers.. $\endgroup$ Oct 26, 2020 at 11:47
  • 2
    $\begingroup$ @MartinVesely There are variable coupling technologies in superconducting qubits but as far as I know none of them use real variable capacitance. Superconducting quantum devices universally use the Josephson junction which can be made to act like a variable inductor. Annealers typically use inductive coupling between qubits and the Josephson junction, configured into a SQUID, to provide adjustable coupling. The gmon qubit also uses a SQUID to achieve adjustable coupling but in that case the coupling is capacitive. $\endgroup$
    – DanielSank
    Oct 26, 2020 at 15:26
  • $\begingroup$ Thank you for the comment. $\endgroup$ Oct 27, 2020 at 8:21

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