Nonreciprocity and Circulation in a Passive Josephson-Junction Ring.

Building large-scale superconducting quantum circuits will require miniaturization and integration of supporting devices including microwave circulators, which are currently bulky, stand-alone components. Here, we report the measurement of microwave scattering from a ring of Josephson junctions, with dc-only control fields. We detect the effect of quasiparticle tunneling, and dynamically classify the system at its operating design point into different quasiparticle sectors. We optimize the device within one of the quasiparticle sectors, where we observe an unambiguous signature of nonreciprocal 3-port scattering within that sector. This enables operation as a circulator, and at the optimal circulation point, we observe on-resonance insertion loss of 2 dB, isolation of 14 dB, power reflectance of -11 dB, and a bandwidth of 200 MHz, averaged over the 3 input ports.

Characterization of a Scalable Donor-Based Singlet-Triplet Qubit Architecture in Silicon.

We present a donor-based quadruple-quantum-dot device, designed to host two singlet-triplet qubits fabricated by scanning tunnelling microscope lithography, with just two leads per qubit. The design is geometrically compact, with each pair of dots independently controlled via one gate and one reservoir. The reservoirs both supply electrons for the dots and measure the singlet-triplet state of each qubit via dispersive sensing. We verify the locations of the four phosphorus donor dots via an electrostatic model of the device. We study one of the observed singlet-triplet states with a tunnel coupling of 39 GHz and a S0-to- T- decay of 2 ms at zero detuning. We measure a 5 GHz electrostatic interaction between two pairs of dots separated by 65 nm. The results outline a low-gate-density pathway to a scalable 1D building block of atomic-precision singlet-triplet qubits using donors with dispersive readout.

Coherent control of a donor-molecule electron spin qubit in silicon.

Donor spins in silicon provide a promising material platform for large scale quantum computing. Excellent electron spin coherence times of [Formula: see text] µs with fidelities of 99.9% have been demonstrated for isolated phosphorus donors in isotopically pure 28Si, where donors are local-area-implanted in a nanoscale MOS device. Despite robust single qubit gates, realising two-qubit exchange gates using this technique is challenging due to the statistical nature of the dopant implant and placement process. In parallel a precision scanning probe lithography route has been developed to place single donors and donor molecules on one atomic plane of silicon with high accuracy aligned to heavily phosphorus doped silicon in-plane gates. Recent results using this technique have demonstrated a fast (0.8 ns) two-qubit gate with two P donor molecules placed 13 nm apart in natSi. In this paper we demonstrate a single qubit gate with coherent oscillations of the electron spin on a P donor molecule in natSi patterned by scanning tunneling microscope (STM) lithography. The electron spin exhibits excellent coherence properties, with a [Formula: see text] decoherence time of 298 ± 30 µs, and [Formula: see text] dephasing time of 295 ± 23 ns.

Spin read-out in atomic qubits in an all-epitaxial three-dimensional transistor.

The realization of the surface code for topological error correction is an essential step towards a universal quantum computer1-3. For single-atom qubits in silicon4-7, the need to control and read out qubits synchronously and in parallel requires the formation of a two-dimensional array of qubits with control electrodes patterned above and below this qubit layer. This vertical three-dimensional device architecture8 requires the ability to pattern dopants in multiple, vertically separated planes of the silicon crystal with nanometre precision interlayer alignment. Additionally, the dopants must not diffuse or segregate during the silicon encapsulation. Critical components of this architecture-such as nanowires9, single-atom transistors4 and single-electron transistors10-have been realized on one atomic plane by patterning phosphorus dopants in silicon using scanning tunnelling microscope hydrogen resist lithography11,12. Here, we extend this to three dimensions and demonstrate single-shot spin read-out with 97.9% measurement fidelity of a phosphorus dopant qubit within a vertically gated single-electron transistor with <5 nm interlayer alignment accuracy. Our strategy ensures the formation of a fully crystalline transistor using just two atomic species: phosphorus and silicon.