Nelson Leung

Multimode photon blockade

  1. Srivatsan Chakram,
  2. Kevin He,
  3. Akash V. Dixit,
  4. Andrew E. Oriani,
  5. Ravi K. Naik,
  6. Nelson Leung,
  7. Hyeokshin Kwon,
  8. Wen-Long Ma,
  9. Liang Jiang,
  10. and David I. Schuster
Interactions are essential for the creation of correlated quantum many-body states. While two-body interactions underlie most natural phenomena, three- and four-body interactions are
important for the physics of nuclei [1], exotic few-body states in ultracold quantum gases [2], the fractional quantum Hall effect [3], quantum error correction [4], and holography [5, 6]. Recently, a number of artificial quantum systems have emerged as simulators for many-body physics, featuring the ability to engineer strong interactions. However, the interactions in these systems have largely been limited to the two-body paradigm, and require building up multi-body interactions by combining two-body forces. Here, we demonstrate a pure N-body interaction between microwave photons stored in an arbitrary number of electromagnetic modes of a multimode cavity. The system is dressed such that there is collectively no interaction until a target total photon number is reached across multiple distinct modes, at which point they interact strongly. The microwave cavity features 9 modes with photon lifetimes of ∼2 ms coupled to a superconducting transmon circuit, forming a multimode circuit QED system with single photon cooperativities of ∼109. We generate multimode interactions by using cavity photon number resolved drives on the transmon circuit to blockade any multiphoton state with a chosen total photon number distributed across the target modes. We harness the interaction for state preparation, preparing Fock states of increasing photon number via quantum optimal control pulses acting only on the cavity modes. We demonstrate multimode interactions by generating entanglement purely with uniform cavity drives and multimode photon blockade, and characterize the resulting two- and three-mode W states using a new protocol for multimode Wigner tomography.
arxiv pdf

Universal stabilization of a parametrically coupled qubit

  1. Yao Lu,
  2. Srivatsan Chakram,
  3. Nelson Leung,
  4. Nathan Earnest,
  5. Ravi K. Naik,
  6. Ziwen Huang,
  7. Peter Groszkowski,
  8. Eliot Kapit,
  9. Jens Koch,
  10. and David I. Schuster
We autonomously stabilize arbitrary states of a qubit through parametric modulation of the coupling between a fixed frequency qubit and resonator. The coupling modulation is achieved
with a tunable coupler design, in which the qubit and the resonator are connected in parallel to a superconducting quantum interference device. This allows for quasi-static tuning of the qubit-cavity coupling strength from 12 MHz to more than 300 MHz. Additionally, the coupling can be dynamically modulated, allowing for single photon exchange in 6 ns. Qubit coherence times exceeding 20 μs are maintained over the majority of the range of tuning, limited primarily by the Purcell effect. The parametric stabilization technique realized using the tunable coupler involves engineering the qubit bath through a combination of photon non-conserving sideband interactions realized by flux modulation, and direct qubit Rabi driving. We demonstrate that the qubit can be stabilized to arbitrary states on the Bloch sphere with a worst-case fidelity exceeding 80 %.
arxiv pdf

Realization of a Λ system with metastable states of a capacitively-shunted fluxonium

  1. Nathan Earnest,
  2. Srivatsan Chakram,
  3. Yao Lu,
  4. Nicholas Irons,
  5. Ravi K. Naik,
  6. Nelson Leung,
  7. Jay Lawrence,
  8. Jens Koch,
  9. and David I. Schuster
We realize a Λ system in a superconducting circuit, with metastable states exhibiting lifetimes up to 7ms. We exponentially suppress the tunneling matrix elements involved in spontaneous
energy relaxation by creating a „heavy“ fluxonium, realized by adding a capacitive shunt to the original circuit design. The device allows for both cavity-assisted and direct fluorescent readout, as well as state preparation schemes akin to optical pumping. Since direct transitions between the metastable states are strongly suppressed, we utilize Raman transitions for coherent manipulation of the states.
arxiv pdf