A CNOT gate between multiphoton qubits encoded in two cavities

  1. Serge Rosenblum,
  2. Yvonne Gao,
  3. Philip Reinhold,
  4. Chen Wang,
  5. Christopher Axline,
  6. Luigi Frunzio,
  7. Steven Girvin,
  8. Liang Jiang,
  9. Mazyar Mirrahimi,
  10. Michel Devoret,
  11. and Robert Schoelkopf
Entangling gates between qubits are a crucial component for performing algorithms in quantum computers. However, any quantum algorithm will ultimately have to operate on error-protected
logical qubits, which are effective qubits encoded in a high-dimensional Hilbert space. A common approach is to encode logical qubits in collective states of multiple two-level systems, but algorithms operating on multiple logical qubits are highly complex and have not yet been demonstrated. Here, we experimentally realize a controlled NOT (CNOT) gate between two multiphoton qubits in two microwave cavities. In this approach, we encode a qubit in the large Hilbert space of a single cavity mode, rather than in multiple two-level systems. We couple two such encoded qubits together through a transmon, which is driven with an RF pump to apply the CNOT gate within 190 ns. This is two orders of magnitude shorter than the decoherence time of any part of the system, enabling high-fidelity operations comparable to state-of-the-art gates between two-level systems. These results are an important step towards universal algorithms on error-corrected logical qubits.

Josephson directional amplifier for quantum measurement of superconducting circuits

  1. Baleegh Abdo,
  2. Katrina Sliwa,
  3. S. Shankar,
  4. Michael Hatridge,
  5. Luigi Frunzio,
  6. Robert Schoelkopf,
  7. and Michel Devoret
We have realized a microwave quantum-limited amplifier that is directional and can therefore function without the front circulator needed in many quantum measurements. The amplification
takes place in only one direction between the input and output ports. Directionality is achieved by multi-pump parametric amplification combined with wave interference. We have verified the device noise performances by using it to readout a superconducting qubit and observed quantum jumps. With an improved version of this device, qubit and preamplifer could be integrated on the same chip.

Black-box superconducting circuit quantization

  1. Simon E. Nigg,
  2. Hanhee Paik,
  3. Brian Vlastakis,
  4. Gerhard Kirchmair,
  5. Shyam Shankar,
  6. Luigi Frunzio,
  7. Michel Devoret,
  8. Robert Schoelkopf,
  9. and Steven Girvin
We present a semi-classical method for determining the effective low-energy quantum Hamiltonian of weakly anharmonic superconducting circuits containing mesoscopic Josephson junctions
coupled to electromagnetic environments made of an arbitrary combination of distributed and lumped elements. A convenient basis, capturing the multi-mode physics, is given by the quantized eigenmodes of the linearized circuit and is fully determined by a classical linear response function. The method is used to calculate numerically the low-energy spectrum of a 3D-transmon system, and quantitative agreement with measurements is found.