Hector Hutin

Absence of Charge Offset Drift in a Transmon Qubit

  1. Adria Rospars,
  2. Hector Hutin,
  3. Yannick Seis,
  4. Cristóbal Lledó,
  5. Réouven Assouly,
  6. Romain Cazali,
  7. Rémy Dassonneville,
  8. Ambroise Peugeot,
  9. Alexandre Blais,
  10. Audrey Bienfait,
  11. and Benjamin Huard
Superconducting quantum circuits are sensitive to their electrostatic environment: uncontrolled charges accumulating on the electrodes of a Josephson junction shift the energy levels
of a qubit, perturbing its operation and restricting their design. This effect is captured by a single parameter – the charge offset – whose slow, unpredictable drift has proven difficult to eliminate in practice. Here, we report a tantalum-based transmon qubit in which the charge offset remains pinned at zero over nearly three months of measurements, including two thermal cycles, with no observable compromise to the qubit lifetime. This exceptional stability disappears in later cooldowns, indicating a fragile mechanism at play. We attribute it to the inductance of a thin superconducting layer inadvertently formed in parallel with the Josephson junction during fabrication. X-ray surface spectroscopy suggests this layer arises from an incomplete wet-etch of tantalum on sapphire. Deliberately engineering such a layer offers a route to eliminating charge-offset drift in superconducting circuits more broadly.
arxiv pdf

Monitoring the energy of a cavity by observing the emission of a repeatedly excited qubit

  1. Hector Hutin,
  2. Antoine Essig,
  3. Réouven Assouly,
  4. Pierre Rouchon,
  5. Audrey Bienfait,
  6. and Benjamin Huard
The number of excitations in a large quantum system (harmonic oscillator or qudit) can be measured in a quantum non demolition manner using a dispersively coupled qubit. It typically
requires a series of qubit pulses that encode various binary questions about the photon number. Recently, a method based on the fluorescence measurement of a qubit driven by a train of identical pulses was introduced to track the photon number in a cavity, hence simplifying its monitoring and raising interesting questions about the measurement backaction of this scheme. A first realization with superconducting circuits demonstrated how the average number of photons could be measured in this way. Here we present an experiment that reaches single shot photocounting and number tracking owing to a cavity decay rate 4 orders of magnitude smaller than both the dispersive coupling rate and the qubit emission rate. An innovative notch filter and pogo-pin based galvanic contact makes possible these seemingly incompatible features. The qubit dynamics under the pulse train is characterized. We observe quantum jumps by monitoring the photon number via the qubit fluorescence as photons leave the cavity one at a time. Besides, we extract the measurement rate and induced dephasing rate and compare them to theoretical models. Our method could be applied to quantum error correction protocols on bosonic codes or qudits.
arxiv pdf