Abstract:
A solid-state quantum computing structure includes a d-wave superconductor in sets of islands that clean Josephson junctions separate from a first superconducting bank. The d-wave superconductor causes the ground state for the supercurrent at each junction to be doubly degenerate, with two supercurrent ground states having distinct magnetic moments. These quantum states of the supercurrents at the junctions create qubits for quantum computing. The quantum states can be uniformly initialized from the bank, and the crystal orientations of the islands relative to the bank influence the initial quantum state and tunneling probabilities between the ground states. A second bank, which a Josephson junction separates from the first bank, can be coupled to the islands through single electron transistors for selectably initializing one or more of the supercurrents in a different quantum state. Single electron transistors can also be used between the islands to control entanglements while the quantum states evolve. After the quantum states have evolved to complete a calculation, grounding the islands, for example, through yet another set of single electron transistors, fixes the junctions in states having definite magnetic moments and facilitates measurement of the supercurrent when determining a result of the quantum computing.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This is a divisional of U.S. pat. app. Ser. No. 09/452,749, filed on Dec. 1, 1999. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     This invention relates to quantum computing and to solid state devices that use superconducting materials to create and maintain coherent quantum states such as required for quantum computing. 
     2. Description of Related Art 
     Research on what is now called quantum computing traces back to Richard Feynman, [R. Feynman, Int. J. Theor. Phys., 21, 467-488 (1982)]. Feynman noted that quantum systems are inherently difficult to simulate with conventional computers but that observing the evolution of a quantum system could provide a much faster way to solve the same computational problems. In particular, solving a theory for the behavior of a quantum system commonly involves solving a differential equation related to the Hamiltonian of the quantum system. Observing the behavior of the quantum system provides information regarding the solutions to the equation. 
     Further efforts in quantum computing were initially concentrated on “software development” or building of the formal theory of quantum computing. Software for quantum computing attempts to set the Hamiltonian of a quantum system to correspond to a problem requiring solution. Milestones in these efforts were the discoveries of the Shor and Grover algorithms. [See P. Shor, SIAM J. of Comput., 26:5, 1484-1509 (1997); L. Grover, Proc. 28th STOC, 212-219 (1996); and A. Kitaev, LANL preprint quant-ph/9511026 (1995)]. In particular, the Shor algorithm permits a quantum computer to factorize natural numbers. The showing that fault-tolerant quantum computation is theoretically possible opened the way for attempts at practical realizations of quantum computers. [See E. Knill, R. Laflamme, and W. Zurek, Science, 279, p. 342 (1998).] 
     One proposed application of a quantum computer is factoring of large numbers. In such an application, a quantum computer could render obsolete all existing encryption schemes that use the “public key” method. In another application, quantum computers (or even a smaller scale device, a quantum repeater) could allow absolutely safe communication channels, where a message, in principle, cannot be intercepted without being destroyed in the process. [See H. J. Briegel et al., LANL preprint quant-ph/9803056 (1998) and the references therein.] 
     Quantum computing generally involves initializing the states of N qubits (quantum bits), creating controlled entanglements among the N qubits, allowing the quantum states of the qubits to evolve under the influence of the entanglements, and reading the qubits after they have evolved. A qubit is conventionally a system having two degenerate quantum states, and the initial state of the qubit typically has non-zero probabilities of being found in either degenerate state. Thus, N qubits define an initial state that is a combination of 2 N  degenerate states. The entanglements control the evolution of the distinguishable quantum states and define calculations that the evolution of the quantum states perform. This evolution, in effect, performs 2 N  simultaneous calculations. Reading the qubits after evolution is complete determines the states of the qubits and the results of the calculations. 
     Several physical systems have been proposed for the qubits in a quantum computer. One system uses chemicals having degenerate spin states. Nuclear magnetic resonance (NMR) techniques can read the spin states. These systems have successfully implemented the Shor algorithm for factoring of a natural number ( 15 ). However, efforts to expand such systems up to a commercially useful number of qubits face difficult challenges. 
     Another physical system for implementing a qubit includes a superconducting reservoir, a superconducting island, and a dirty Josephson junction that can transmit a Cooper pair (of electrons) from the reservoir into the island. The island has two degenerate states. One state is electrically neutral, but the other state has an extra Cooper pair on the island. A problem with this system is that the charge of the island in the state having the extra Cooper pair causes long range electric interactions that interfere with the coherence of the state of the qubit. The electric interactions can force the island into a state that definitely has or lacks an extra Cooper pair. Accordingly, the electric interactions can end the evolution of the state before calculations are complete or qubits are read. This phenomenon is commonly referred to as collapsing the wavefunction, loss of coherence, or decoherence. 
     Research is continuing and seeking a structure that implements a quantum computer having a sufficient number of qubits to perform useful calculations. 
     SUMMARY 
     In accordance with the invention, a qubit includes a superconducting island that a Josephson junction separates from a superconducting bank. The island has a crystal orientation that differs from the crystal orientation of the reservoir, and a grain boundary between the island and reservoir forms a clean (ballistic) Josephson junction. One or both of the island and the bank are D-wave superconductors so that a ground state current flows at the Josephson junction. The ground state of the supercurrent at the Josephson junction is twice degenerate with the magnetic moment produced by the supercurrent distinguishing the two states. The crystal orientation of the island relative to the bank controls the equilibrium phase difference in the order parameter across the junction and therefore the tunneling probabilities between the ground states. 
     To read the supercurrent state associated with the island, a single electron transistor (SET) or parity key can connect the island to ground. When the SET is biased to conduct, the current through the SET collapses supercurrent state to a state with fixed magnetic moment and fixes the supercurrent in that state. Thus, upon completion of a calculation, a control circuit biases the SET to conduct, and the magnetic moment at the Josephson junction is fixed in a particular state and can be dependably read. 
     To form a quantum register, multiple Josephson junctions can couple respective superconducting islands to a superconducting bank, and a current through the bank can initialize the quantum states of the supercurrents at the junctions. Single electron transistors (SETs) or parity keys interconnect the islands to create controlled entanglements as required for quantum computing. After completion of the computing, other SETs or parity keys connect the islands to ground and freeze the supercurrents at the Josephson junctions into states having definite magnetic moments. This freezing maintains the states for subsequent read operations that measure the local magnetic moments or magnetic flux. 
     One embodiment of the invention is a quantum computing structure such as a quantum coherer or a quantum register that includes a bank of a superconducting material and an island of a superconducting material, wherein at least one of the island and the bank is a d-wave superconductor. The normal-conductor portion of a clean Josephson junction can be, for example, a grain boundary between the bank and the island. Optionally, a single electron transistor (SET) or a parity key is between the island and ground. The orientation of the supercurrent through the junction is fixed when the SET is conductive and can evolve when the SET is non-conductive. As another option, the structure also includes a second bank of superconducting material, and a Josephson junction between the first and second banks. Operation of a SET between the second bank and the island selectively initializes the supercurrent&#39;s quantum state according to the phase of the order parameter in the first or second bank. 
     Another embodiment of the invention is a quantum register that includes: a bank of a superconducting material; a plurality of islands of superconducting material; and a plurality of clean Josephson junctions. Each clean Josephson junction is between the bank and a corresponding one of the islands. One or both of the island and the bank include a d-wave superconductor. The quantum register optionally includes three sets of SETs. Each SET in a first set is between ground and a corresponding one of the islands. Each SET in the second set is between a corresponding pair of the islands. Each SET in the third set is between a second bank and a corresponding one of the islands. The Josephson junction creates an order parameter phase difference between the first and second banks. The second bank and the third set of SETs can be used for selective initialization of supercurrents at the junctions according to the phase of the second bank. 
     In accordance with another embodiment of the invention, a quantum computing method cools a structure including a bank and an island to a temperature that makes the bank and the island superconducting and suppresses the decoherence processes in the system. The structure includes a junction that is a clean Josephson junction between the island and the bank. After the structure is at the appropriate temperature, the method establishes a supercurrent at the junction in a quantum state that is an admixture of a first state having a first magnetic moment and a second state having a second magnetic moment. The supercurrent at the junction is a ground state current arising from use of a d-wave superconductor in the structure and can be set by running a current through the bank. The quantum state evolves according to probabilities for tunneling between the first and second ground states. The evolution performs the quantum computing. Determining a measured magnetic moment or flux due to the supercurrent at the junction determines a result from the quantum computing. 
     In accordance with another aspect of the invention, determining the measured magnetic moment includes: grounding the island to fix the supercurrent in the first or second state; and measuring the magnetic flux produced by the supercurrent while the island is grounded. 
     Typically, the quantum register further includes a plurality of islands and a plurality of junctions, each junction being a clean Josephson junction between the bank and a corresponding island. The quantum states of the supercurrents at the junctions evolve according to the conductivities of transistors that couple islands together. These transistors create entanglements of the quantum states of the islands. The manufacturer of the quantum register can select for each island, a crystal orientation according to the initial quantum state desired for the island. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A,  1 B, and  1 C are plan views of quantum coherer having a horizontal architecture in accordance with an embodiment of the invention. 
     FIGS. 2A,  2 B, and  2 C are cross-sectional views of horizontal quantum coherers that in accordance with embodiments of the invention. 
     FIGS. 3A and 3B are respectively plan and cross-sectional views of a vertical quantum coherer in accordance with an embodiment of the invention. 
     FIGS. 4A and 4B are respectively plan and cross-sectional views of a vertical quantum coherer in accordance with another embodiment of the invention. 
     FIGS. 5 is cross-sectional views of a hybrid vertical/horizontal quantum coherer in accordance with an embodiment of the invention. 
     FIG. 6 shows a qubit having a single electron transistor that freezes the state of the qubit. 
     FIG. 7 shows a structure including a collection of qubits having single electron transistors that create entanglements among the qubits and facilitate read out of the qubits. 
     FIG. 8 illustrates a system having a double bus capable of applying different phases of the order parameter from the buses to the qubit. 
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION 
     In accordance with an aspect of the invention, quantum computing uses qubits based on the degenerate ground states of the supercurrent at a DD, DND, or SND Josephson junction. The Josephson junctions can be fabricated in useful numbers in a solid state structure. With a d-wave superconductor on at least one side of the Josephson junction, the Josephson junction has non-zero ground state supercurrent in the vicinity of the junction. This ground state supercurrent is either clockwise or counterclockwise in the preferred (so called ab-) plane of the d-wave superconductor. The ground-state supercurrent in the vicinity of each Josephson junction is thus doubly degenerate and provides the basis for a quantum coherer or a qubit for quantum computing in accordance with an embodiment of the invention. 
     FIG. 1A is a plan view of horizontal quantum coherers  100  in accordance with exemplary embodiments of the invention. Quantum coherer  100  provides a basic block for construction of a qubit but can also be an independent device allowing demonstration of macroscopic quantum tunneling and incoherent quantum noise in a solid state system. As described further below, the macroscopic quantum tunneling in a set of independent quantum coherers permits construction of a random number generator that generates random series with zero correlation. 
     Quantum coherer  100  includes a Josephson junction  130  between a larger superconducting bank  110  and a mesoscopic, superconducting island  120  formed on an insulating substrate  140 . At least one of bank  110  and island  120  is a d-wave superconductor, for example, a high-Tc cuprate such as YBa 2 Cu 3 O 7−x  or any superconductor, in which the Cooper pairs are in a state with non-zero orbital angular momentum. In a first exemplary embodiment of the invention, both bank  110  and island  120  are made of d-wave superconductor. In this embodiment, junction  130  is clean in that the junction is conducting (e.g., a normal conducting layer or a grain boundary) and lacks scattering sites. As described further below, a grain boundary between a d-wave superconductor bank  110  and a d-wave superconductor island  120  can create a Josephson junction  130 . 
     In a second exemplary embodiment, bank  110  is an s-wave superconducting material such as Niobium (Nb), and island  120  is a d-wave superconductor. In a third embodiment, bank  110  is a d-wave superconducting material, and island  120  is an swave superconductor. For the second and third embodiments, junction  130  includes a normal conductor between bank  110  and island  120 . The normal conductor can be any conductive material that forms a good contact with both the d-wave and s-wave superconductors, has a large elastic scattering length, and remains a normal conductor at the operating temperature of quantum coherer  100  (typically between about 10° K and about 1° K). In particular, gold (Au) is a suitable normal conductor for junction  130 . 
     In the exemplary embodiments, bank  110  is a chip of superconducting material about 1 μm or more in length and width. The thickness of bank  110  is not critical but generally should not exceed that of the mesoscopic island  120 . Island  120  is mesoscopic (i.e., has a size such that a single excess Cooper pair is noticeable) and typically has a width W about 0.2 μm or less, a length L about 0.5 μm or less, and thickness about 0.2 μm or less. 
     Quantum coherer  100  can be formed using conventional techniques. In the first exemplary embodiment where both bank  110  and island  120  are d-wave superconductors, substrate  140  is a bi-crystal substrate such as a strontium-titanate bi-crystal substrate available from KagakuGijutsu-sha of Tokyo, Japan. The fabrication process begins by growing a film of a high-Tc cuprate having a thickness of about 0.2 microns on substrate  140 . Regions of the high-Tc cuprate film inherit different crystal orientation from underlying substrate  140 , and a grain boundary forms between the two different regions. Such a film can be grown using pulsed laser deposition, which uses a laser beam to sputter the high-Tc cuprate onto substrate  140 . A photolithographic process then masks and etches the film to form island  120  (typically as one of several islands) adjacent bank  110 . For islands  120  of the small size desired, the etching or patterning process can use an electron beam to remove part of the wave superconductor and leave island  120  with the desired dimensions. Il&#39;ichev et al., cond-mat/9811017, p.2 describes known fabrication technique using high-Tc cuprates and is hereby incorporated by reference in its entirety. 
     In the second and third embodiments where one of bank  110  or island  120  is an s-wave superconductor, the fabrication process starts by depositing a film of d-wave superconductor on substrate  140 . The film is etched (if necessary) to limit the boundaries of the d-wave superconductor to the desired boundaries of bank  110  or island  120 . Alternatively, bank  110  or island  120  can be etched from a bulk d-wave film. A normal conductor such as gold is deposited and patterned to leave material for junctions  130 . Finally, a film of s-wave superconductor is deposited and patterned (if necessary) to limit the boundaries of the s-wave superconductor for bank  110  or island  120 . 
     For operation, quantum coherer  100  is cooled to a temperature less than about 10° K so that bank  110  and island  120  are superconducting and Josephson junction  130  is operative. The operating temperature of quantum coherer  100  is far below the threshold temperature for superconductivity of the d-wave superconductor to suppress thermal sources of decoherence. In particular, the low temperature suppresses decoherence processes due to inelastic scattering. If quantum coherer  100  contains an s-wave superconductor, the operating temperature is below the transition temperature of the s-wave superconductor (e.g., below 9.25° K for pure Nb). 
     At junction  130 , the d-wave superconductor causes a non-zero supercurrent in the ground state, and the ground state of the supercurrent is twice degenerate if no external electromagnetic field is applied. Two degenerate states having the ground state energy and definite magnetic moment correspond to minimal supercurrents circulating through Josephson junction  130  in clockwise and counter-clockwise senses, in a preferred plane of the crystal structures of bank  110  and/or island  120 . In accordance with current theoretical descriptions, e.g., the Ginzburg-Landau theory, of superconductivity, an order parameter Ψ describes supercurrents in superconductors, and a phase difference Δφ in the order parameter when crossing junction  130  indicates the state or direction of the supercurrent. The two states associated with the supercurrent in island  120  permit quantum computing as described further below. 
     Quantum coherer  100  operates at a temperature below about 10° K so that bank  110  and island  120  are superconducting and thermal excitations do not interfere with the coherence of the quantum state associated with the supercurrent in island  120 . An external circuit (not shown) can generate an electric field that causes a current through bank  110  to the right or left that initializes quantum coherer  100  to a quantum state corresponding to a known superposition of the clockwise and counterclockwise supercurrent states at junction  130 . Alternatively, temporary application of a magnetic field can also initialize the state of island  120  by temporarily breaking the degeneracy in the two ground state energies. Subsequent quantum tunneling between the ground states causes the state of island  120  to evolve. 
     In the first exemplary, island  120  is a d-wave superconductor with a crystal orientation that differs from that of bank  130 . Since the Josephson junction is a clean junction, the difference in crystal orientation is a primary factor in determining the magnitude of the equilibrium phase difference Δφ in the order parameter Ψ at the junction, and the magnitude of the phase difference Δφ is not restricted to π/2 as typically would be the case with a tunneling junction. (The two degenerate states of the junction respectively correspond to positive and negative phase differences Δφ.) Accordingly, the choice of lattice mismatch between bank  110  and island  120  selects the phase difference Δφ. This permits selection of tunneling rates between the ground states within an exponentially wide range. 
     Another advantage of having a clean junction is a difference in crystal orientations (or Δφ) can restrict the ground states to having a low probability of being in states having excess charge on island  120 . Thus, the state of island  120  has weaker electrostatic interactions with the surroundings. This reduces or eliminates a source of decoherence in the state of island  120 , and the state of island  120  can continue to evolve for a relatively long period without collapsing the wavefunction. The spontaneous supercurrent at Josephson junction  130  creates spontaneous magnetization, and the direction of the current and the magnetization distinguish the working quantum states of quantum coherer  100 . However, the magnetic reactions with the surroundings are weak enough to avoid significant problems with decoherence. 
     The geometry or architecture of Josephson junction  130  in quantum coherer  100  can be varied in variety of ways that facilitate selection of the phase difference Δφ in the superconducting order parameter. FIG. 1B is a plan view of a quantum coherer  100 B according to another embodiment of the invention. Quantum coherer  100 B includes a Josephson junction  130 B that separates bank  110  from mesoscopic island  120 . Josephson junction  130 B is particularly suited for the embodiments of the invention where one of bank  110  and island  120  is an s-wave superconductor and the other one of bank  110  and island  120  is a d-wave superconductor. Bank  110 , island  120 , and junction  130 B are respectively formed in the same manner described above for bank  110 , island  120 , and junction  130  of FIG.  1 A. 
     Quantum coherer  100 B differs from quantum coherer  100  in crystal orientation of island  120  relative to bank  11 O across junction  130 B. The a-b plane of the d-wave superconductor lies in the plane of FIG.  1 B. In coherer  100 B, junction  130 B has three regions S 1 , S 2 , and S 3  where the relative crystal orientation of bank  110  and island  120  across region S 1  differs from the relative crystal orientation across regions S 2  and S 3 . The lengths of regions S 1 , S 2 , and S 3  can be changed to adjust the equilibrium phase difference in the superconducting order parameter across junction  213  and the magnitude of the magnetic flux of the ground state supercurrent. 
     FIG. 1C shows a plan view of another horizontal quantum coherer  100 C in which a Josephson junction  130 C has two regions S 1  and S 2  with different crystal orientations across the regions S 1  and S 2 . As in coherer  100 B, changing the orientation of island  120  and the lengths of regions S 1  and S 2  can adjust the phase difference in the superconducting order parameter between bank  110  and island  120 . 
     The cross-section of the junction  130  also has several alternative configurations. FIG. 2A shows a cross-sectional view of a horizontal quantum coherer where both bank  110  and island  120  are d-wave superconductors and a grain boundary forms Josephson junction  130 . FIG. 2B shows a cross-sectional view of a horizontal quantum coherer where a normal conductor between bank  110  and island  120  forms Josephson junction  130 . The normal conductor is suitable when both bank  110  and island  120  are d-wave superconductors or when one of bank  110  and island  120  is an s-wave superconductor. 
     FIG. 2C illustrates that surface of the Josephson junction  130  is not required to be perpendicular to the preferred plane (the ab-plane) of the supercurrent in the d-wave superconductor. Current techniques for growing a high-Tc superconductor on insulating substrate  140  typically keep the ab-plane of the high-Tc superconductor substantially parallel to the surface of substrate  140 . In FIG. 2C, junction  130  is at an angle relative to the c-direction of the d-wave superconductor. Normally, the deposition of the d-wave superconductor film on substrate  140  keeps the ab-plane of the d-wave superconductor parallel to the surface of substrate  140  and the c-direction perpendicular to the surface. Conventional patterning of the film creates an edge parallel to the c-direction. However, an anisotropic etch process such as electron beam etching with substrate  140  at an angle to the beam direction can create a non-zero angle between the edge of the d-wave film and the c-direction. This angle provides another degree of freedom in selecting a configuration that provides the desired phase difference Δφ in the superconducting order parameter. 
     FIGS. 3A and 3B respectively show a plan view and a cross-sectional view of a vertical quantum coherer  300  in accordance with an embodiment of the invention. The terms “horizontal” and “vertical” as applied to the quantum coherers described herein indicate the predominant plane of the ground state supercurrents. Quantum coherer  300  includes an insulating substrate  340 , a superconducting bank  310 , a Josephson junction  330 , and a mesoscopic superconducting island  320 . A fabrication process for quantum coherer  300  grows a d-wave superconductor film to a thickness between about 0.2 μm and about 0.5 μm on substrate  340 . Deposition of a normal conductor such as gold on the d-wave superconductor film forms a normal conductor film between about 0.1 μm and about 0.3 μm thick. Deposition of an s-wave superconductor such as Nb on the normal conductive film forms an s-wave superconductor film less than about 0.2 μm thick. Finally, patterning of the s-wave superconductor film and the normal conductor film creates mesoscopic, superconductive island  320  that Josephson junction  330  separated from superconductive bank  310 . 
     FIGS. 4A and 4B respectively show plan and cross-sectional views of a quantum coherer  400  having a vertical architecture according to another embodiment of the invention. Quantum coherer  400  includes a superconductor bank  410 , a mesoscopic superconductor island  420 , and a Josephson junction  430 , formed on an insulating substrate  440 . A fabrication process for quantum coherer  400  grows a d-wave superconductor film on substrate  440  to a thickness less than about 0.2 μm and patterns the film to form island  420 . Insulative sidewall spacers  450  are then formed on island  420 . Such spacers can be conventionally formed by depositing and patterning an insulative layer or by a self-aligned process that anisotropically etches a conformal insulative layer formed on substrate  440  and island  420 . A layer of a normal conductor such as gold is deposited on the resulting structure to a thickness between about 0.1 μm and about 0.3 μm and patterned to form a normal conductive region of Josephson junction  430 . The normal conductive region extends over island  420  and at least part of sidewall spacers  440 . Finally, a layer of an s-wave superconductor is deposited on the structure and patterned (if necessary) to form bank  410 . The thickness of bank  410  is not critical to the operation of quantum coherer  400 . 
     FIG. 5 shows a cross-sectional view of a quantum coherer  500  having a hybrid vertical/horizontal architecture according to another embodiment of the invention. Quantum coherer  500  includes a superconductor bank  510 , a mesoscopic superconductor island  520 , and a Josephson junction  530 , formed on an insulating substrate  540 . A fabrication process for quantum coherer  500  grows a d-wave superconductor film on substrate  440  to a thickness less than about 0.2 μm and patterns the film to form island  520 . The patterning can leave sides of island  520  perpendicular to the surface of substrate  540  or any desired angle. A layer of a normal conductor such as gold is deposited on the resulting structure to a thickness between about 0.1 μm and about 0.3 μm and patterned to form a normal conductive region of Josephson junction  530 . In this embodiment, the normal conductive region extends over island  520  and is in contact with at least one sidewall of island  520 . Finally, a layer of an s-wave superconductor is deposited on the structure and patterned (if necessary) to form bank  510 . The phase difference in the superconducting order parameter from bank  510  to island  520  depends on the relative crystal orientation between the top surface of island  520  and the overlying part of bank  510  and the relative crystal orientation of the side of island  520  and the adjacent part of bank  510 . 
     The quantum coherers such as described above avoid the destructive effects of low energy thermal excitations for several reasons. In particular, the superconducting gap (between the ground state energy of Cooper pairs and the higher energy states of electrons) and the small phase volume available in the nodes of the d-wave order parameter in the superconducting island and the bank suppress the low energy elementary excitations. Moreover, near the boundary, there is a possibility of specific admixture of s-wave superconductivity restoring the finite energy gap on all of the Fermi surface. In a normal layer of the junction, where the order parameter is suppressed, the elementary excitations are gapped due to size quantization. 
     One application of the quantum coherers is in a random number generator. In this application, the quantum states of a set of quantum coherers evolve to a state where each quantum coherer has an equal (or at least known) probability of being in each of the current direction states. The current-directions states are then determined, for example, by observing each quantum coherer with a magnetic force microscope or another magnetic probe. Each determined state (clockwise or counterclockwise) corresponds to a bit value (0 or 1) so that the collection of determined states provides a random binary value having as many bits as there are quantum coherers in the set. Quantum theory indicates that a series of bits thus generated are random without correlation or repetition. 
     FIG. 6 shows an embodiment of a qubit  600  based on the architecture of quantum coherer  100 . Qubit  600  is merely an illustrative embodiment of a qubit in accordance with the invention, and other embodiments of qubit can employ other quantum coherer architectures such as but not limited to those described above. 
     Qubit  600  combines quantum coherer  100  with external circuitry that allows freezing of the quantum tunneling between the two degenerate supercurrent ground states. To freeze the quantum state of the supercurrent, a parity key or single electron transistor (SET)  640  connects island  120  to ground (normal or superconducting). The free passage of electrons between island  120  and ground collapses the wavefunction of the supercurrent at junction  130  into one of the ground states (a state corresponding to either phase difference Δφ or −Δφ) having definite magnetic moment. (The probability of collapsing to a particular phase difference Δφ or −Δφ depends on probability amplitudes in the ground state before the collapse.) Island  120  remains in the definite magnetic moment state while SET  640  continues to connect island  120  to ground, and that state, while frozen, can be measured to read out and determine the results of a calculation. Changing the gate voltage of SET  640  can stop the flow of electrons to or from ground and thereby allows island  120  to evolve according to the tunneling rate between the ground states. 
     Single electron transistors are known and described, for example, by A. Zagoskin, “Quantum Theory of Many-Body Processes”, which is hereby incorporated by reference in its entirety. SETs include a grain capacitively coupled to two devices (e.g., island  120  and ground). An electron or Cooper pair can tunnel from either device onto the grain when the grain is uncharged. However, the grain is small enough that once an electron or Cooper pair tunnels onto the grain, the charging of the grain electrically repels and prevents further tunneling onto the grain. A gate associated with the grain can change the voltage of grain to shut off or otherwise control the tunneling rate. P. Joyez et al., “Observation of Parity-Induced Suppression of Josephson Tunneling in the Superconducting Single Electron Transistor”, Physical Review Letters, Vol. 72, No. 15, Apr. 11, 1994 describes operation and manufacture of single electron transistors and is also incorporated by reference herein in its entirety. 
     Qubit  600  is referred to herein as a permanent readout superconducting qubit (PRSQ) because barring thermal fluctuations, the spontaneous flux of a frozen (grounded and collapsed) qubit remains fixed. Accordingly, a readout device such as a magnetic force microscope (MFM) tip or a superconducting quantum interferometer device (SQUID) loop can contact the system when the decohering effects of the read out device will not disrupt the qubit result. The readout device measures the weak local magnetic fields that the spontaneous supercurrents (clockwise and counterclockwise) cause in the vicinity of the Josephson junction  130 . More particularly, the MFM scans a microscopic magnetized tip attached to a cantilever across the surface and measures deformation of the cantilever as the mechanical force that acts on the magnetized tip. Alternatively, a SQUID loop detects the magnetic flux in the vicinity of the Josephson junction  130 . Another possible read system may use a difference in the absorption of circularly polarized microwave radiation due to the clockwise or counterclockwise currents at the junction. 
     FIG. 7 shows a PRSQ register  700  including several islands  120 - 1  to  120 -N in contact with a bank  110 . In the exemplary embodiment, islands  120 - 1  to  120 -N and bank  110  are made of a d-wave superconductor at a temperature of about 10° K as described above. Grain boundaries are between bank  110  and respective islands  120 - 1  to  120 -N and form clean Josephson junctions  130 - 1  to  130 -N, respectively. The crystal orientations of islands  120 - 1  to  120 -N differ from the crystal orientation of bank  110  and control equilibrium phase differences Δφ 1  to Δφ N  between the phase of the order parameter in bank  110  and the phases of the order parameter in islands  120 - 1  to  120 -N. Phases Δφ 1  to Δφ N  can differ from each other or all be the same. A manufacturer of PRSQ register  700  selects phases Δφ 1  to Δφ N  according to the application of register  700  and designs a substrate that will create the desired grain boundaries when d-wave superconductive material is deposited on the substrate. 
     To facilitate readout from PRSQ register  700 , SETs  640 - 1  to  640 -N are between islands  120 - 1  to  120 -N and ground. Turning on SETs  640 - 1  to  640 -N permits free current between ground and respective islands  120 - 1  to  120 -N to collapse and freeze the quantum states of the supercurrents at respective junctions  130 - 1  to  130 -N. The techniques described above can then read the quantum states. 
     Register  700  also includes SETs  750 - 2  to  750 -N that connect adjacent islands  120 - 1  to  120 -N. Voltages applied to the gates of SETs  750 - 2  to  750 -N control currents or tunneling probabilities between islands and thereby create controllable entanglements among the quantum states of supercurrents in register  700 . 
     In FIG. 7, islands  120 - 1  to  120 -N are in a linear array, and each island  120 - 2  to  120 -N has a corresponding SET  750 - 2  to  750 -N that connects to the respective preceding islands  750 - 1  to  750 -(N- 1 ) in the linear array. Alternative configurations are possible, for example, an additional SET can connect island  120 - 1  to island  120 -N in a ring. In another embodiment, each island connects through multiple SETs to other islands, for example, in a two-dimensional array of qubits. The configuration and connections of islands can be selected according to the function of or program for PRSQ register  700 . 
     To execute quantum computing with PRSQ register  700 , the states of the qubits corresponding to islands  120 - 1  to  120 -N are first initialized in the same manner as described above, for example, by running a current through bank  110 . All of SETs  640 - 1  to  640 -N are off to prevent interaction with ground, and the voltages on the gates of SETs  750 - 2  to  750 -N are adjusted according to the desired calculation. SETs  750 - 2  to  750 -N create entanglements that enable tunneling between the ground states of PRSQ register  700 . After the quantum state of PRSQ register  700  evolves to complete the desired calculation, SETs  750 - 2  to  750 -N are turned off to decouple the qubits, and then SETs  640 - 1  to  640 -N are turned on. This collapses the wavefunction so that the supercurrent at each Josephson junction  130 - 1  to  130 -N has a definite magnetic moment. One or more read out devices sense the magnetic moments of the supercurrents at junctions  130 - 1  to  130 -N to determine the results of the quantum computing. 
     The time required for a calculation and the interpretation of the read out results depends on the calculation performed. Such issues are the subject of many papers on quantum computing. The structures described herein can perform such calculations provided that the structures provide a sufficient number of qubits and a decoherence time that is longer than the required calculation time. The structures can typically achieve longer coherence times by decreasing the operating temperature. 
     FIG. 8 illustrates a quantum register  800  having a double bus configuration. Quantum register  800  includes a first superconducting bank  110  and a second superconducting bank  810  with a Josephson junction  830  between the banks. Josephson junction  830  creates a phase difference Δφ between the order parameter in bank  110  and the order parameter in bank  120 . Josephson junction  830  is preferably a clean Josephson junction so that phase difference Δφ depends on the relative crystal orientations of banks  110  and  120 , but junction  830  is alternatively an insulative or dirty Josephson junction. Superconducting islands  120 - 1  to  120 -N connect to bank  110  via respective clean Josephson junctions  130 - 1  to  130 -N. 
     Quantum register  800  includes three sets of SETs. SETs  640 - 1  to  640 -N connect to respective islands  120 - 1  to  120 -N to ground. SETs  750 - 2  to  750 -N connect adjacent islands for controlled entanglements. SETs  840 - 1  to  840 -N are between respective islands  120 - 1  to  120 -N and bank  810 . An advantage of quantum register  800  is the ability to change the initialization and ground-state tunneling probabilities by selecting which, if any, of SETs  840 - 1  to  840 -N connect corresponding islands  120 - 1  to  120 -N to bank  810 . 
     To illustrate an initialization process using double-bus quantum register  800 , let the phase of the superconducting order parameter in bus  110  be zero. The relative phase χ of bus  810  can be created by connecting bus  110  and  810  on the left of FIG.  8  and passing an external magnetic field through the left most portion of the resulting loop. Opening selected keys  840 - 1  to  840 -N (while keys  640 - 1  to  640 -N remain closed) creates an energy difference between the two previously degenerate ground states in the corresponding islands  120 - 1  to  120 -N. In particular, the states with phases +Δφ and −Δφ when connected to bus  810  differ in energy with the energy difference being proportional to [cos(Δφ+χ)−cos(Δφ−χ)]. The connected islands  120 - 1  to  120 -N eventually settle to the lowest energy state +Δφ or −Δφ depending on the phase χ of bus  810 . 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.