Gated Conditional Displacement Readout of Superconducting Qubits

Abstract

We have realized a new interaction between superconducting qubits and a readout cavity that results in the displacement of a coherent state in the cavity, conditioned on the state of the qubit. This conditional state, when it reaches the cavity-following, phase-sensitive amplifier, matches its measured observable, namely the in-phase quadrature. In a setup where several qubits are coupled to the same readout resonator, we show it is possible to measure the state of a target qubit with minimal dephasing of the other qubits. Our results suggest novel directions for faster readout of superconducting qubits and implementations of bosonic quantum error-correcting codes.

Fast High-Fidelity Quantum Nondemolition Qubit Readout via a Nonperturbative Cross-Kerr Coupling

“…Moreover, each probability has a statistical uncertainty, due to finite number of realizations of ±0.6 %. These results are comparable to the QND fidelity obtained in Touzard et al [15] using a parametric modulation scheme and corresponds, to the best of our knowledge, to the state-of-the-art values.…”

“…We define four conditional probabilities, P α,β , the probability to measure α in the first measurement and β in the second measurement, where α, β = g, e can correspond to ground or excited states. From these probabilities, the QND fidelity [15] is obtained to be Q = Pg,g+Pe,e 2 = 99 %. In P e,e = 98.3 %, we estimate 0.7 % to be explained by relaxation during measurement, and in P g,g = 99.6 %, we estimate only 0.02 % to be due to thermal excitation during measurement.…”

Repetition Cat Qubits for Fault-Tolerant Quantum Computation

“…In the presence of twophoton pumping, this Hamiltonian is an approximation of π/T | − α −α| ⊗ (b †b −n), rotating the target cat qubit conditional to the control cat qubit being in the state | − α . Such Hamiltonians have been already realized using parametric methods [50], similar to those used in driven two-photon dissipation. Toffoli gate.…”

“…The phase-flip probability induced by the nonadiabaticity of the evolution can be reduced by adding the effective Hamiltonian H CNOT = 1 2 π Tâ −α 2α ⊗ (b †b − |α| 2 )+ H.c. . Such a Hamiltonian has also been recently implemented using a detuned parametric pumping method [50].…”

Native three-body interaction in superconducting circuits

“…We now add capacitive couplings to external control and ground. The external control could consist of resonators for readout, flux lines for tuning, and drive lines for preparation [39]. Each external coupling adds to the diagonal of the capacitance matrix, and we assume that the total contribution for each node is identical and equal to K. The external couplings are drawn in red in Fig.…”