Quantum computing and condensed matter physics with microwave photons

Generating single microwave photons in a circuit

A. A. Houck, D. I. Schuster, J. M. Gambetta, J. A. Schreier, B. R. Johnson, J. M. Chow, L. Frunzio, J. Majer, M. H. Devoret, S. M. Girvin and R. J. Schoelkopf

Microwaves have widespread use in classical communication technologies, from long-distance broadcasts to short-distance signals within a computer chip. Like all forms of light, microwaves, even those guided by the wires of an integrated circuit, consist of discrete photons. To enable quantum communication between distant parts of a quantum computer, the signals must also be quantum, consisting of single photons, for example. However, conventional sources can generate only classical light, not single photons.

Resolving photon number states in a superconducting circuit

D. I. Schuster, A. A. Houck, J. A. Schreier, A. Wallraff, J. M. Gambetta, A. Blais, L. Frunzio, J. Majer, B. Johnson, M. H. Devoret, S. M. Girvin and R. J. Schoelkopf

Electromagnetic signals are always composed of photons, although in the circuit domain those signals are carried as voltages and currents on wires, and the discreteness of the photon’s energy is usually not evident. However, by coupling a superconducting quantum bit (qubit) to signals on a microwave transmission line, it is possible to construct an integrated circuit in which the presence or absence of even a single photon can have a dramatic effect.

Kondo Effect in the Presence of Magnetic Impurities

H. B. Heersche, Z. de Groot, J. A. Folk, L. P. Kouwenhoven, H. S. J. van der Zant, A. A. Houck, J. Labaziewicz, I. L. Chuang

We measure transport through gold grain quantum dots fabricated using electromigration, with magnetic impurities in the leads. A Kondo interaction is observed between dot and leads, but the presence of magnetic impurities results in a gate-dependent zero-bias conductance peak that is split due to a RKKY interaction between the spin of the dot and the static spins of the impurities. A magnetic field restores the single Kondo peak in the case of an antiferromagnetic RKKY interaction.

Kondo Effect in Electromigrated Gold Break Junctions

A. A. Houck, J. Labaziewicz, E. K. Chan, J. A. Folk, and I. L. Chuang

We present gate-dependent transport measurements of Kondo impurities in bare gold break junctions, generated with high yield using an electromigration process that is actively controlled. Thirty percent of measured devices show zero-bias conductance peaks. Temperature dependence suggests Kondo temperatures 7 K. The peak splitting in magnetic field is consistent with theoretical predictions for g = 2, though in many devices the splitting is offset from 2gμBby a fixed energy.

Focusing inside negative index materials

Jeffrey B. Brock, Andrew A. Houck, and Isaac L. Chuang

Two key theoretical predictions for flat negative index lenses are that thicker lenses should have both internal and external foci, and that these foci should move linearly with small deviations in frequency. Two-dimensional electric field profiles of microwave transmission through rectangular slabs of composite wire and split-ring resonator material are presented. By scanning inside and outside the material, both internal and external foci are revealed.

Experimental Observations of a Left-Handed Material That Obeys Snell’s Law

Andrew A. Houck, Jeffrey B. Brock, and Isaac L. Chuang

We measure two-dimensional profiles of collimated microwave beams transmitted through composite wire and split-ring resonator prisms. Prior experiments suggest these structures have a negative index of refraction, though these claims have been questioned. Our 2D measurements demonstrate that transmission obeys Snell’s law with a negative index, confirming the refractive nature of this signal and refuting alternatives posed in the criticisms. In addition, we present preliminary evidence that a flat rectangular slab of this material can focus power from a point source.