M. Stern

Strong coupling of a superconducting flux qubit to single bismuth donors

  1. T. Chang,
  2. I. Holzman,
  3. S. Q. Lim,
  4. D. Holmes,
  5. B.C. Johnson,
  6. D.N. Jamieson,
  7. and M. Stern
The realization of a quantum computer represents a tremendous scientific and technological challenge due to the extreme fragility of quantum information. The physical support of information,
namely the quantum bit or qubit, must at the same time be strongly coupled to other qubits by gates to compute information, and well decoupled from its environment to keep its quantum behavior. An interesting physical system for realizing such qubits are magnetic impurities in semiconductors, such as bismuth donors in silicon. Indeed, spins associated to bismuth donors can reach an extremely long coherence time — of the order of seconds. Yet it is extremely difficult to establish and control efficient gates between these spins. Here we demonstrate a protocol where single bismuth donors can coherently transfer their quantum information to a superconducting flux qubit, which acts as a mediator or quantum bus. This superconducting device allows to connect distant spins on-demand with little impact on their coherent behavior.
arxiv pdf

Tunable superconducting flux qubits with long coherence times

  1. T. Chang,
  2. T. Cohen,
  3. I. Holzman,
  4. G. Catelani,
  5. and M. Stern
In this work, we study a series of tunable flux qubits inductively coupled to a coplanar waveguide resonator fabricated on a sapphire substrate. Each qubit includes an asymmetric superconducting
quantum interference device which is controlled by the application of an external magnetic field and acts as a tunable Josephson junction. The tunability of the qubits is typically ±3.5 GHz around their central gap frequency. The measured relaxation times are limited by dielectric losses in the substrate and can attain T1∼8μs. The echo dephasing times are limited by flux noise even at optimal points and reach T2E∼4μs, almost an order of magnitude longer than state of the art for tunable flux qubits.
arxiv pdf

Reproducibility and control of superconducting flux qubits

  1. T. Chang,
  2. I. Holzman,
  3. T. Cohen,
  4. B.C. Johnson,
  5. D.N. Jamieson,
  6. and M. Stern
Superconducting flux qubits are promising candidates for the physical realization of a scalable quantum processor. Indeed, these circuits may have both a small decoherence rate and
a large anharmonicity. These properties enable the application of fast quantum gates with high fidelity and reduce scaling limitations due to frequency crowding. The major difficulty of flux qubits‘ design consists of controlling precisely their transition energy – the so-called qubit gap – while keeping long and reproducible relaxation times. Solving this problem is challenging and requires extremely good control of e-beam lithography, oxidation parameters of the junctions and sample surface. Here we present measurements of a large batch of flux qubits and demonstrate a high level of reproducibility and control of qubit gaps, relaxation times and pure echo dephasing times. These results open the way for potential applications in the fields of quantum hybrid circuits and quantum computation.
arxiv pdf

Storage and retrieval of microwave fields at the single-photon level in a spin ensemble

  1. C. Grezes,
  2. B. Julsgaard,
  3. Y. Kubo,
  4. W. L. Ma,
  5. M. Stern,
  6. A. Bienfait,
  7. K. Nakamura,
  8. J. Isoya,
  9. S. Onoda,
  10. T. Ohshima,
  11. V. Jacques,
  12. D. Vion,
  13. D. Esteve,
  14. R. B. Liu,
  15. K. Mølmer,
  16. and P. Bertet
We report the storage of microwave pulses at the single-photon level in a spin-ensemble memory consisting of 1010 NV centers in a diamond crystal coupled to a superconducting LC resonator.
The energy of the signal, retrieved 100μs later by spin-echo techniques, reaches 0.3% of the energy absorbed by the spins, and this storage efficiency is quantitatively accounted for by simulations. This figure of merit is sufficient to envision first implementations of a quantum memory for superconducting qubits.
arxiv pdf