Masahiro Kitagawa

Low power, fast and broadband ESR quantum control using a stripline resonator

  1. Yung Szen Yap,
  2. Makoto Negoro,
  3. Mayuko Kuno,
  4. Yoshikiyo Sakamoto,
  5. Akinori Kagawa,
  6. and Masahiro Kitagawa
Using a home-built Ku band ESR spectrometer equipped with an arbitrary waveform generator and a stripline resonator, we implement two types of pulses that would benefit quantum computers:
BB1 composite pulse and a microwave frequency comb. Broadband type 1 (BB1) composite pulse is commonly used to combat systematic errors but previous experiments were carried out only on extremely narrow linewidth samples. Using a sample with a linewidth of 9.35 MHz, we demonstrate that BB1 composite pulse is still effective against pulse length errors at a Rabi frequency of 38.46 MHz. The fast control is realized with low microwave power which is required for initialization of electron spin qubits at 0.6 T. We also digitally design and implement a microwave frequency comb to excite multiple spin packets of a different sample. Using this pulse, we demonstrate coherent and well resolved excitations spanning over the entire spectrum of the sample (ranging from -20 to 20 MHz). In anticipation of scaling up to a system with large number of qubits, this approach provides an efficient technique to selectively and simultaneously control multiple qubits defined in the frequency-domain.
arxiv pdf

Nuclear magnetic resonance spectroscopy with a superconducting flux qubit

  1. Koichiro Miyanishi,
  2. Yuichiro Matsuzaki,
  3. Hiraku Toida,
  4. Kosuke Kakuyanagi,
  5. Makoto Negoro,
  6. Masahiro Kitagawa,
  7. and Shiro Saito
We theoretically analyze the performance of the nuclear magnetic resonance (NMR) spectroscopy with a superconducting flux qubit (FQ). Such NMR with the FQ is attractive because of the
possibility to detect the relatively small number of nuclear spins in a local region (∼μm) with low temperatures (∼ mK) and low magnetic fields (∼ mT), in which other types of quantum sensing schemes cannot easily access. A sample containing nuclear spins is directly attached on the FQ, and the FQ is used as a magnetometer to detect magnetic fields from the nuclear spins. Especially, we consider two types of approaches to NMR with the FQ. One of them is to use spatially inhomogeneous excitations of the nuclear spins, which are induced by a spatially asymmetric driving with radio frequency~(RF) pulses. Such an inhomogeneity causes a change in the DC magnetic flux penetrating a loop of the FQ, which can be detected by a standard Ramsey measurement on the FQ. The other approach is to use a dynamical decoupling on the FQ to measure AC magnetic fields induced by Larmor precession of the nuclear spins. In this case, neither a spin excitation nor a spin polarization is required since the signal comes from fluctuating magnetic fields of the nuclear spins. We calculate the minimum detectable density (number) of the nuclear spins for the FQ with experimentally feasible parameters. We show that the minimum detectable density (number) of the nuclear spins with these approaches is around 1021 /cm3 (108) with an accumulation time of a second.
arxiv pdf