IBM recently announced one with 16 qubits—the components needed to build a quantum computer—and Google is gunning for around 50 qubits this year. Rigetti has made chips with 8 qubits; it says the new fab will speed up the experimentation needed to increase that number.
Rigetti has developed a highly coherent and scalable quantum integrated circuit architecture. Two key ingredients are a new fab process for superconducting through-silicon vias, and a low-temperature bonding process for 3D integration.
As part of a 3D quantum integrated circuit architecture, a cap chip forms the upper half of an enclosure that provides isolation, increases vacuum participation ratio, and improves performance of individual resonant elements. They have demonstrated that such caps can be reliably fabricated, placed on a circuit chip, and form superconducting connections to the circuit.
Rigetti has developed and demonstrated a new two-qubit gate scheme based on direct parametric modulation of qubit frequencies. This gate scheme can be faster and more selective than previous methods, making it better suited for scaled-up chips with many qubits.
Forest is built on top of Quil™, the first instruction language for hybrid quantum/classical computing. Hybrid quantum/classical algorithms take advantage of the best aspects of a classical computer and a quantum computer simultaneously. Classical computers are best at rote sequences of arithmetic, while quantum computers are best at manipulating extremely large ensembles of information at once.
Quil is an open and portable instruction set, using a shared memory model that is optimized for near-term algorithms and hardware. Forest 1.0 includes pyQuil, a set of open-source python tools for building and running Quil programs.
Rigetti has shown that parametric coupling techniques can be used to generate selective entangling interactions for multi-qubit processors. By inducing coherent population exchange between adjacent qubits under frequency modulation, we implement a universal gateset for a linear array of four superconducting qubits. An average process fidelity of =93% is estimated for three two-qubit gates via quantum process tomography. We establish the suitability of these techniques for computation by preparing a four-qubit maximally entangled state and comparing the estimated state fidelity against the expected performance of the individual entangling gates. In addition, we prepare an eight-qubit register in all possible bitstring permutations and monitor the fidelity of a two-qubit gate across one pair of these qubits. Across all such permutations, an average fidelity of =91.6±2.6% is observed. These results thus offer a path to a scalable architecture with high selectivity and low crosstalk.
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