Superconducting circuits offer tremendous design flexibility in the quantum regime culminating most recently in the demonstration of few qubit systems supposedly approaching the threshold for fault-tolerant quantum information processing. Competition in the solid-state comes from semiconductor qubits, where nature has bestowed some very useful properties which can be utilized for spin qubit-based quantum computing. Here we begin to explore how selective design principles deduced from spin-based systems could be used to advance superconducting qubit science. We take an initial step along this path proposing an encoded qubit approach realizable with state-of-the-art tunable Josephson junction qubits. Our results show that this design philosophy holds promise, enables microwave-free control, and offers a pathway to future qubit designs with new capabilities such as with higher fidelity or, perhaps, operation at higher temperature. The approach is also especially suited to qubits based on variable super-semi junctions.
![Encoded superconducting qubits and tunable Josephson junctions. (a), Schematic diagram of a possi- ble encoded superconducting qubit scheme as described in the text. An encoded qubit consists of two tunable physical SC qubits (e.g. tunable transmons such as xmons or gatemons), with the encoded qubit states |0⟩Q=|01⟩ and |1⟩Q=|10⟩. In this picture, two encoded qubits are shown (e.g., physical qubits 1a and 1b form an encoded qubit) and more encoded qubits can be introduced in a straightforward manner. Each SC qubit has a z-control line which tunes the Josephson en- ergy EJ, and there are no additional microwave xy-control lines. All manipulation of the qubit states are done by the z-control pulses. Each transmons are capacitively coupled to neighboring transmons, and also coupled to a transmission line resonator for readout. (b), a double JJs in a loop acts as a tunable JJ, controlled by an externally applied magnetic flux. In the SQUID tunable approach, one of the xmons in each en- coded qubit needs a separate voltage control to tune the gate charge number ng which is needed to initialize the encoded qubit state.(c), electrostatically tunable JJ based on a prox- imitized superconducting-semiconductor nanowire connecting two superconductors [31] used for gatemons. The nanowire is coated with SC for clean contact and a portion is lifted off to form a semiconductor nanowire weak link. The JJ energy EJ is tuned by a side-gate voltage VG, which can also serve as capacitive tuning for initialization.](http://research.tahan.com/wp-content/uploads/2015/11/Fig12-590x512.jpg)
Encoded superconducting qubits and tunable Josephson junctions. (a), Schematic diagram of a possi- ble encoded superconducting qubit scheme as described in the text. An encoded qubit consists of two tunable physical SC qubits (e.g. tunable transmons such as xmons or gatemons), with the encoded qubit states |0⟩Q=|01⟩ and |1⟩Q=|10⟩. In this picture, two encoded qubits are shown (e.g., physical qubits 1a and 1b form an encoded qubit) and more encoded qubits can be introduced in a straightforward manner. Each SC qubit has a z-control line which tunes the Josephson en- ergy EJ, and there are no additional microwave xy-control lines. All manipulation of the qubit states are done by the z-control pulses. Each transmons are capacitively coupled to neighboring transmons, and also coupled to a transmission line resonator for readout. (b), a double JJs in a loop acts as a tunable JJ, controlled by an externally applied magnetic flux. In the SQUID tunable approach, one of the xmons in each en- coded qubit needs a separate voltage control to tune the gate charge number ng which is needed to initialize the encoded qubit state.(c), electrostatically tunable JJ based on a prox- imitized superconducting-semiconductor nanowire connecting two superconductors [31] used for gatemons. The nanowire is coated with SC for clean contact and a portion is lifted off to form a semiconductor nanowire weak link. The JJ energy EJ is tuned by a side-gate voltage VG, which can also serve as capacitive tuning for initialization.