Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session F09: Recent Advances with Superconducting CircuitsInvited Live

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Sponsoring Units: DQI Chair: Anja Metelmann, Freie Univ Berlin 
Tuesday, March 16, 2021 11:30AM  12:06PM Live 
F09.00001: Hardware efficient quantum computing using repetition cat qubits. 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, their actual implementation usually comes at the price of a tremendous physical resource overhead. Our approach to reduce the physical cost of quantum computing is the use of catqubits in a simple repetition code. The catqubits, 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, a universal set of fully protected logical gates can be implemented on the repetition code, achieving very low logical error rates with a reasonable physical overhead. In this talk, I will present the key points of the approach and give estimates of the expected performance based on numerical simulations of the circuits including realistic error models for all the gates and operations. 
Tuesday, March 16, 2021 12:06PM  12:42PM Live 
F09.00002: Experimental realization of the soft 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. One of the most promising candidates for such a fullyprotected superconducting qubit is the 0π circuit [Brooks et al., Phys. Rev. A 87, 052306 (2013)]. Here, we realize the proposed circuit topology in an experimentally obtainable parameter regime and show that the device, which we call as the soft 0π qubit, hosts logical states with disjoint support that are exponentially (firstorder) protected against charge (flux) noise. Multitone spectroscopy measurements reveal the energylevel structure of the system, which can be precisely described by a simple twomode Hamiltonian. Using a Ramantype protocol, we exploit a higherlying chargeinsensitive energy level of the device to realize coherent population transfer and logical operations. The measured relaxation (T_{1 }= 1.6 ms) and dephasing (T_{R }= 9 μs, T_{2E }= 25 µs) times demonstrate that the soft 0π circuit not only broadens the family of superconducting qubits, but also constitutes an important step towards quantum computing with intrinsically protected superconducting qubits. 
Tuesday, March 16, 2021 12:42PM  1:18PM Live 
F09.00003: Optical photon generation from a superconducting qubit Invited Speaker: Alp Sipahigil Superconducting circuits based on Josephson junctions emerged as a powerful platform for processing quantum information. However, these systems operate at low temperatures and microwave frequencies, and require a coherent interface with optical photons to transfer quantum information across long distances. In this talk, I will present our recent experiments demonstrating quantum transduction of a superconducting qubit excitation to an optical photon [1]. I will describe how we use mesoscopic mechanical oscillators in their quantum ground states to convert single photons from microwave frequencies to the optical domain. I will conclude by discussing the prospects of this approach for realizing future quantum networks based on superconducting quantum processors. [1] M. Mirhosseini*, A. Sipahigil*, M. Kalaee*, O. Painter, arXiv:2004.04838 (2020) 
Tuesday, March 16, 2021 1:18PM  1:54PM Live 
F09.00004: Stabilization and operation of a Kerrcat qubit. 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 16, 2021 1:54PM  2:30PM Live 
F09.00005: Multimode photon blockade Invited Speaker: Srivatsan Chakram Strong interactions and the resulting correlations are responsible for much of the richness observed in quantum many body systems. We present a new nonlocal manybody interaction between microwave photons stored in the electromagnetic modes of a multimode cavity [1]. 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 ~1 billion [2]. This multimode photon blockade interaction is similar to Coulomb blockade for electrons, but in this case acts on the total number of photons shared across a group of cavity modes. The system is dressed using cavity photon number resolved transmon drives 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. 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 also use multimode photon blockade to generate entanglement only with uniform cavity drives, and characterize the resulting 2 and 3mode W states using a new protocol for multimode Wigner tomography. These interactions are promising for studies of many body quantum optics, and for quantum information processing using multimode circuit QED. 
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