Fully Directional Quantum-limited Phase-Preserving Amplifier

  1. Gangqiang Liu,
  2. Andrew Lingenfelter,
  3. Vidul Joshi,
  4. Nicholas E. Frattini,
  5. Volodymyr V. Sivak,
  6. Shyam Shankar,
  7. and Michel H. Devoret
We present a way to achieve fully directional, quantum-limited phase-preserving amplification in a four-port, four-mode superconducting Josephson circuit by utilizing interference between
six parametric processes that couple all four modes. Full directionality, defined as the reverse isolation surpassing forward gain between the matched input and output ports of the amplifier, ensures its robustness against impedance mismatch that might be present at its output port during applications. Unlike existing directional phase-preserving amplifiers, both the minimal back-action and the quantum-limited added noise of this amplifier remains unaffected by noise incident on its output port. In addition, the matched input and output ports allow direct on-chip integration of these amplifiers with other circuit QED components, facilitating scaling up of superconducting quantum processors.

The Kerr-Cat Qubit: Stabilization, Readout, and Gates

  1. Alexander Grimm,
  2. Nicholas E. Frattini,
  3. Shruti Puri,
  4. Shantanu O. Mundhada,
  5. Steven Touzard,
  6. Mazyar Mirrahimi,
  7. Steven M. Girvin,
  8. Shyam Shankar,
  9. and Michel H. Devoret
Quantum superpositions of macroscopically distinct classical states, so-called Schrödinger cat states, are a resource for quantum metrology, quantum communication, and quantum computation.
In particular, the superpositions of two opposite-phase coherent states in an oscillator encode a qubit protected against phase-flip errors. However, several challenges have to be overcome in order for this concept to become a practical way to encode and manipulate error-protected quantum information. The protection must be maintained by stabilizing these highly excited states and, at the same time, the system has to be compatible with fast gates on the encoded qubit and a quantum non-demolition readout of the encoded information. Here, we experimentally demonstrate a novel method for the generation and stabilization of Schrödinger cat states based on the interplay between Kerr nonlinearity and single-mode squeezing in a superconducting microwave resonator. We show an increase in transverse relaxation time of the stabilized, error-protected qubit over the single-photon Fock-state encoding by more than one order of magnitude. We perform all single-qubit gate operations on time-scales more than sixty times faster than the shortest coherence time and demonstrate single-shot readout of the protected qubit under stabilization. Our results showcase the combination of fast quantum control with the robustness against errors intrinsic to stabilized macroscopic states and open up the possibility of using these states as resources in quantum information processing.

Comparing and combining measurement-based and driven-dissipative entanglement stabilization

  1. Yehan Liu,
  2. Shyam Shankar,
  3. Nissim Ofek,
  4. Michael Hatridge,
  5. Anirudh Narla,
  6. Katrina Sliwa,
  7. Luigi Frunzio,
  8. Robert J. Schoelkopf,
  9. and Michel H. Devoret
We demonstrate and contrast two approaches to the stabilization of qubit entanglement by feedback. Our demonstration is built on a feedback platform consisting of two superconducting
qubits coupled to a cavity which are measured by a nearly-quantum-limited measurement chain and controlled by high-speed classical logic circuits. This platform is used to stabilize entanglement by two nominally distinct schemes: a „passive“ reservoir engineering method and an „active“ correction based on conditional parity measurements. In view of the instrumental roles that these two feedback paradigms play in quantum error-correction and quantum control, we directly compare them on the same experimental setup. Further, we show that a second layer of feedback can be added to each of these schemes, which heralds the presence of a high-fidelity entangled state in realtime. This „nested“ feedback brings about a marked entanglement fidelity improvement without sacrificing success probability.

Confining the state of light to a quantum manifold by engineered two-photon loss

  1. Zaki Leghtas,
  2. Steven Touzard,
  3. Ioan M. Pop,
  4. Angela Kou,
  5. Brian Vlastakis,
  6. Andrei Petrenko,
  7. Katrina M. Sliwa,
  8. Anirudh Narla,
  9. Shyam Shankar,
  10. Michael J. Hatridge,
  11. Matthew Reagor,
  12. Luigi Frunzio,
  13. Robert J. Schoelkopf,
  14. Mazyar Mirrahimi,
  15. and Michel H. Devoret
Physical systems usually exhibit quantum behavior, such as superpositions and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially
engineered interaction with the environment can become a resource for the generation and protection of quantum states. This notion can be generalized to the confinement of a system into a manifold of quantum states, consisting of all coherent superpositions of multiple stable steady states. We have experimentally confined the state of a harmonic oscillator to the quantum manifold spanned by two coherent states of opposite phases. In particular, we have observed a Schrodinger cat state spontaneously squeeze out of vacuum, before decaying into a classical mixture. This was accomplished by designing a superconducting microwave resonator whose coupling to a cold bath is dominated by photon pair exchange. This experiment opens new avenues in the fields of nonlinear quantum optics and quantum information, where systems with multi-dimensional steady state manifolds can be used as error corrected logical qubits.

Non-Poissonian Quantum Jumps of a Fluxonium Qubit due to Quasiparticle Excitations

  1. Uri Vool,
  2. Ioan M. Pop,
  3. Katrina Sliwa,
  4. Baleegh Abdo,
  5. Chen Wang,
  6. Teresa Brecht,
  7. Yvonne Y. Gao,
  8. Shyam Shankar,
  9. Michael Hatridge,
  10. Gianluigi Catelani,
  11. Mazyar Mirrahimi,
  12. Luigi Frunzio,
  13. Robert J. Schoelkopf,
  14. Leonid I. Glazman,
  15. and Michel H. Devoret
As the energy relaxation time of superconducting qubits steadily improves, non-equilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevant
limit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum non-demolition projective measurements within a time interval much shorter than T1, using a quantum limited amplifier (Josephson Parametric Converter). The quantum jumps statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in magnetic field.

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.