Toshiba double-transmon coupler

Raveh Neeman

doi:10.5281/zenodo.22674814

Toshiba + RIKEN RQC + University of Tokyo

Toshiba double-transmon coupler

Toshiba + RIKEN RQC + University of Tokyo · superconducting circuits · DEMONSTRATED (Nov 2024) · 2 physical qubits · access: lab-only, two-qubit test devices

architecture: Transmon lattice with tunable couplers · Open on the map

Register profile — the machine's variant of its architecture

qubit type: fixed-frequency transmon

gate mechanism: double-transmon coupler (two transmons sharing a three-junction loop), external flux switches the interaction; CZ 48 ns

connectivity: two-qubit test pair

control: room-temperature RF; a shunt capacitance variant removes DC bias current at idle

control placement: room-temp

readout: dispersive

gate evidence: a benchmark paper with its protocol

flags: a component, not a machine

Technologies by layer — the machine's cell per layer

1 Qubit carrier	Transmon · primary ▣ Fig. 1a-b (circuit schematic: data qubits Q1, Q2 + coupler transmons C3, C4; false-color device picture); §II Device setup ✅ two fixed-frequency transmons: Q1 4.314 GHz (T1 230 µs, T2 360 µs), Q2 4.778 GHz (T1 210 µs, T2 130 µs) (Nov 2024)
2 Encoding	Bare physical qubit (no encoding) · primary ◌ 🔎 Bare two-level subspace — Computational subspace of an anharmonic oscillator; leakage to \|2⟩ is the price.
3 Gate mechanism	Tunable-coupler CZ / iSWAP · primary ▣ Fig. 1a (DTC circuit); §IV CZ-gate implementation; Fig. 4c-d (interleaved RB; 12-hour CZ-error run); Fig. 5 (error vs gate length) ✅ double-transmon coupler (two transmons, three-junction loop), flux-switched; CZ 99.90% averaged over 12 h at 48 ns; residual ZZ ≈6 kHz off (Nov 2024)
4 Connectivity / transport	Static nearest-neighbour lattice · primary ▣ Fig. 1b (false-color picture of the two-qubit device: Q1-Q2 joined by the DTC) ✅ two-qubit test pair (degree 1); no multi-qubit DTC processor exists
5 Control	Room-temperature electronics + per-qubit coax/flex · primary ▣ Fig. 6 (schematic diagram of the measurement setup); Appendix B Measurement setup; Fig. 2 (Z-pulse on coupler loop) ✅ room-temperature RF with external flux bias; shunt-capacitance variant idles at zero flux, no DC bias current (PRApplied 23, 064069, Jun 2025)
6 Readout	Dispersive microwave readout (+TWPA, Purcell) · primary ▣ Fig. 1a (readout elements); Fig. 10 (single-shot readouts of the three resonators); Appendix F State discrimination ✅ dispersive
7 Code	— none — nothing in this layer ▦ 🔎 no QEC code run; two-qubit component test devices only
8 Decoder	— none — nothing in this layer ▦ 🔎 no decoder (no QEC)
9 Interconnect	— none — nothing in this layer ◌ 🔎 monolithic two-qubit test device, no interconnect
10 Manufacturing	Superconducting-qubit lithography (Al/AlOx/Al junctions, Nb wiring, 300 mm) · primary ▤ Appendix A: Fabrication ✅ Superconducting-qubit lithography (Nb/Al JJ, 300 mm) — IBM Albany 300 mm / Anderon foundry; Google Santa Barbara; Rigetti ABAA trimming (97.4% frequency-targeting success); OQC 500-qubit wafer-scale…

Standard records

crosstalk → 6.0e+3 Hz · best published for superconducting (residual ZZ in the off state)

Nov 2024 · source

2Q error under full-width parallel operation → 1.00e-3 · best-of-line (99.90% CZ averaged over 12 h on a 2-qubit device)

Nov 2024 · source

T1 relaxation → 230 µs · best

Nov 2024 · source

T2 coherence (echo/DD) → 360 µs · best

Nov 2024 · source

evidence: ✅ 6 / 10

Pictures

Every picture carries the story of what it shows and its credit line.

Fig. 1 — Figure 1: (a) CAD rendering of the transmon emitter/detector (TED) layout and (b) its corresponding circuit, indexed by x∈{s,m}x\{s,m\} to distingui… — A four-panel figure: a CAD rendering of a superconducting chip layout (200 µm scale bar), its circuit diagram, a measured plot of frequency against applied flux, and a block schematic of source and detector devices. It comes from the arXiv paper "Transmon Architecture for Emission and Detection of Single Microwave Photons" by Daniel L. Campbell and co-authors. It shows three coupled transmons — storage, waveguide-linked, and a flux-driven coupler between them — that release or catch single microwave photons; notice the storage transmon's frequency barely moves. “Fig. 1 — Figure 1: (a) CAD rendering of the transmon emitter/detector (TED) layout and (b) its corresponding circuit, indexed by x∈{s et al., arxiv.org/abs/2601.11378, CC BY 4.0 (creativecommons.org/licenses/by/4.0/)