Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J36: DQI Invited Session: Quantum Memories for Superconducting QubitsInvited

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Sponsoring Units: DQI Chair: Anja Metelmann, Princeton University Room: 601/603 
Tuesday, March 3, 2020 2:30PM  3:06PM 
J36.00001: The KerrCat Qubit: Stabilization, Readout, and Gates. Invited Speaker: Alexander Grimm Superpositions of two oppositephase coherent states in an oscillator, socalled Schrödingercat states, can be used to encode a qubit protected against phaseflip errors. Such a protected “cat qubit” has the potential to significantly reduce the overhead associated with quantum error correction. However, the practical operation of a cat qubit faces several challenges: Its basis states are highly excited states of the oscillator and need to be stabilized in order to maintain the protection. At the same time, the system has to be compatible with fast gates on the encoded qubit and a quantum nondemolition (QND) readout of the encoded information. 
Tuesday, March 3, 2020 3:06PM  3:42PM 
J36.00002: Faulttolerant quantum computation with repetition catqubits. Invited Speaker: Jeremie Guillaud Quantum error correcting codes provide, when operated below the threshold, an arbitrary good protection against noise, thus solving the decoherence problem for quantum information processing. However, the actual implementation of the most promising ones, such as the surface code, comes at the price of tremendous physical resources to reach a sufficient level of protection. We present a 1D repetition code based on the socalled catqubits as a viable candidate for a massive reduction in the hardware requirements for universal and faulttolerant quantum computation. The catqubits that are stabilized by a twophoton driven dissipative process, exhibit a tunable noise bias where the effective bitflips are exponentially suppressed with the average number of photons. Exploiting this noise bias, we build, at the level of the repetition catqubit, a universal set of fully protected logical gates. Remarkably, this construction avoids the costly magic states preparation, distillation and injection, even for nonClifford gates. 
Tuesday, March 3, 2020 3:42PM  4:18PM 
J36.00003: Detection of optical photons from a superconducting qubit Invited Speaker: Alp Sipahigil The ability to convert a superconducting qubit excitation to an optical photon would enable the incorporation of superconducting quantum processors in optical quantum networks. In this talk, we present an integrated device platform for converting a qubit excitation to an optical photon via piezooptomechanical transduction. We first describe the design and fabrication of a nanomechanical resonator with strong piezoelectric coupling to a transmon qubit and a large optomechanical coupling rate. We then discuss experiments using this mechanical mode as an intermediary to convert a qubit excitation to an optical photon in two steps. First, we dynamically tune the qubit frequency to perform a swap operation between the qubit and the mechanical mode. This is followed by an optical pulse that upconverts the phonon to an optical photon via the optomechanical interaction. We detect the resulting photons and demonstrate optical photon generation from a transmon qubit. We measure qubit Rabi oscillations using single optical photon detection to characterize the transducer’s noise and efficiency. Finally, we discuss prospects for optically mediated entanglement generation between remote superconducting circuits using this platform. 
Tuesday, March 3, 2020 4:18PM  4:54PM 
J36.00004: Quantum information processing using multimode cavities Invited Speaker: Srivatsan Chakram Multimode superconducting microwave cavities provide a hardware efficient means of engineering a large Hilbert space with high coherence, suitable for quantum simulations and information processing. Coupling to a superconducting transmon circuit results in random access control [1], with logic gates possible between arbitrary pairs of cavity modes. I will present our progress towards realizing such a processor using the quantum flute  a seamless rectangular 3D multimode cavity, with a tailored mode dispersion and lifetimes approaching a millisecond for ~10 modes. We present various schemes for controlling the cavity states using interactions mediated by the dispersively coupled transmon. 4wave mixing processes induced by the nonlinearity of the transmon can be used to exchange quantum states between the transmon and the cavity modes in a few hundred nanoseconds. When driven offresonantly, these sideband drives can be used to controllably dress the cavity states, and engineer novel multimode interactions that are useful for quantum simulations. These interactions can also be used to compensate multimodestatedependent Stark shifts, and perform generalized parity measurements, crucial for high fidelity gate operations and quantum error correction. 
Tuesday, March 3, 2020 4:54PM  5:30PM 
J36.00005: Experimental Demonstration of a Superconducting 0π Qubit Invited Speaker: Andras Gyenis Encoding a qubit in logical quantum states with wavefunctions characterized by disjoint support and robust energies can offer simultaneous protection against relaxation and pure dephasing. Using a twodimensional circuitquantumelectrodynamics architecture, we experimentally realize a superconducting 0π qubit, which hosts protected states suitable for quantuminformation processing. Our multitone spectroscopy measurements reveal the energy level structure of the system, which can be precisely described by a simple twomode Hamiltonian. We find that the circuit realizes an effective onedimensional crystal with two sublattices, where the geometrical phase difference between Wannier states localized at adjacent phase unit cells leads to interference effects associated with tunneling of pairs of Cooper pairs. The parity symmetry of the qubit results in chargeinsensitive levels connecting the protected states, allowing for logical operations. The measured relaxation (1.6 ms) and dephasing times (25 μs) demonstrate that our implementation of the 0π circuit not only broadens the family of superconducting qubits but also represents a promising candidate for the building block of a faulttolerant quantum computer. 
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