A chemical potential for transmons
Three-wave parametric couplings have traditionally been used to create parametric gain between two low-Q modes by driving at the sum of their frequencies, and photon conversion by driving at their difference. In this project we extend this idea to qubits by coupling a low-Q mode containing a SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement) to a high-Q transmon. Driving the SNAIL at the sum of the SNAIL and transmon 0 to 1 photon transition frequencies creates a pair of photons. The SNAIL is designed to promptly lose its photon, and this, together with the transmon anharmonicity, creates a parametrically controlled heating rate from the ground to first excited state. Driving the SNAIL and qubit 0 to 1 transition difference frequency creates cooling via a similar process. Heating and cooling between higher qubit states are also accessible via distinct drive frequencies. By combining these drives we can create quite unusual transmon steady states, with full control over both the transmon temperature and relaxation time scale.
A modular quantum computer based on a quantum state router
In this project, we have designed and implemented a superconducting, microwave quantum state router which can realize all-to-all couplings among four quantum modules. Each module consists of a single transmon, readout mode, and communication mode coupled to the router. The router design centers on a parametrically driven, Josephson-junction based three-wave mixing element which generates photon exchange among the modules' communication modes. We have demonstrated SWAP operations among the four communication modes, with an average full-SWAP time of 760 ns and average inter-module gate fidelity of 0.97, limited by our modes' coherences. We have also demonstrated photon transfer and pairwise entanglement between the modules' qubits, and parallel operation of simultaneous SWAP gates across the router. We are working to extend these results to faster and higher fidelity router operations, as well as scaled to support larger networks of quantum modules.
Nearly quantum-limited Josephson-junction frequency-comb synthesizer
While coherently driven Kerr microcavities have rapidly matured as a platform for frequency-comb formation, such microresonators generally possess weak Kerr coefficients; consequently, triggering comb generation requires millions of photons to be circulating inside the cavity. This suppresses the role of quantum fluctuations in the dynamics of the comb. In this project, we realize a minimal version of coherently driven Kerr-mediated microwave-frequency combs in the circuit quantum electrodynamics (cQED) architecture, where the fluctuations of the quantum vacuum are the primary limitation on comb coherence. We have achieved a comb phase coherence of up to 35 us, approaching the theoretical device quantum limit of 55 us and vastly longer than the inherent lifetimes of the modes, of 13 ns. The ability within cQED to engineer stronger nonlinearities than optical microresonators, together with operation at cryogenic temperatures, and the excellent agreement of comb dynamics with quantum theory indicates a promising platform for the study of complex dynamics of quantum nonlinear systems.
Increasing qubit readout fidelity with two-mode squeezed light
Implementing quantum information processing on a large scale with flawed components requires highly efficient, quantum non-demolition (QND) qubit readout. In superconducting circuits, qubit readout using coherent light with fidelity above 99% has been achieved by using a quantum-limited parametric amplifier such as the Josephson Parametric Converter (JPC), as the first stage amplifier. However, further improvement of such measurement is fundamentally limited by the vacuum fluctuations on the ports of the JPC. Alternatively, readout with squeezed input can entangle the vacuum fluctuations in different modes, thus allowing for the reduction of the noise by controlling their interference. In our lab, we have demonstrated a dispersive qubit readout scheme which exploits the two-mode squeezed light generated by a first JPC and processed by a second JPC to form an amplified interferometer . We have observed a 22% improvement in the voltage Signal-to-Noise Ratio (SNR) of the measurement compared to coherent light. We can extend this scheme to generate remote entanglement by placing a qubit-cavity in each arm of the interferometer.
Multi-parametric operation for realization of directional devices
Josephson parametric amplifiers, although nearly quantum limited, typically operate in reflection and have a fixed gain-bandwidth product. Recently, there have been both theoretical and experimental efforts to address these shortcomings by combining multiple parametric processes within a single multimode device. In our lab, we focus on using multiple parametric processes to couple two modes of a Josephson Parametric Converter. We regularly use paired, detuned gain processes as well as two separate methods of combining gain and photon conversion, all in the same physical device. All three schemes avoid the fixed gain-bandwidth product of a singly pumped JPC. We are currently working to extend these ideas to circuits with three or more modes which should allow us to create a broadband, directional quantum-limited amplifier.