We demonstrate a transmon-qubit readout based on the nonlinear response to a drive of polaritonic meters in situ coupled to the qubit. Inside a three-dimensional readout cavity, we place a transmon molecule consisting of a transmon qubit and an ancilla mode interacting via nonperturbative cross-Kerr-coupling. The cavity couples strongly only to the ancilla mode, leading to hybridized lower and upper polaritonic meters. Both polaritons are anharmonic and dissipative, as they inherit a self-Kerr nonlinearity $U$ from the ancilla and effective decay $\ensuremath{\kappa}$ from the open cavity. Via the ancilla, the polariton meters also inherit the nonperturbative cross-Kerr-coupling to the qubit. This results in a high qubit-dependent displacement $2\ensuremath{\chi}>\ensuremath{\kappa},U$ that can be read out via the cavity without causing Purcell decay. Moreover, the polariton meters, being nonlinear resonators, present bistability, and bifurcation behavior when the probing power increases. In this work, we focus on the bifurcation at low power in the few-photon regime, called the mesoscopic regime, which is accessible when the self-Kerr and decay rates of the polariton meter are similar, $U\ensuremath{\sim}\ensuremath{\kappa}$. Capitalizing on a latching mechanism by bifurcation, the readout is sensitive to transmon-qubit relaxation error only in the first tens of nanoseconds. We thus report a single-shot fidelity of 98.6% while having an integration time of 500 ns and no requirement for an external quantum-limited amplifier.We demonstrate a transmon-qubit readout based on the nonlinear response to a drive of polaritonic meters in situ coupled to the qubit. Inside a three-dimensional readout cavity, we place a transmon molecule consisting of a transmon qubit and an ancilla mode interacting via nonperturbative cross-Kerr-coupling. The cavity couples strongly only to the ancilla mode, leading to hybridized lower and upper polaritonic meters. Both polaritons are anharmonic and dissipative, as they inherit a self-Kerr nonlinearity $U$ from the ancilla and effective decay $\ensuremath{\kappa}$ from the open cavity. Via the ancilla, the polariton meters also inherit the nonperturbative cross-Kerr-coupling to the qubit. This results in a high qubit-dependent displacement $2\ensuremath{\chi}>\ensuremath{\kappa},U$ that can be read out via the cavity without causing Purcell decay. Moreover, the polariton meters, being nonlinear resonators, present bistability, and bifurcation behavior when the probing power increases. In this work, we focus on the bifurcation at low power in the few-photon regime, called the mesoscopic regime, which is accessible when the self-Kerr and decay rates of the polariton meter are similar, $U\ensuremath{\sim}\ensuremath{\kappa}$. Capitalizing on a latching mechanism by bifurcation, the readout is sensitive to transmon-qubit relaxation error only in the first tens of nanoseconds. We thus report a single-shot fidelity of 98.6% while having an integration time of 500 ns and no requirement for an external quantum-limited amplifier.

arXiv:2210.04793Physical Review AppliedOct 19, 20235 citations

Millimeter Wave Readout of a Superconducting Qubit

Akash V. Dixit, Zachary L. Parrott, Dennis Chunikhin +3 more

Millimeter waves are emerging as an enabling technology for connecting and enhancing different quantum platforms such as Rydberg atoms, optomechanics, and superconducting qubits. In this work, we explore the interaction between millimeter wave photons and conventional transmon qubits, specifically for qubit readout. We study a circuit quantum electrodynamic (cQED) system consisting of a millimeter wave cavity at $ω_r = 2π\times 34.7$ GHz and a transmon qubit at $ω_q = 2π\times 3.1$ GHz coupled at rate $g = 2π\times 1.3$ . With such a large detuning between cavity and qubit, $ω_r/ω_q > 10$ , we are able to suppress drive induced unwanted resonant state transitions, enabling strong drives for qubit readout. We measure no resonant state transitions even when driving with more than $100$ photons and read out the qubit state with more than $100$ photons to achieve a measurement fidelity greater than $99\%$ without the aid of a quantum limited amplifier.Millimeter waves are emerging as an enabling technology for connecting and enhancing different quantum platforms such as Rydberg atoms, optomechanics, and superconducting qubits. In this work, we explore the interaction between millimeter wave photons and conventional transmon qubits, specifically for qubit readout. We study a circuit quantum electrodynamic (cQED) system consisting of a millimeter wave cavity at $ω_r = 2π\times 34.7$ GHz and a transmon qubit at $ω_q = 2π\times 3.1$ GHz coupled at rate $g = 2π\times 1.3$. With such a large detuning between cavity and qubit, $ω_r/ω_q > 10$, we are able to suppress drive induced unwanted resonant state transitions, enabling strong drives for qubit readout. We measure no resonant state transitions even when driving with more than $100$ photons and read out the qubit state with more than $100$ photons to achieve a measurement fidelity greater than $99\%$ without the aid of a quantum limited amplifier.

quant-pharXiv:2603.13837Mar 17, 20260 citations

Readout Induced Transitions in a Driven Superconducting Transmon Qubit

Zijun Chen, Mostafa Khezri, B. Chiaro +6 more

APSJan 1, 20180 citations

U

from the ancilla and effective decay $\ensuremath{\kappa}$ from the open cavity. Via the ancilla, the polariton meters also inherit the nonperturbative cross-Kerr-coupling to the qubit. This results in a high qubit-dependent displacement $2\ensuremath{\chi}>\ensuremath{\kappa},U$ that can be read out via the cavity without causing Purcell decay. Moreover, the polariton meters, being nonlinear resonators, present bistability, and bifurcation behavior when the probing power increases. In this work, we focus on the bifurcation at low power in the few-photon regime, called the mesoscopic regime, which is accessible when the self-Kerr and decay rates of the polariton meter are similar, $U\ensuremath{\sim}\ensuremath{\kappa}$. Capitalizing on a latching mechanism by bifurcation, the readout is sensitive to transmon-qubit relaxation error only in the first tens of nanoseconds. We thus report a single-shot fidelity of 98.6% while having an integration time of 500 ns and no requirement for an external quantum-limited amplifier.

ω_r = 2π\times 34.7

GHz and a transmon qubit at

ω_q = 2π\times 3.1

GHz coupled at rate

g = 2π\times 1.3

. With such a large detuning between cavity and qubit,

ω_r/ω_q > 10

, we are able to suppress drive induced unwanted resonant state transitions, enabling strong drives for qubit readout. We measure no resonant state transitions even when driving with more than

100

photons and read out the qubit state with more than

100

photons to achieve a measurement fidelity greater than

99\%

without the aid of a quantum limited amplifier.