Engineering cross resonance interaction in multi-modal quantum circuits

Abstract

Existing scalable superconducting quantum processors have only nearest-neighbor coupling. This leads to reduced circuit depth, requiring large series of gates to perform an arbitrary unitary operation in such systems. Recently, multi-modal devices have been demonstrated as a promising candidate for small quantum processor units. Always on longitudinal coupling in such circuits leads to implementation of native high fidelity multi-qubit gates. We propose an architecture using such devices as building blocks for a highly connected larger quantum circuit. To demonstrate a quantum operation between such blocks, a standard transmon is coupled to the multi-modal circuit using a 3D bus cavity giving rise to small exchange interaction between the transmon and one of the modes. We study the cross resonance interaction in such systems and characterize the entangling operation as well as the unitary imperfections and cross-talk as a function of device parameters. Finally, we tune up the cross resonance drive to implement multi-qubit gates in this architecture.Superconducting qubits have become one of the most promising platforms for quantum computation and quantum information processing 1 in the near term. Over the past decade small quantum processors with superconducting qubits have shown tremendous improvement in terms of coherence times reaching milliseconds 2,3 and scalability up to 10-70 qubits 4,5 . However almost all the existing architectures 6-9 in superconducting qubits have only nearest-neighbor coupling. With limited connectivity this often imposes strong constraints on available multi-qubit operations in such architectures and leads to inefficient implementation of quantum algorithms and quantum simulations 10 . On the other hand, always on all-to-all interaction in longitudinally coupled multimodal devices 11,12 leads to implementation of fast high fidelity N-qubit gates in the circuit. Previous experiments have demonstrated such devices as an effective three qubit processor with efficient implementation of small quantum algorithms 13 . Using multi-modal devices as building blocks for a larger quantum processor could enable greater interqubit connectivity and increased circuit depth for quantum information processing. This is a useful approach to enhance the performance of near-term imperfect quantum processors 14,15 without fault tolerance.In this letter, we demonstrate a circuit QED architecture consisting of a multi-modal superconducting circuit 12 and a transmon 16 qubit coupled via an exchange (σ x σ x ) coupling mediated via a bus cavity. We numerically analyze the effect of a cross resonance 17,18 drive in such systems and estimate the elements of the effective Hamiltonian 19 for experimentally realizable parameters. Then we use a frequency tunable transmon to experimentally study the cross resonance effect as a function of detuning between the two qubits. We identify the optimum detuning range and tune up cross resonance interaction for a multi-qubit entangling gate. We characterize the performance of ...

Long-range connectivity in a superconducting quantum processor using a ring resonator

“…Our estimate of cross-talk is in congruence with a previous result [37] showing significantly lower cross-talk in 3D cQED architecture compared to the existing 2D designs [36] due to superior microwave isolation between the cavities.…”

Removing leakage-induced correlated errors in superconducting quantum error correction

“…Superconducting transmon qubits are an appealing platform for the implementation of quantum error correction [4][5][6][7][8][9][10][11][12][13]. However, the fundamental operations, such as single-qubit gates [14,15], entangling gates [16][17][18][19][20], and measurement [21] are known to populate non-computational levels, creating a demand for a reset protocol [22][23][24][25][26][27] that can remove leakage population from these higher states without adversely impacting performance in a large scale system. Directly quantifying leakage during normal operation presents another challenge, as optimizing measurement for detecting multiple levels is hard to combine with high speed and fidelity.…”

Hybrid Quantum-Classical Eigensolver Without Variation or Parametric Gates

“…In essence, the implementation of the circuits described in this paper will depend on the ability to limit circuit depth and associated error rates by NISQ hardware circuit optimization (i.e., scheduling). However, significant improvements in qubit connectivity of various modalities (e.g., ion traps) [22,42] or optimizing quantum circuits against decoherence [43,44] may blunt this concern.…”