Conveyor-belt superconducting quantum computer

TL;DR

The paper proposes a globally driven superconducting quantum processor with a conveyor-belt architecture that uses always-on longitudinal ZZ interactions to achieve universal quantum computation with $O(N)$ physical qubits for $N$ logical qubits. Computation is encoded on a string of information-carrying qubits separated by non-active sectors, while global control lines drive two qubit species (regular and crossed) to realize selective operations, including a novel one-shot Toffoli gate. A key mechanism is a Rydberg-like blockade induced by ZZ couplings, enabling effective local operations under global pulses, and a programmable exchange protocol that relocates qubits without disturbing data. Universality is demonstrated by combining arbitrary single-qubit gates, the exchange operation, and the Toffoli gate, with scalable swap generation on the IC qubit graph; the architecture supports fault-tolerant strategies and may be adapted to multiple platforms, potentially benefiting from advanced quantum error-correcting codes such as qLDPC codes.

Abstract

The processing unit of a solid-state quantum computer consists in an array of coupled qubits, each locally driven with on-chip microwave lines that route carefully-engineered control signals to the qubits in order to perform logical operations. This approach to quantum computing comes with two major problems. On the one hand, it greatly hampers scalability towards fault-tolerant quantum computers, which are estimated to need a number of qubits -- and, therefore driving lines -- on the order of $10^6$. On the other hand, these lines are a source of electromagnetic noise, exacerbating frequency crowding and crosstalk, while also contributing to power dissipation inside the dilution fridge. We here tackle these two overwhelming challenges by presenting a novel quantum processing unit (QPU) for a universal quantum computer which is globally (rather than locally) driven. Our QPU relies on a string of superconducting qubits with always-on ZZ interactions, enclosed into a closed geometry, which we dub ``conveyor belt''. Strikingly, this architecture requires only $\mathcal{O}(N)$ physical qubits to run a computation on $N$ computational qubits, in contrast to previous $\mathcal{O}(N^2)$ proposals for global quantum computation. Additionally, universality is achieved via the implementation of single-qubit gates and a one-shot Toffoli gate. The ability to perform multi-qubit operations in a single step could vastly improve the fidelity and execution time of many algorithms.

Figures (4)

• Figure 1: Schematic description of the proposed architecture. Two types, $A$ and $B$, of superconducting qubits (red and blue squares, respectively) are separately driven by two classical sources $V_{A,B}(t)$ (red and blue continuous lines). They are coupled via a longitudinal ZZ coupling (black and grey springs). Black triangles inside regular and crossed qubits denote local corrections in the resonance frequency of the qubits (see main text). The $A$-type crossed qubit (red square inside the loop) enables one-shot Toffoli gate (three-qubit gate) -- the corresponding interactions are depicted in gray. The $B$-type crossed qubit performs single-qubit gates. The elements highlighted in yellow indicate the information carrying sites $Q_1$, $Q_2$, $\cdots$, $Q_N$, separated by three-qubits sectors $S_1$, $S_2$, $\cdots$, $S_N$. The $Q_j$'s host the computational qubits through well-formed configurations (\ref{['encoding']}) where the $S_j$'s are initialized in alternating sequences of paramagnetic ($geg$) and ferromagnetic ($ggg$) phases. Since ZZ interactions critically depend on the relative distance between two qubits, we have used the honeycomb structure to make sure that the couplings among adjacent qubits are (nominally) all identical. The figure pertains to a $N=8$ qubit quantum computer.
• Figure 2: (a) Schematic description of the unitary evolution induced by $\hat{\Pi}_{\rm exc}$ on a ${\rm FP}$ well-formed state where the $Q_7$ and $Q_8$ sites are in the input states $\vert \psi_{7 ,8} \rangle$. This unitary operation acts as a swap between two qubits, separated by a ferromagnetic region. Moreover, after the unitary, paramagnetic and ferromagnetic phases are exchanged. The reverse unitary transformation is $\hat{\Pi}^{-1}_{\rm exc} := \hat{\Pi}_B \hat{\Pi}_{\rm exc} \hat{\Pi}_B$. (b) Schematic representation of the universality proof.
• Figure 3: Alternative design with respect to the conveyor-belt quantum computer of the main text. Two two-qubit gates are present in the setup to ensure the universality of the computation. In order to control them independently we employ a third Rabi frequency (A-type double-crossed qubit), see Eq. \ref{['double-crossed']}.
• Figure 4: Alternative design with respect to the conveyor-belt quantum computer of the main text. Two two-qubit gates are present in the setup to ensure the universality of the computation. The two qubits which mediate the two-qubit gates are driven by a third control source $V_C(t)$, i.e. they belong to a third, namely $C$, species (green elements): in order to control them independently the Rabi frequency must be different (one of them is regular, the other one is crossed), see Eq. \ref{['crossed_C']}.