Noise Insensitive Superconducting Cavity Qubtis Pan Zheng, Thomas Alexander, Ivar Taminiau, Han Le, Michal Kononenko, Hamid Mohebbi, Vadiraj Ananthapadmanabha Rao, Karthik Sampath Kumar, Thomas McConkey, Evan Peters, Joseph Emerson, Matteo Mariantoni, Christopher Wilson, David G. Cory - - When driven in the single-photon level, the entire system is in Introduction and Motivation The Transmon Qubits the superposition of the states where the photon exists in either - Circuit quantum electrodynamics (QED) studies the - When the detuning between a qubit and a cavity is much larger of the cavities. The symmetric and antisymmetric superposition interactions between artificial atoms (superconducting qubits) than the coupling strength, the second order dispersive coupling of this collective mode can be used to encode a two level system, and electromagnetic fields in the microwave regime at low causes the cavity frequency depending on the state of the qubit realizing a logical qubit: temperatures [1]. It provides a fundamental architecture for [1]. The dispersive coupling provides possible links between realizing quantum computation. isolated cavities. - When a qubit is placed in a 3D superconducting cavity, the - A transmon qubit is a superconducting charge qubit insensitive cavity acts as both a resonator with low loss and a package to charge noise [5]. In the project they will be fabricated from providing an isolated environment [2]. Coherence times of evaporation deposited aluminum on a sapphire chip. transmon qubits up to ~100 μs have been observed [3]. - Advantages of the logical qubits: - Quantum memories made of 3D cavities have achieved 1) A logical qubit of this form will be immune to the collective coherence times as long as 1 ms [4]. dephasing noise on both cavities as the noise will apply an - We aim to make the long lived cavities into qubits with a identical random rotation on any linear superpostion of the transmon acting as a nonlinear actuator on the cavities. This logical states. gives us the best of both worlds, long lifetimes with control of 2) The logical qubit is expected to have a long coherence time as single photon states. all the quantum information is stored in the high-Q cavities, not the lossy transmon qubit. The 3D Superconducting Cavities 3) The number of photons stored in both cavities may be observed without measuring the logical qubit. This allows - The cylindrical 3D cavity consists of a coaxial cavity and a photon loss to be detected and corrected for. circular waveguide [3]. The Combined System 4) The logical qubit may be controlled by detuning the cavities. - The quarter wavelength (λ/4) coaxial cavity is fabricated from - The two cavities fabricated in a single piece have identical This requires microwave tones only for initialization and ultra high purity aluminum. It has one end shorted and the other frequencies and do not communicate directly with each other. measurement. open. The height of the stub defines the resonance frequency. - The sapphire chip with a transmon qubit on it is placed in the - The cavity seam at the top of the cylinder is coupled to the bridge channel connecting the two cavities. The two ends of the Acknowledgements quarter wavelength stub via a cutoff circular mode in the chip intrude into the cavities at the height of the stub, with the waveguide which exponentially suppresses photon seam losses. coupling strength adjustable via changing the intruding length. - The cavity can be driven by a port opened at the height of the - The transmon qubit in the channel couples to the two cavities stub where the electrical component of the field reaches its in the dispersive regime. It will mix the energy levels in the maximum.. individual cavities into collective energy states . Cavity A Cavity B Cavity Cavity Drive Coupler Drive References [1] S. M. Girvin, Circuit QED: Superconducting Qubits Coupled to Port to drive the cavity Microwave Photons (2014) [2] H. Paik et al. Phys. Rev. Lett. 107, 240501 (2011) [3] C. Rigetti et al. Phys. Rev. B 86, 100506(R) (2012) Bridge [4] M. Reagor et al. Phys. Rev. B 94, 014506 (2016) [5] J. Koch et al. Phys. Rev. A 76, 042319 (2007) .