Extra-substrate control system

#This invention relates to quantum computing devices, and more specifically to initializing, controlling, coupling, and measuring the state of structures that could be used as qubits in quantum computing devices.

#Research in the field of quantum computing began in 1982 when Richard Feynman introduced the concept of a “quantum simulator.” See R. P. Feynman, “Simulating Physics with Computers”, Int. J. Theor. Phys., 21:467-488 (1982). Soon it was determined that a quantum system could be used to yield a potentially exponential time saving in certain types of intensive computations. See D. Deutsch, “Quantum Theory, the Church—Turing Principle and the Universal Quantum Computer”, Proc. of the Roy. Soc. of London A400:97-117 (1985). Since then, research has progressed to include significant software and hardware advances. As the speed of classical computers approaches a projected upper bound due to the natural limits of miniaturization of integrated circuits, so interest in quantum computers has intensified. Indeed many algorithms to run on quantum computers have been written, two notable examples being the Shor and Grover algorithms. See P. Shor, “Polynomial-time Algorithms for Prime Factorization and Discrete Logarithms on a Quantum Computer”, SIAM J. of Comput., 26(5): 1484-1509 (1997), and U.S. Pat. No. 6,317,766 entitled “Fast Quantum Mechanical Algorithms”; and L. Grover, “A Fast Quantum Mechanical Algorithm for Database Search”, Proc. 28th STOC, 212-219 (1996). Nevertheless, sizeable obstacles remain to the development of large-scale quantum computing devices that are both practical and capable of out-performing currently available classical computers. See, for example, “Quantum Dreams”, The Economist, Mar. 10, 2001, pp. 81-82.

#In fact, the field of quantum computing remained theoretical until the late 1990's when several hardware proposals were tested. Of these proposals, the most scalable physical systems are those that are superconducting structures. Superconducting material is material that has zero electrical resistance below critical levels of current, magnetic field and temperature. Josephson junctions are examples of such structures that have been of special interest because their observable properties are macroscopic manifestations of underlying quantum mechanical principles.

#FIGS. 1A and 1B display two embodiments of the present invention in which superconducting material on a first substrate is coupled to circuitry on a second substrate.

#FIG. 2 shows a schematic view of a flux shield, illustrating aspects of the flux shield including features that enable its application as a flux focuser.

#FIG. 3A shows an embodiment of a flux focuser with several apertures and FIG. 3B shows exemplary control, coupling and measurement circuitry.

#When a high degree of isolation and robustness is required for the interaction of some structures, for example for quantum computing structures, embodiments of the present invention find application. Specifically, a need exists for controlling qubits to perform quantum computing. The present invention allows one structure, say a qubit, to interact with another structure, say a measurement circuit, where each may be formed from different materials, and by different processes, and reside on different substrates. Other aspects of the present invention relate to the need to have initialization, control, coupling and measurement circuitry for a given qubit or plurality of qubits. The present invention provides a flexible, inexpensive and scalable means to accomplish this.

#In general, an extra substrate control system comprises a first substrate on which one or more superconducting structures, for example qubits, are placed such that the superconducting structures interact with a control system on a second substrate. An embodiment of the present invention includes the use of a flux focuser that permits the selective coupling of a single superconducting structure such as a qubit from a plurality of superconducting structures without coupling to other superconducting structures of the plurality. Accordingly, the present invention permits various circuitries to interact with a superconducting structure in a selectable manner.

#An embodiment of the present invention includes a substrate with a superconducting structure on it, a second substrate with circuitry on it, and a mechanism for controllably coupling the circuitry to the superconducting structure. The superconducting structure can be a quantum computing structure such as a qubit, an array of qubits, or a quantum register comprised of qubits and associated circuitry. The present invention is generally applicable to phase qubits and charge qubits. In the case where the superconducting structure is a qubit, the circuitry often is used for initialization, control, coupling, and measurement. The circuitry can be made from normal, semi- or super-conducting material. The mechanism for coupling the superconducting structure to the circuitry includes inductive coupling, capacitive coupling. For example, the circuitry can have a loop that shares a mutual inductance with the superconducting structure. Control of this coupling mechanism may involve additional structures. In a Josephson junction, for example, quantum phase usually manifests itself as a magnetic moment which may be inductively coupled to other structures.