Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The $\cos(2\varphi)$ qubit, consisting of an island shunted to ground through a tunneling element that selectively transmits pairs of Cooper pairs, leverages charge-parity symmetry to protect from charge-induced errors. In this experiment, we observe a doublet of states of opposite Cooper-pair parity split by $13.6~\mathrm{MHz}$ . Operating in a soft-transmon regime, this splitting is two orders of magnitude smaller than in previous implementations, pushing charge-induced losses well beyond the measured coherence times. Despite the low transition frequency, we demonstrate coherent qubit control, single-shot readout, and resolve quantum jumps. Charge protection of the qubit is evidenced by a $100-$ fold suppression of the island charge matrix element compared to the unprotected plasmon transition, placing dielectric loss limits above $10~\mathrm{ms}$ . The measured $T_1 = 70~μ\mathrm{s}$ and $T_2^\mathrm{echo}= 2.5~μ\mathrm{s}$ are instead limited by $1/f$ flux noise in the tunnelling element's loop. This experiment shows that pushing Cooper-pair pairing in the transmon regime sets high limits on charge-induced losses while preserving coherent control and single-shot readout of the low-frequency qubit. We identify flux noise as the dominant remaining limitation, calling for gradiometric designs or novel $4e$ -tunneling elements.Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The $\cos(2\varphi)$ qubit, consisting of an island shunted to ground through a tunneling element that selectively transmits pairs of Cooper pairs, leverages charge-parity symmetry to protect from charge-induced errors. In this experiment, we observe a doublet of states of opposite Cooper-pair parity split by $13.6~\mathrm{MHz}$. Operating in a soft-transmon regime, this splitting is two orders of magnitude smaller than in previous implementations, pushing charge-induced losses well beyond the measured coherence times. Despite the low transition frequency, we demonstrate coherent qubit control, single-shot readout, and resolve quantum jumps. Charge protection of the qubit is evidenced by a $100-$fold suppression of the island charge matrix element compared to the unprotected plasmon transition, placing dielectric loss limits above $10~\mathrm{ms}$. The measured $T_1 = 70~μ\mathrm{s}$ and $T_2^\mathrm{echo}= 2.5~μ\mathrm{s}$ are instead limited by $1/f$ flux noise in the tunnelling element's loop. This experiment shows that pushing Cooper-pair pairing in the transmon regime sets high limits on charge-induced losses while preserving coherent control and single-shot readout of the low-frequency qubit. We identify flux noise as the dominant remaining limitation, calling for gradiometric designs or novel $4e$-tunneling elements.

quant-pharXiv:2603.13114Mar 16, 20260 citations

A New Nonlinearity for Superconducting Circuits

Marric Stephens

PhysicsSep 5, 20240 citations

Detection and manipulation of Majorana fermions in circuit QED

Clemens Müller, Jérôme Bourassa, Alexandre Blais

Motivated by recent experimental progress towards the measurement and manipulation of Majorana fermions with superconducting circuits, we propose a device interfacing Majorana fermions with circuit quantum electrodynamics. The proposed circuit acts as a charge parity detector changing the resonance frequency of a superconducting $\ensuremath{\lambda}/4$ resonator conditioned on the parity of charges on nearby gates. Operating at both charge and flux sweet spots, this device is highly insensitive to environmental noise. It enables high-fidelity single-shot quantum nondemolition readout of the state of a pair of Majorana fermions encoding a topologically protected qubit. Additionally, the interaction permits the realization of an arbitrary phase gate on the topological qubit, closing the loop for computational completeness. Away from the charge sweet spot, this device can be used as a highly sensitive charge detector with a sensitivity better than ${10}^{\ensuremath{-}4}\text{e}/\sqrt{\text{Hz}}$ and bandwidth larger than 1 MHz.Motivated by recent experimental progress towards the measurement and manipulation of Majorana fermions with superconducting circuits, we propose a device interfacing Majorana fermions with circuit quantum electrodynamics. The proposed circuit acts as a charge parity detector changing the resonance frequency of a superconducting $\ensuremath{\lambda}/4$ resonator conditioned on the parity of charges on nearby gates. Operating at both charge and flux sweet spots, this device is highly insensitive to environmental noise. It enables high-fidelity single-shot quantum nondemolition readout of the state of a pair of Majorana fermions encoding a topologically protected qubit. Additionally, the interaction permits the realization of an arbitrary phase gate on the topological qubit, closing the loop for computational completeness. Away from the charge sweet spot, this device can be used as a highly sensitive charge detector with a sensitivity better than ${10}^{\ensuremath{-}4}\text{e}/\sqrt{\text{Hz}}$ and bandwidth larger than 1 MHz.

arXiv:1306.1539Physical Review BDec 2, 201337 citations

SUPERCONDUCTING CIRCUITRY. Progress Report

C. R. Vail, John L. Artley

May 1, 19660 citations

Parity-spin superconducting qubit based on topological insulators

Guo-Liang Guo, Han-Bing Leng, Xin Liu

Abstract We propose to utilize two parity-protected qubits which are built based on superconductor/topological-insulator/superconductor (SC/TI/SC) Josephson junction to implement a parity-spin superconducting qubit. The SC/TI/SC Josephson junctions have identical Josephson potential, which is robust against fabrication variations and guarantees the reliable cos ⁡ 2 ϕ energy-phase relation for implementing a parity-protected qubit. By viewing the even and odd parity ground states of a single parity-protected qubit as spin- 1 2 states, we construct the logic qubit states using the total parity odd subspace of two parity-protected qubits, refereed to parity-spin qubit. This parity-spin qubit exhibits robustness against charge noise, similar to a singlet-triplet (S-T) qubit’s immunity to global magnetic field fluctuations. Meanwhile, the flux noise cannot directly couple two states with the same total parity and is significantly suppressed. Benefiting from the simultaneous protection from charge and flux noise, we demonstrate an enhancement of both T 1 and T 2 coherence times. Our work presents a TI-based approach to engineer symmetry-protected superconducting qubits.Abstract We propose to utilize two parity-protected qubits which are built based on superconductor/topological-insulator/superconductor (SC/TI/SC) Josephson junction to implement a parity-spin superconducting qubit. The SC/TI/SC Josephson junctions have identical Josephson potential, which is robust against fabrication variations and guarantees the reliable cos ⁡ 2 ϕ energy-phase relation for implementing a parity-protected qubit. By viewing the even and odd parity ground states of a single parity-protected qubit as spin- 1 2 states, we construct the logic qubit states using the total parity odd subspace of two parity-protected qubits, refereed to parity-spin qubit. This parity-spin qubit exhibits robustness against charge noise, similar to a singlet-triplet (S-T) qubit’s immunity to global magnetic field fluctuations. Meanwhile, the flux noise cannot directly couple two states with the same total parity and is significantly suppressed. Benefiting from the simultaneous protection from charge and flux noise, we demonstrate an enhancement of both T 1 and T 2 coherence times. Our work presents a TI-based approach to engineer symmetry-protected superconducting qubits.

arXiv:2308.12027New Journal of PhysicsMay 14, 20242 citations

Experimental realization of a

\cos(2\varphi)

transmon qubit

Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The

\cos(2\varphi)

qubit, consisting of an island shunted to ground through a tunneling element that selectively transmits pairs of Cooper pairs, leverages charge-parity symmetry to protect from charge-induced errors. In this experiment, we observe a doublet of states of opposite Cooper-pair parity split by

13.6~\mathrm{MHz}

. Operating in a soft-transmon regime, this splitting is two orders of magnitude smaller than in previous implementations, pushing charge-induced losses well beyond the measured coherence times. Despite the low transition frequency, we demonstrate coherent qubit control, single-shot readout, and resolve quantum jumps. Charge protection of the qubit is evidenced by a

100-

fold suppression of the island charge matrix element compared to the unprotected plasmon transition, placing dielectric loss limits above

10~\mathrm{ms}

. The measured

T_1 = 70~μ\mathrm{s}

and

T_2^\mathrm{echo}= 2.5~μ\mathrm{s}

are instead limited by

1/f

flux noise in the tunnelling element's loop. This experiment shows that pushing Cooper-pair pairing in the transmon regime sets high limits on charge-induced losses while preserving coherent control and single-shot readout of the low-frequency qubit. We identify flux noise as the dominant remaining limitation, calling for gradiometric designs or novel

4e

-tunneling elements.