In the "flux-qubit" team, we study mesoscopic superconducting circuits containing tunnel junctions in order to use them as building blocks for a quantum computer.

- Spectroscopy of the qubit
- Coherent dynamics of a qubit
- Inductive measurement of a qubit
- Dynamics of a qubit coupled to a harmonic oscillator
- Dynamics of two coupled flux qubits

We acknowledge support from the following institutions

**THE "PERSISTENT-CURRENT" QUBIT**

Fig. 2 : A) The
persistent-current qubit is a three-junction loop biased by an external flux. B)
Low-energy diagram of the persistent-current qubit C) Detection (in gray) and
manipulation (in purple) of the qubit.

**Inductive measurement of a qubit**

Switching of the measuring DC-SQUID to the finite voltage state strongly perturbs the measurement circuit and the qubit. We investigate a method for the readout of a flux qubit based on a direct measurement of the Josephson inductance of the DC-SQUID. The DC-SQUID is shunted by a capacitor, such that the plasma frequency is in the range 0.5-1 GHz (see fig. 5A). Close to the resonance frequency of this circuit the output voltage is very sensitive to the flux produced by the qubit. We have characterized this method and measured the state of a persistent current qubit, obtaining a relaxation time of the order of 80 ms (see figure 5B). The fidelity of the measurement is 70%, being limited at this stage by the amplifier added noise.

Fig. 5

**Epitaxial Josephson junctions**

Up to now, the Josephson junctions in the superconducting circuits were fabricated with two-angle shadow evaporation. This technique is easy to use and standard practice for making sub-micron structures. However, the reproducibility of the junctions is limited, as the junction region is non-planar and the area is therefore poorly defined. As the sample is covered with resist during evaporation, cleaning the surface by heating is not possible and resist outgassing might also limit the junction quality. In particular, fluctuations in the critical current density make it difficult to achieve more complex qubit circuits. Critical current noise due to impurities in the barrier may lead to additional decoherence. Therefore we try to establish a fabrication scheme based on epitxially grown Al/Oxide/Al trilayers. Single-crystalline aluminum films and trilayers have been grown already. The TEM picture (figure 6A) shows the lattice planes of the aluminum films. The oxide layer in between is also very uniform. Presently, we are setting up a fabrication procedure for making sub-micron Josephson junctions out of these trilayers (see figure 6B).

Fig. 6

- We investigate a new gradiometric geometry for our qubit which should make it immune to flux noise and therefore increase the coherence time
- We study chains of coupled flux qubits in order to transmit quantum information

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[3] Y. Nakamura, Y. Pashkin, J. Tsai, Nature 398, 786
(1999)

[4] J. E. Mooij et al., Science 285, 1036 (1999)

[5] C. H. van
der Wal et al., Science 290, 773 (2000)

[6] I. Chiorescu, Y. Nakamura, C. J.
P. M. Harmans, J. E. Mooij, published online in Science : 13 February 2003 10.1126/science.1081045

Hans Mooij, Kees Harmans (faculty members)

Patrice Bertet, Jonathan Eroms (postdocs) Alexander ter Haar, Adrian Lupascu, Jelle Plantenberg, Floor Paauw (graduate students)

Pieter de Groot Loek Zonnenberg Eduard Driessen Alexander Groot (MS student)

Toeno van der Saar (bachelor student)