Protected superconducting qubits such as the 0- π qubit promise to substantially reduce physical error rates. However, a key challenge in the field is designing gates for these qubits that do not compromise their protection or become infeasibly slow as the protection of the qubit is improved. In this work we propose a protected phase gate that is compatible with the protected regime of the 0- π qubit and does not suffer from spurious coupling to additional circuit modes. Our gate utilizes an internal mode of the circuit as an ancilla and is achieved by varying the qubit-ancilla coupling via a tunable Josephson element. Through numerical simulations, we study how the gate error scales with the circuit parameters of the 0- π qubit and the tunable Josephson element that enacts the gate. Ultimately, we find that a protected gate with the 0- π qubit is possible with near-term circuit parameters. Our work opens up the possibility of performing protected gates on protected superconducting qubits, which may significantly reduce hardware overheads for quantum computation.
Protected superconducting qubits such as the 0- π qubit promise to substantially reduce physical error rates. However, a key challenge in the field is designing gates for these qubits that do not compromise their protection or become infeasibly slow as the protection of the qubit is improved. In this work we propose a protected phase gate that is compatible with the protected regime of the 0- π qubit and does not suffer from spurious coupling to additional circuit modes. Our gate utilizes an internal mode of the circuit as an ancilla and is achieved by varying the qubit-ancilla coupling via a tunable Josephson element. Through numerical simulations, we study how the gate error scales with the circuit parameters of the 0- π qubit and the tunable Josephson element that enacts the gate. Ultimately, we find that a protected gate with the 0- π qubit is possible with near-term circuit parameters. Our work opens up the possibility of performing protected gates on protected superconducting qubits, which may significantly reduce hardware overheads for quantum computation.
We analyze the accuracy of quantum phase gates acting on ``0-$\ensuremath{\pi}$ qubits'' in superconducting circuits, where the gates are protected against thermal and Hamiltonian noise by continuous-variable quantum error-correcting codes. The gates are executed by turning on and off a tunable Josephson coupling between an LC oscillator and a qubit or pair of qubits; assuming perfect qubits, we show that the gate errors are exponentially small when the oscillator's impedance L/C is large compared to $\ensuremath{\hbar}/4{e}^{2}\ensuremath{\approx}1\phantom{\rule{4pt}{0ex}}\mathrm{k}\ensuremath{\Omega}$. The protected gates are not computationally universal by themselves, but a scheme for universal fault-tolerant quantum computation can be constructed by combining them with unprotected noisy operations. We validate our analytic arguments with numerical simulations.
Superconducting circuits rank among some of the most interesting architectures for the implementation of quantum information processing devices.The recently proposed 0-π qubit (Brooks et al 2013 Phys.Rev. A 87 52306) promises increased protection from spontaneous relaxation and dephasing.In this paper we present a detailed theoretical study of the coherence properties of the 0-π device, investigate relevant decoherence channels, and show estimates for achievable coherence times in multiple parameter regimes.In our analysis, we include disorder in circuit parameters, which results in the coupling of the qubit to a low-energy, spurious harmonic mode.We analyze the effects of such coupling on decoherence, in particular dephasing due to photon shot noise, and outline how such a noise channel can be mitigated by appropriate parameter choices.In the end we find that the 0-π qubit performs well and may become an attractive candidate for the implementation of the next-generation superconducting devices for uses in quantum computing and information.
The prospects for a utility-scale, fault-tolerant superconducting quantum computer have grown increasingly promising through demonstrations of quantum advantage and quantum error correction. The next breakthrough will likely be driven by processors at scale, with qubit counts exceeding tens of thousands. Achieving this scale presents a formidable challenge for current architectures, including limitations in chip footprint, cryogenic cooling capacity, and control complexity. One approach to alleviating these constraints is to make each qubit better, thereby reducing the redundancy required for error correction. Protected qubits provide a promising path in this direction by encoding quantum information into a decoherence-free subspace. This can be achieved through Hamiltonian engineering in superconducting circuits, with representative examples including the 0–π qubit, the cos(2φ) qubit, and the GKP qubit. In this thesis, we present the superconducting grid states qubit, or gridium, as a strong candidate for protected qubits. The gridium consists of a Cooper-quartet tunneling element shunted by a quantum phase slip element. We show that the gridium realizes a decoherence-free subspace within a single mode and can achieve strong protection using readily available superconducting circuit elements. We further demonstrate that its key components—Cooper-quartet tunneling and quantum phase slip—can be realized and engineered through appropriate circuit design, as illustrated with a prototype device. By applying Hamiltonian engineering, we construct three gridium devices with varying levels of protection. We show that the measured spectra are consistent with numerical simulations of the theoretical model. From these fits, we extract the corresponding wavefunctions and observe enhanced protection as the number of grids increases. To directly probe this protection, we measure the depolarization times of the devices while dynamically tuning the protection via flux control. Our best device exhibits T1 = 2.4 ms, comparable to state-of-the-art superconducting qubits. The gridium architecture presented in this thesis demonstrates strong potential for realizing fault-tolerant quantum processors based on protected qubits.
Parity-protected superconducting qubits offer intrinsically long coherence, but many current implementations require magnetic-flux biasing, which introduces flux noise, control overhead, and limited scalability. Here we propose a parity-protected qubit based on a gate-tunable superconductor-altermagnet-superconductor Josephson junction. Altermagnets are compensated magnets with momentum-dependent spin splitting and zero net magnetization, providing spin-dependent functionality without external magnetic fields. In the proposed junction, the two spin sectors acquire opposite phase shifts, generating two Josephson channels whose interference is controlled electrically by the chemical potential. At the tuned 0-π transition, the first Josephson harmonic is strongly suppressed while the second harmonic dominates, yielding a double-well potential with two nearly degenerate states of opposite Cooper-pair parity. For realistic gatemon-compatible parameters, we estimate coherence times of up to tens of milliseconds while maintaining fully gate-controlled qubit operations. These results establish altermagnetic Josephson junctions as a promising route toward protected superconducting qubits with local, scalable, and all-electrical control.
Identifying a materials platform for creating qubits that are both tunable and resilient towards environmental noise is one of the main hurdles that need to be overcome to realize quantum computation that is practically useful. One pursued avenue to this end is to use superconducting qubits with intrinsic spin-dependent interactions, such as spin-orbit coupling or magnetism. However, the recently discovered class of materials known as altermagnets remains largely unexplored in this context. We here use microscopic calculations to determine how the properties of superconducting qubits are modified when altermagnetic Josephson junctions are included. The key qubit performance parameters, including splitting, anharmonicity, decoherence, and single/coupled-qubit gate operation times, display rich behavior depending on the characteristic properties of the altermagnetic material, such as the strength of the Néel field and the crystallographic orientation of the altermagnetic relative to the interfaces in the system. We focus in particular on the transmon design and show that the qubit is very well protected against decoherence and simultaneously shows superior anharmonicity both near 0-π transition points and when it is in a φ-state. We propose that by using strain, the altermagnetic qubit can be moved out of its protected regime to enable faster gate operation times, and then moved back to its protected state. We establish the physical mechanism underlying the behavior of all central qubit metrics, clarifying how real devices interpolate between an altermagnetic double-well regime exhibiting barrier-induced protection and a conventional transmon-like single-well regime. We also discuss how the altermagnetic properties influence flux qubits and fluxonium.
Multi-qubit parity measurements are at the core of many quantum error correction schemes. Extracting multi-qubit parity information typically involves using a sequence of multiple two-qubit gates. In this paper, we propose a superconducting circuit device with native support for multi-qubit parity-controlled gates (PCG). These are gates that perform rotations on a parity ancilla based on the multi-qubit parity operator of adjacent qubits, and can be directly used to perform multi-qubit parity measurements. The circuit consists of a set of concatenated Josephson ring modulators and effectively realizes a set of transmon-like qubits with strong longitudinal nearest-neighbor couplings. PCGs are implemented by applying microwave drives to the parity ancilla at specific frequencies. We investigate the scheme's performance with numerical simulation using realistic parameter choices and decoherence rates, and find that the device can perform four-qubit PCGs in 30 ns with process fidelity surpassing 99%. Furthermore, we study the effects of parameter disorder and spurious coupling between next-nearest neighboring qubits. Our results indicate that this approach to realizing PCGs constitute an interesting candidate for near-term quantum error correction experiments.