The ability to switch the coupling between quantum bits (qubits) on and off is essential for implementing many quantum-computing algorithms. We demonstrated such control with two flux qubits coupled together through their mutual inductances and through the dc superconducting quantum interference device (SQUID) that reads out their magnetic flux states. A bias current applied to the SQUID in the zero-voltage state induced a change in the dynamic inductance, reducing the coupling energy controllably to zero and reversing its sign.

We compute the decoherence caused by 1/f fluctuations at low frequency f in the critical current I0 of Josephson junctions incorporated into flux, phase, charge and hybrid flux-charge superconducting quantum bits (qubits). The dephasing time τ φ scales as I0/ΩΛS 1/2 I 0(1 Hz), where Ω/2π is the energy level splitting frequency, SI 0 (1 Hz) is the spectral density of the critical current noise at 1 Hz, and Λ ≡ |I0dΩ/ΩdI0| is a parameter computed for given parameters for each type of qubit that specifies the sensitivity of the level splitting to critical current fluctuations. Computer simulations show that the envelope of the coherent oscillations of any qubit after time t scales as exp(−t 2 /2τ 2 φ ) when the dephasing due to critical current noise dominates the dephasing from all sources of dissipation. We compile published results for fluctuations in the critical current of Josephson tunnel junctions fabricated with different technologies and a wide range in I0 and A, and show that their values of SI 0 (1 Hz) scale to within a factor of three of 144 (I0/µA) 2 / A/µmWe empirically extrapolate S 1/2 I 0(1 Hz) to lower temperatures using a scaling T (K)/4.2. Using this result, we find that the predicted values of τ φ at 100 mK range from 0.8 to 12 µs, and are usually substantially longer than values measured experimentally at lower temperatures.

We propose a scheme to implement variable coupling between two flux qubits using the screening current response of a dc Superconducting QUantum Interference Device (SQUID). The coupling strength is adjusted by the current bias applied to the SQUID and can be varied continuously from positive to negative values, allowing cancellation of the direct mutual inductance between the qubits. We show that this variable coupling scheme permits efficient realization of universal quantum logic. The same SQUID can be used to determine the flux states of the qubits.Comment: 4 pages, 4 figure

We report measurements on two superconducting flux qubits coupled to a readout Superconducting QUantum Interference Device (SQUID). Two on-chip flux bias lines allow independent flux control of any two of the three elements, as illustrated by a two-dimensional qubit flux map. The application of microwaves yields a frequency-flux dispersion curve for 1-and 2-photon driving of the single-qubit excited state, and coherent manipulation of the single-qubit state results in Rabi oscillations and Ramsey fringes. This architecture should be scalable to many qubits and SQUIDs on a single chip.PACS numbers: 03.67. Lx, 85.25.Cp, 85.25.Dq Superconducting quantum bits (qubits) based on charge [1,2], magnetic flux [3,4], and phase difference across a Josephson junction [5,6] are attractive candidates for the basis of a quantum computer because of their inherent scalability using established thin-film fabrication techniques. Advantages of the flux qubit include its immunity to the ubiquitous charge noise in the substrate and that it can be configured with no direct electrical connections. One type of flux qubit consists of a superconducting loop interrupted by three Josephson junctions with critical currents I 0 , I 0 , and αI 0 (α < 1) [7]. When the applied flux bias Φ Q is at a degeneracy point (n+1/2)Φ 0 (n is an integer such that |Φ Q −nΦ 0 | ≤ Φ 0 /2; Φ 0 ≡ h/2e is the flux quantum), a screening supercurrent J Q can flow in either direction around the loop. The ground and first excited states of the qubit correspond to symmetric and antisymmetric superpositions of the two current states and are separated by an energy ∆. Here, ∆/h is the tunnel frequency between the current states, typically a few GHz. When Φ Q is away from a degeneracy point, the energy difference between the two superposed states is ν = (The state of the qubit is measured by coupling the screening flux generated by J Q to a hysteretic dc superconducting quantum interference device (SQUID). This flux determines the bias current at which the SQUID switches out of the zero-voltage state.In addition to the development of scalable interqubit couplings [8], a prerequisite for scaling to a system of many qubits is that the attendant readout, filtering, and bias circuitry also scale. A particular challenge is that the flux bias must be settable for each element individually. This mandates the use of on-chip flux-bias lines in an arrangement that enables one to apply a combination of currents to address any given qubit or SQUID while maintaining all other flux biases at constant values. Furthermore, the bias currents required to change the flux over (say) ±1Φ 0 should not be so large that it becomes impractical to deliver them to a chip cooled to millikelvin temperatures. This requirement establishes minimum self-inductances of the qubit and readout SQUID that are substantially larger than values used previously in 3-junction qubits, which have relied on external coils to generate large magnetic fields [4,9,10]. At the same time, the mutual inductance between ...