Abstract: The disclosure relates to a repetition code for a cat qubit comprising a number d of data cat-qubits and at least ancillary cat-qubits, wherein the number d is greater than or equal to 3, each ancillary cat-qubit is connected to two data cat-qubits by two respective CNOT gates such that no data cat-qubit is connected to more than two ancillary cat-qubits, and the two-photon dissipation rate of the ancillary cat-qubits is greater than the two-photon dissipation factor of the data cat-qubits. Said repetition code is implemented by carrying out, for each ancillary cat-qubit, error correction cycles comprising at least the following steps: preparing the ancillary cat-qubit in a state suitable for the operator X: “I+>” or “I?>”; activating one of the two CNOT gates connected to said ancillary cat-qubit (6); activating the other CNOT gate connected to this ancillary cat-qubit; and measuring the photon-number parity of said ancillary cat-qubit.

Abstract: A superconducting microwave quantum circuit includes a controllable-energy Josephson junction element connected to a linear passive circuit portion exhibiting a plurality of resonant modes, of which the Foster's first-form decomposition across the terminals of the Josephson junction element includes a target resonant mode exhibiting an impedance Z higher than 13 kohm and a pulsation w. The energy of the Josephson junction element is controllable and modulated by at least two respective pulse trains within which the pulses are separated by a duration 2?/w and have a width of less than one tenth of this duration, the amplitude of the pulses within each respective pulse train being modulated by a respective sinusoidal carrier and the pulse trains are respectively offset pairwise by a duration ?t such that |sin(w*At)| equals 13 kohm/Z, so that the resonant mode of pulsation w is stabilised in one of two GKP states encoding a qubit.

Abstract: A confinement device for a cat qubit includes a two-photon exchanger, a low quality-factor buffer oscillator and a high-quality factor anharmonic buffer oscillator. The oscillators are connected to the exchanger such that, when they are driven at their respective resonance frequency (?l, ?h) and the exchanger is connected to a cat qubit oscillator, an exchange of two photons from the cat qubit oscillator with one photon from the low-quality oscillator and an exchange of two photons from the cat qubit oscillator with one photon from the high-quality oscillator respectively take place. The device implements a Hamiltonian of formula g2h((a2??2)bh†+(a2??2)+bh)+g2l((a2??2)†bl+(a2??2)bl†) where g2h and g2l are Hamiltonian forces, a is the photon annihilation operator of the cat qubit oscillator, ? is the amplitude of the cat state, bh is the photon annihilation operator of the high-quality oscillator, bl is the photon annihilation operator of the low-quality oscillator.

Abstract: Techniques for modifying the Josephson potential of a transmon qubit by shunting the transmon with an inductance are described. The inclusion of this inductance may increase the confined potential of the qubit system compared with the conventional transmon, which may lead to a transmon qubit that is stable at much higher drive energies. The inductive shunt may serve the purpose of blocking some or all phase-slips between the electrodes of the qubit. As a result, the inductively shunted transmon may offer an advantage over conventional devices when used for applications involving high energy drives, whilst offering few to no drawbacks in comparison to conventional devices when used at lower drive energies.

Abstract: Techniques for modifying the Josephson potential of a transmon qubit by shunting the transmon with an inductance are described. The inclusion of this inductance may increase the confined potential of the qubit system compared with the conventional transmon, which may lead to a transmon qubit that is stable at much higher drive energies. The inductive shunt may serve the purpose of blocking some or all phase-slips between the electrodes of the qubit. As a result, the inductively shunted transmon may offer an advantage over conventional devices when used for applications involving high energy drives, whilst offering few to no drawbacks in comparison to conventional devices when used at lower drive energies.

Abstract: Techniques for modifying the Josephson potential of a transmon qubit by shunting the transmon with an inductance are described. The inclusion of this inductance may increase the confined potential of the qubit system compared with the conventional transmon, which may lead to a transmon qubit that is stable at much higher drive energies. The inductive shunt may serve the purpose of blocking some or all phase-slips between the electrodes of the qubit. As a result, the inductively shunted transmon may offer an advantage over conventional devices when used for applications involving high energy drives, whilst offering few to no drawbacks in comparison to conventional devices when used at lower drive energies.