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Physical Review B

Josephson Persistent-Current Qubit

Johan E. Mooij, Terry P. Orlando, Leonid S Levitov, Lin Tian, Caspar H. van der Wal, Seth Lloyd

Published Jan 1, 1999• 12 citations
OpenAlex

Abstract

A qubit was designed that can be fabricated with conventional electron beam lithography and is suited for integration into a large quantum computer. The qubit consists of a micrometer-sized loop with three or four Josephson junctions; the two qubit states have persistent currents of opposite direction. Quantum superpositions of these states are obtained by pulsed microwave modulation of the enclosed magnetic flux by currents in control lines. A superconducting flux transporter allows for controlled transfer between qubits of the flux that is generated by the persistent currents, leading to entanglement of qubit information.

Related papers

Josephson Persistent-Current Qubit

Johan E. Mooij, Terry P. Orlando, Leonid S Levitov +3 more

A qubit was designed that can be fabricated with conventional electron beam lithography and is suited for integration into a large quantum computer. The qubit consists of a micrometer-sized loop with three or four Josephson junctions; the two qubit states have persistent currents of opposite direction. Quantum superpositions of these states are obtained by pulsed microwave modulation of the enclosed magnetic flux by currents in control lines. A superconducting flux transporter allows for controlled transfer between qubits of the flux that is generated by the persistent currents, leading to entanglement of qubit information.

Science•Aug 13, 1999•1,373 citations

Superconducting persistent-current qubit

Terry P. Orlando, Johan E. Mooij, Lin Tian +4 more

We present the design of a superconducting qubit that has circulating currents of opposite sign as its two states. The circuit consists of three nanoscale aluminum Josephson junctions connected in a superconducting loop and controlled by magnetic fields. The advantages of this qubit are that it can be made insensitive to background charges in the substrate, the flux in the two states can be detected with a superconducting quantum interference device, and the states can be manipulated with magnetic fields. Coupled systems of qubits are also discussed as well as sources of decoherence.

arXiv:cond-mat/9908283•Physical review. B, Condensed matter•Dec 1, 1999•754 citations

Novel Computing Architecture on Arrays of Josephson Persistent Current Bits

Jie Han, Pieter P. Jonker

A superconducting qubit (or quantum bit), which consists of a micrometer-sized loop with three Josephson junctions, has two persistent currents of opposite direction as its two states. The states of the qubit can be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops, making it possible to base a classical computer architecture. We study a novel computing structure based on these Josephson Persistent Current (PC) Bits, starting from elementary logic gates to a Random Access Memory (RAM). The investigation shows that the Josephson PC Bit technology would not surpass semiconductor technology in term of the device density, while it is a promising candidate for unltra-fast memory, which can be integrated with other technologies. The classical computer might also serve as pre and post processor for the quantum computing performed in the heart of the array. The Josephson PC circuits, therefore, seems a good vehicle for the study of the quantum computer paradigm.

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On quantum and classical computing with arrays of superconducting persistent current qubits

Pieter P. Jonker, Jie Han

A superconducting qubit (or quantum bit), which consists of a micrometer-sized loop with three or four Josephson junctions, has two persistent currents of opposite direction as its two states. The states of the qubits, manipulated with magnetic fields and measured with a SQUID, can be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops by increasing its critical current, making it possible to base a processor array architecture on these cubits (quantum bits used in a classical way). Such a classical computer might also serve as pre and post processor for the quantum computing performed in the heart of the array. Because classical and quantum computing based on the same device can be studied now simultaneously, architecture of arrays of qubits and cubits seems a good vehicle for studying the quantum computer paradigm, independently from the question whether superconducting loops will be the ultimate implementation vehicle. Other (e.g. spin-based) devices may be more successful. In this paper, the architectural issues of a heterogeneous quantum/classical computer for an implementation of Shor's factoring algorithm based on arrays of qubits and cubits, are presented.

International Workshop on Computer Architecture for Machine Perception•Nov 7, 2002•3 citations

Josephson Persistent Current Qubit and Quantum Flux Measurements

Caspar H. van der Wal, Johan E. Mooij, A. C. Wallast +10 more

. In circuits of underdamped, small-capacitance Josephson junctions, the charge and phase variables behave quantum mechanically. Superpositions of charge states and flux states have been demonstrated, as well as microwave induced quantum dynamics (see e.g. refs [3, 4]). These e#ects have their origin in the conjugation of the phase and charge variables of Josephson circuits. The qubit design that we present here also contains small-capacitance Josephson junctions. However, even though charge-phase conjugation underlies the quantum behavior of the qubit, it was explicitly designed to be very insensitive to charge e#ects. In practice, random background charges and 1/f charge noise are always present in Josephson circuits, but these charge e#ects have negligible influence on the qubit. The qubit loop contains three Josephson junctions. Two junctions have Josephson coupling E J , the third junction has a coupling of #E J .Weaim

•Nov 21, 2007•0 citations

Single and coupled Josephson junction quantum bits

A. C. J. ter Haar

In this thesis measurements on single and coupled Josephson persistent current qubits are presented. The Josephson persistent current qubit consists of a superconducting loop intersected by three Josephson junctions. When half a flux quantum is applied to the loop this system acts as a quantum two state system. The two states are associated with a clock or anti clockwise current flowing in the loop and can be measured using a SQUID magnetometer. The research presented in this thesis aimed at achieving coupling between two Josephson persistent current qubits and to show conditional gate operations in such a coupled qubit system. Also it was tried to improve the coherence properties of a single qubit by using a gradiometer type configuration to minimize influence from fluxnoise which would cause a disturbance of the energy bias of the qubit system. The basic principles of the Josephson persistent current qubit are discussed, as well as design criteria to minimize decoherence due to the environment seen by the squid or microwave lines. Spectroscopy measurements have been performed on coupled qubit systems mapping out the energy level structure of these systems. For two coupled qubits the origin and magnitude of the coupling J is derived for coupling via the geometrical inductance between the qubits. Also it is shown that coupling via a shared junction or a shared line acts the same as the geometrical coupling. Spectroscopy measurements show coupling for qubits which are mainly geometrical coupled and spectroscopy measurements on an other sample show a coupling for qubits coupled via a large shared Josephson junction. Using such a shared junction allows for the coupling to be designed within a large range. For the coupled qubit system with coupling via a large junction spectroscopy measurements from both the ground state to higher states as well as from the partially populated first excited state to higher states have been performed and the level structure of this system is mapped out. The energy level structure is in good agreement with the 4 level approximation for two coupled qubits. Coherent Rabi oscillations between two states flipping either qubit of this system have been performed with an oscillation decay time of up to 70ns. Also conditional spectroscopy has been performed by first going coherently from the ground state to a higher state and then incoherently pumping the system to other states. Spectroscopy measurements on a gradiometer type Josephson persistent current qubit show trapped fluxoid biasing can be used to create a phase bias of pi over the qubit junctions. The results of this thesis show that coupling between Josephson persistent current qubits can be achieved and designed within the wanted range. Superpositions between the ground state and higher states in such a coupled qubit system have been created by performing Rabi oscillations. Also conditional spectroscopy has been performed.

•Feb 2, 2005•3 citations
A qubit was designed that can be fabricated with conventional electron beam lithography and is suited for integration into a large quantum computer. The qubit consists of a micrometer-sized loop with three or four Josephson junctions; the two qubit states have persistent currents of opposite direction. Quantum superpositions of these states are obtained by pulsed microwave modulation of the enclosed magnetic flux by currents in control lines. A superconducting flux transporter allows for controlled transfer between qubits of the flux that is generated by the persistent currents, leading to entanglement of qubit information.
A qubit was designed that can be fabricated with conventional electron beam lithography and is suited for integration into a large quantum computer. The qubit consists of a micrometer-sized loop with three or four Josephson junctions; the two qubit states have persistent currents of opposite direction. Quantum superpositions of these states are obtained by pulsed microwave modulation of the enclosed magnetic flux by currents in control lines. A superconducting flux transporter allows for controlled transfer between qubits of the flux that is generated by the persistent currents, leading to entanglement of qubit information.
We present the design of a superconducting qubit that has circulating currents of opposite sign as its two states. The circuit consists of three nanoscale aluminum Josephson junctions connected in a superconducting loop and controlled by magnetic fields. The advantages of this qubit are that it can be made insensitive to background charges in the substrate, the flux in the two states can be detected with a superconducting quantum interference device, and the states can be manipulated with magnetic fields. Coupled systems of qubits are also discussed as well as sources of decoherence.
A superconducting qubit (or quantum bit), which consists of a micrometer-sized loop with three Josephson junctions, has two persistent currents of opposite direction as its two states. The states of the qubit can be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops, making it possible to base a classical computer architecture. We study a novel computing structure based on these Josephson Persistent Current (PC) Bits, starting from elementary logic gates to a Random Access Memory (RAM). The investigation shows that the Josephson PC Bit technology would not surpass semiconductor technology in term of the device density, while it is a promising candidate for unltra-fast memory, which can be integrated with other technologies. The classical computer might also serve as pre and post processor for the quantum computing performed in the heart of the array. The Josephson PC circuits, therefore, seems a good vehicle for the study of the quantum computer paradigm.
A superconducting qubit (or quantum bit), which consists of a micrometer-sized loop with three or four Josephson junctions, has two persistent currents of opposite direction as its two states. The states of the qubits, manipulated with magnetic fields and measured with a SQUID, can be brought into quantum coherence to perform quantum computing. Classical bits can also be obtained from these superconducting loops by increasing its critical current, making it possible to base a processor array architecture on these cubits (quantum bits used in a classical way). Such a classical computer might also serve as pre and post processor for the quantum computing performed in the heart of the array. Because classical and quantum computing based on the same device can be studied now simultaneously, architecture of arrays of qubits and cubits seems a good vehicle for studying the quantum computer paradigm, independently from the question whether superconducting loops will be the ultimate implementation vehicle. Other (e.g. spin-based) devices may be more successful. In this paper, the architectural issues of a heterogeneous quantum/classical computer for an implementation of Shor's factoring algorithm based on arrays of qubits and cubits, are presented.
. In circuits of underdamped, small-capacitance Josephson junctions, the charge and phase variables behave quantum mechanically. Superpositions of charge states and flux states have been demonstrated, as well as microwave induced quantum dynamics (see e.g. refs [3, 4]). These e#ects have their origin in the conjugation of the phase and charge variables of Josephson circuits. The qubit design that we present here also contains small-capacitance Josephson junctions. However, even though charge-phase conjugation underlies the quantum behavior of the qubit, it was explicitly designed to be very insensitive to charge e#ects. In practice, random background charges and 1/f charge noise are always present in Josephson circuits, but these charge e#ects have negligible influence on the qubit. The qubit loop contains three Josephson junctions. Two junctions have Josephson coupling E J , the third junction has a coupling of #E J .Weaim
In this thesis measurements on single and coupled Josephson persistent current qubits are presented. The Josephson persistent current qubit consists of a superconducting loop intersected by three Josephson junctions. When half a flux quantum is applied to the loop this system acts as a quantum two state system. The two states are associated with a clock or anti clockwise current flowing in the loop and can be measured using a SQUID magnetometer. The research presented in this thesis aimed at achieving coupling between two Josephson persistent current qubits and to show conditional gate operations in such a coupled qubit system. Also it was tried to improve the coherence properties of a single qubit by using a gradiometer type configuration to minimize influence from fluxnoise which would cause a disturbance of the energy bias of the qubit system. The basic principles of the Josephson persistent current qubit are discussed, as well as design criteria to minimize decoherence due to the environment seen by the squid or microwave lines. Spectroscopy measurements have been performed on coupled qubit systems mapping out the energy level structure of these systems. For two coupled qubits the origin and magnitude of the coupling J is derived for coupling via the geometrical inductance between the qubits. Also it is shown that coupling via a shared junction or a shared line acts the same as the geometrical coupling. Spectroscopy measurements show coupling for qubits which are mainly geometrical coupled and spectroscopy measurements on an other sample show a coupling for qubits coupled via a large shared Josephson junction. Using such a shared junction allows for the coupling to be designed within a large range. For the coupled qubit system with coupling via a large junction spectroscopy measurements from both the ground state to higher states as well as from the partially populated first excited state to higher states have been performed and the level structure of this system is mapped out. The energy level structure is in good agreement with the 4 level approximation for two coupled qubits. Coherent Rabi oscillations between two states flipping either qubit of this system have been performed with an oscillation decay time of up to 70ns. Also conditional spectroscopy has been performed by first going coherently from the ground state to a higher state and then incoherently pumping the system to other states. Spectroscopy measurements on a gradiometer type Josephson persistent current qubit show trapped fluxoid biasing can be used to create a phase bias of pi over the qubit junctions. The results of this thesis show that coupling between Josephson persistent current qubits can be achieved and designed within the wanted range. Superpositions between the ground state and higher states in such a coupled qubit system have been created by performing Rabi oscillations. Also conditional spectroscopy has been performed.