A research team led by the RIKEN Center for Emergent Matter Science (CEMS) has constructed a hybrid device for quantum computing, made from two different types of qubits, each with distinct advantages. The new system can be quickly initialized and read out, while simultaneously maintaining high control fidelity. In 1998, Daniel Loss, one of the authors of the current study, proposed, along with David DiVincenzo of IBM, to build a quantum computer by using the spins of electrons embedded in a quantum dot. The first type of qubit used for the new system — a single-spin qubit called a Loss-DiVincenzo (LD) qubit — has high control fidelity, which makes it good for calculations. It also has a long decoherence time, so that it will stay in a given state for a relatively long time before losing its signal to the environment. The downside of the LD qubit is that it cannot be quickly initialized into a state or read out. The second type of qubit used, called a singlet-triplet (ST) qubit, can be quickly initialized and read out, but it quickly becomes decoherent. For the study, the scientists implemented a controlled-phase (CPHASE) gate between an LD qubit and an ST qubit in a quantum dot array. This allowed spin states to be entangled between the qubits in a time fast enough to maintain the coherence, allowing the state of the single-spin qubit to be read out by the fast singlet-triplet qubit measurement. Schematic of the device. Courtesy of RIKEN CEMS. The controlled-phase gate acted within 5.5 nanoseconds (ns). Although the researchers did not pursue benchmarking protocols, they point out that the gate time is much shorter than the corresponding dephasing time (211 ns), indicating that the fidelity of this type of gate could be high. Spin-based quantum computers have the potential to tackle difficult mathematical problems that cannot be solved using ordinary computers, but it has proven difficult to scale them up to the size required for performing calculations. The team believes that its hybrid architecture could help solve scalability issues when building spin-based quantum computers. According to researcher Akito Noiri, “With this study we have demonstrated that different types of quantum dots can be combined on a single device to overcome their respective limitations. This offers important insights that can contribute to the scalability of quantum computers.” The research was published in Nature Communications (https://doi.org/10.1038/s41467-018-07522-1).