Session JJ04: V: Superconducting Quantum Information: Gates and Bosonic Qubits

Double-Transmon Coupler I: Elimination of Residual ZZ Coupling for Highly Detuned Transmon Qubits

Tunable couplers have attracted much attention as a key device to achieve high performance in superconducting quantum computers. Recently, we have proposed a new kind of tunable coupler composed of two fixed-frequency transmons coupled through a loop with an additional Josephson junction, which we call a double-transmon coupler (DTC) [1]. By tuning the flux in the loop, we can completely eliminate residual ZZ coupling between highly detuned qubits. This is a remarkable feature of the DTC, because conventional tunable couplers with a single frequency-tunable transmon (single-transmon couplers) can eliminate the ZZ coupling only if the detuning is smaller than the qubit anharmonicity [2] and therefore they have finite residual ZZ coupling for highly detuned qubits [3,4]. The DTC also allows us to perform fast, high-fidelity two-qubit gates [1], as explained in the next talk.

[1] H. Goto, Phys. Rev. Appl. 18, 034038 (2022) (Editors’ Suggestion)

[2] J. Stehlik et al., Phys. Rev. Lett. 127, 080505 (2021)

[3] M. C. Collodo et al., Phys. Rev. Lett. 125, 240502 (2020)

[4] Y. Xu et al., Phys. Rev. Lett. 125, 240503 (2020)   

Double-Transmon Coupler II: High-Fidelity CPHASE and Parametric Gates

As explained in the previous talk (Double-Transmon Coupler I), we proposed a new coupler, the double-transmon coupler (DTC), that can eliminate the ZZ coupling of the highly detuned qubits. As numerically shown in Ref. [1], we can achieve a fast and high-fidelity CPHASE gate by controlling the DC magnetic flux in the loop of the DTC. In this talk, we first show another important application of the DTC: a parametric gate by applying the AC magnetic flux to the loop of the DTC. The CPHASE gate in Ref. [1] and the parametric gate in this talk are implemented by utilizing ZZ and transverse coupling, respectively. To use both gates interchangeably with only flux control of the DTC, we tune various parameters for not only ZZ but also transverse coupling. As a result of tuning the circuit design parameters and the flux waveform, we successfully implemented CPHASE and parametric gates only by applying the DC and AC magnetic fluxes, respectively. We show numerical results that both gates can be implemented with high-fidelity for highly detuned, fixed-frequency qubits. We expect that a high-performance quantum computer with the DTC can be realized by using these two kinds of gates for different purposes.

[1] H. Goto, Phys. Rev. Appl. 18, 034038 (2022).   

Three-qubit interaction in superconducting circuits

Current circuit QED theory for superconducting qubits evaluates the Hamiltonian of quantum subsystem up to qubit-qubit interactions. In multiqubit circuits we consider further precision in modeling analysis by considering many-body interaction terms, whose presence can either help reduce the algorithmic depth, or manifest the error rate of spectators. In this talk we propose extended circuit QED modeling and the major differences it can make.

    

A universal set of quantum gates on dynamically protected fluxonium qubits

Fluxonium qubits are a promising platform for quantum computation since they have one of the highest coherence times among superconducting qubits. But, when choosing their parameters, we face the same problem as we do with all the other types of qubits: we need to make a compromise between coherence and control. If the matrix element of a transition between the ground and excited state is low (that is in a heavy fluxonium), the qubit can become almost insensitive to dielectric losses, but it would be impossible to control it. In the opposite regime, when the matrix element is high (light fluxonium), the qubit can be well-controlled, but the relaxation is high. For example, the fluxon states of the ultra-heavy IST qubit have relaxation times on the order of hours [1], but coherent control is not feasible with a conventional approach. We propose to solve this problem by introducing a dynamically tunable fluxonium architecture that is able to switch between these two regimes in-situ. Base band flux pulses can be used to turn the initially heavy fluxonium into the light regime with suitable interactions to form a universal set of single and two-qubit quantum gates.

[1] F. Hassani, M. Peruzzo, L. N. Kapoor, A. Trioni, M. Zemlicka, and J. M. Fink, A superconducting qubit with noise-insensitive plasmon levels and decay-protected fluxon states (2022).   

Fast and high-fidelity qubit initialization based on auxiliary energy levels in fluxonium

Fast and high-fidelity qubit initialization is a necessary operation for the implementation of quantum error correction. In circuit quantum electrodynamics, the initialization is realized by implementing a state transfer between the qubit and its readout cavity. However, to ensure the qubit does not predominantly decohere through the cavity, the coupling between them is often kept in the dispersive regime, rendering the microwave-activated state transfer a two-photon process which ultimately limits the initialization speed. Leveraging the flux-tunability and the large anharmonicity of our fluxonium qubit, we circumvent this limitation using a resonant interaction between the cavity and a noncomputational qubit transition at no cost to the nominal qubit coherence. Specifically, we utilize the second-excited energy level of the fluxonium as an auxiliary level to perform the initialization protocol with single-frequency microwave driving. Within hundreds of nanoseconds, we achieve a high-fidelity initialization of the first- and second-excited states of a fluxonium qubit. We also demonstrate that the protocol is suitable for initializing multiple-qubits.   

Non-perturbative analysis of parametric two-qubit and three-qubit gates

Realizing high fidelity entanglement gates is a major task for near-term quantum hardware. With higher fidelity gates achieved in experiments, more accurate theoretical methods are needed. Here, using non-perturbative formalism, we theoretically study an iSWAP gate activated by frequency modulation in a transmon-transmon pair. We make a comprehensive analysis to directly solve the time-dependency and introduce a continuous set of Fermionic Simulation gates by tuning qubit-qubit detuning and pulse phase. We propose a generalized fast and high fidelity three-qubit-gate that only requires one control pulse, thus eliminating time-misalignment error, which is one of the disadvantages of existing proposals for such gates. Our analysis can serve as an example for studying gates beyond the perturbative regime and RWA.

    

Nonlinear response of a transmon using the Schwinger oscillator model of angular momentum

As an alternative to the Jaynes-Cummings model of a qubit, we consider the Schwinger oscillator model of angular momentum [1] to define the pseudo spin of a transmon. Specifically, we treat the transmon as a strong anharmonic oscillator capacitively coupled to a readout resonator, itself represented by a linear oscillator. We then calculate the nonlinear frequency response of the Schwinger pseudo spin in the presence of a driving sinusoidal voltage and surrounding heat bath. We discuss the coherence of the resulting nonlinear many-body state that underlies the pseudo spin, as it arises in the steady state, and consider its broader application to quantum computation. 1) J. Schwinger, On Angular Momentum, U. S. Atomic Energy Commission Report NYO-3071 (1952).   

Detuned two-qubit gate in stabilized cat qubits

Stabilized cat qubits, together with a set of bias-preserving (BP) gates, can provide a biased noise channel having a high code capacity threshold for error correction. Compared to qubits without such structured noise channel, cat qubits require less resource overhead for fault-tolerant quantum computation as the probability of X and Y errors are exponentially suppressed compared to Z errors. However, fast single and multi-qubit gate operations are challenging to execute while ensuring that the noise bias is preserved since they do not necessarily commute with the dominant noise. Recently, a scheme for adiabatically implementing BP Controlled-NOT gate (CNOT) has been proposed by exploiting the freedom of moving outside the computational subspace during gate execution. The scheme uses a detuning term to dynamically compensate for the accumulated geometric phase on the target qubit during adiabatic evolution. The detuning however shifts the eigenstates of the target qubit, impacting the overall fidelity of the gate. Here we study the implications of this term on the overall CNOT gate and explore possible improvements in gate fidelity.   

Schrödinger cat in a Kerr-free SNAIL-terminated resonator

Schrödinger cat states offer variety of application in quantum information processing [1]. Schrödinger cat states were normally observed in 3D cavity with permanent Kerr nonlinearity [2] or concurrent squeezing drive [1]. However, the nonlinearity from the ancillary qubit may induce the collapse of the cat states. Here, we demonstrate a method to generate Schrödinger cat states in a coplanar SNAIL-terminated resonator biased at Kerr-free point. To manipulate and stabilize the non-classical superposition of microwave coherent states, we apply a pulse to tune the Kerr coefficient fastly. Through this strategy, multi-component Schrödinger cat states can be implanted and maintained passively without additional microwave signal. Wigner function measurement on the cat state via an ancillary transmon qubit verifies the fidelity and evolution processes of the non-classical states of microwave field. Our result shows the potential of a scalable 2D structure for the continuous-variable quantum computing based on Schrödinger cats.

[1]. A. Grimm et al., Stabilization and operation of a Kerr-cat qubit, Nature 584, 205-209 (2020).
[2]. G. Kirchmair et al., Observation of quantum state collapse and revival due to the single-photon Kerr effect, Nature 495, 205–209 (2013).   

An ultra-high gain single-photon transistor in the microwave regime

Photon-photon interaction at the single-photon level enables the realization of a single-photon transistor, in analogy to an electronic transistor, that can switch or amplify "source" photons controlled by a single "gate" photon. Circuit quantum electrodynamics provides great flexibility to generate such an interaction, and thus could serve as an effective platform to realize a high performance single-photon transistor. In this presentation, we will present our approach to the realization of such a photonic transistor in the microwave regime with circuit QED architectures, which consists of two microwave cavities dispersively coupled to a superconducting qubit. A single gate photon imprints a phase shift on the qubit state through one cavity as a controlled-phase gate, and further shifts the resonance frequency of the other cavity to trigger the switch of the input signal.

In this way, we realize a gain of the transistor up to 53.4 dB, with an extinction ratio better than 20 dB. Our device outperforms previous devices in the optical regime by several orders in terms of optical gain, which indicates a great potential for application in the field of microwave quantum photonics, such as coherent state manipulation and the realization of hardware-efficient quantum random access memory.