C. Dickel

Experimentally simulating the dynamics of quantum light and matter at deep-strong coupling

The quantum Rabi model describing the fundamental interaction between light and matter is a cornerstone of quantum physics. It predicts exotic phenomena like quantum phase transitions and ground-state entanglement in ultrastrong and deep-strong coupling regimes, where coupling strengths are comparable to or larger than subsystem energies. Demonstrating dynamics remains an outstanding challenge, the few experiments reaching these regimes being limited to spectroscopy. Here, we employ a circuit quantum electrodynamics chip with moderate coupling between a resonator and transmon qubit to realise accurate digital quantum simulation of deep-strong coupling dynamics. We advance the state of the art in solid-state digital quantum simulation by using up to 90 second-order Trotter steps and probing both subsystems in a combined Hilbert space dimension of ∼80, demonstrating characteristic Schrödinger-cat-like entanglement and large photon build-up. Our approach will enable exploration of extreme coupling regimes and quantum phase transitions, and demonstrates a clear first step towards larger complexities such as in the Dicke model.Realising deep-strong coupling phenomena for interacting light-matter systems remains an experimental challenge. Here, Langford et al. employ a circuit quantum electrodynamics chip with moderate coupling between a resonator and transmon qubit to realise digital quantum simulation of deep-strong coupling dynamics.

Active Resonator Reset in the Nonlinear Dispersive Regime of Circuit QED

We present two pulse schemes for actively depleting measurement photons from a readout resonator in the nonlinear dispersive regime of circuit QED. One method uses digital feedback conditioned on the measurement outcome while the other is unconditional. In the absence of analytic forms and symmetries to exploit in this nonlinear regime, the depletion pulses are numerically optimized using the Powell method. We shorten the photon depletion time by more than six inverse resonator linewidths compared to passive depletion by waiting. We quantify the benefit by emulating an ancilla qubit performing repeated quantum parity checks in a repetition code. Fast depletion increases the mean number of cycles to a spurious error detection event from order 1 to 75 at a 1 microsecond cycle time.

Independent, extensible control of same-frequency superconducting qubits by selective broadcasting

A critical ingredient for realising large-scale quantum information processors will be the ability to make economical use of qubit control hardware. We demonstrate an extensible strategy for reusing control hardware on same-frequency transmon qubits in a circuit QED chip with surface-code-compatible connectivity. A vector switch matrix enables selective broadcasting of input pulses to multiple transmons with individual tailoring of pulse quadratures for each, as required to minimise the effects of leakage on weakly anharmonic qubits. Using randomised benchmarking, we compare multiple broadcasting strategies that each pass the surface-code error threshold for single-qubit gates. In particular, we introduce a selective broadcasting control strategy using five pulse primitives, which allows independent, simultaneous Clifford gates on arbitrary numbers of qubits.

Tuneable hopping and nonlinear cross-Kerr interactions in a high-coherence superconducting circuit

Analog quantum simulations offer rich opportunities for exploring complex quantum systems and phenomena through the use of specially engineered, well-controlled quantum systems. A critical element, increasing the scope and flexibility of such experimental platforms, is the ability to access and tune in situ different interaction regimes. Here, we present a superconducting circuit building block of two highly coherent transmons featuring in situ tuneable photon hopping and nonlinear cross-Kerr couplings. The interactions are mediated via a nonlinear coupler, consisting of a large capacitor in parallel with a tuneable superconducting quantum interference device (SQUID). We demonstrate the working principle by experimentally characterising the system in the single-excitation and two-excitation manifolds, and derive a full theoretical model that accurately describes our measurements. Both qubits have high coherence properties, with typical relaxation times in the range of 15 to 40 μs at all bias points of the coupler. Our device could be used as a scalable building block in analog quantum simulators of extended Bose-Hubbard and Heisenberg XXZ models, and may also have applications in quantum computing such as realising fast two-qubit gates and perfect state transfer protocols.Analogue Quantum Simulators: scalable building block with tuneable capabilitiesA superconducting circuit composed of two transmons can be devised to have tuneable photon hopping and nonlinear couplings with adjustable ratios, giving access to previously unexplored interaction regimes, while maintaining high qubit coherence. The realisation comes from a team of researchers from Delft University of Technology and University of Technology Sydney, led by Marios Kounalakis, and relies on the addition of a capacitor and a tuneable nonlinear inductor: analogously to LC filtering, photon hopping can be tuned and suppressed at the filtering condition, while the Josephson nonlinearity of the circuit can be used to implement tuneable cross-Kerr interactions. This design allows changing the ratio between the two coupling strengths. Such a device could constitute a fundamental unit to build large quantum devices able to simulate condensed matter models which are not efficiently computable with classical computers.

Corrigendum: Independent, extensible control of same-frequency superconducting qubits by selective broadcasting

Correction to: npj Quantum Information (2016) 2, 16029. doi:10.1038/npjqi.2016.29; published online 23 August 2016 The original version of this Article contained an error in one of the calculations within the Results section. Although the authors note that this leakage rate is per Clifford, they actually quote the value per nanosecond: