Although quantum mechanics applies to many macroscopic superconducting devices, one basic prediction remained controversial for decades. Namely, a Josephson junction connected to aresistor must undergo a dissipation-induced quantum phase transition from superconductor to insulator once the resistor’s value exceeds h/4e2≈6.5 kΩ (h is Planck’s constant, e is the electron charge). Here we finally demonstrate this transition by observing the resistor’s internal dynamics. Implementing our resistor as a long transmission line section, we find that a junction scatters electromagnetic excitations in the line as either inductance (superconductor) or capacitance (insulator), depending solely on the line’s wave impedance. At the phase boundary, the junction itself acts as ideal resistance: in addition to elastic scattering, incident photons can spontaneously down-convert with a frequency-independent probability, which provides a novel marker of quantum-critical behavior.
There is a tremendous interest in fabricating superconducting flux circuits that are nonstoquastic—i.e., have positive off-diagonal matrix elements—in their qubit representation,as these circuits are thought to be unsimulable by classical approaches and thus could play a key role in the demonstration of speedups in quantum annealing protocols. We show however that the efficient simulation of these systems is possible by the direct simulation of the flux circuits. Our approach not only obviates the reduction to a qubit representation but also produces results that are more in the spirit of the experimental setup. We discuss the implications of our work. Specifically we argue that our results cast doubt on the conception that superconducting flux circuits represent the correct avenue for universal adiabatic quantum computers.
Instantons, spacetime-localized quantum field tunneling events, are ubiquitous in correlated condensed matter and high energy systems. However, their direct observation through collisionswith conventional particles has not been considered possible. We show how recent advance in circuit quantum electrodynamics, specifically, the realization of galvanic coupling of a transmon qubit to a high-impedance transmission line, allows the observation of inelastic collisions of single microwave photons with instantons (phase slips). We develop the formalism for calculating the photon-instanton cross section, which should be useful in other quantum field theoretical contexts. In particular, we show that the inelastic scattering probability can significantly exceed the effect of conventional Josephson quartic anharmonicity, and reach order unity values.
Light does not typically scatter light, as witnessed by the linearity of Maxwell’s equations. We constructed a superconducting circuit, in which microwave photons have well-definedenergy and momentum, but their lifetime is finite due to decay into lower energy photons. The inelastic photon-photon interaction originates from quantum phase-slip fluctuation in a single Josephson junction and has no analogs in quantum optics. Instead, the surprisingly high decay rate is explained by mapping the system to a Luttinger liquid containing an impurity. Our result connects circuit quantum electrodynamics to the topic of boundary quantum field theories in two dimensions, influential to both high-energy and condensed matter physics. The photon lifetime data is a rare example of a verified and useful quantum many-body simulation.
Quantum fluctuations in an anharmonic superconducting circuit enable
frequency conversion of individual incoming photons. This effect, linear in the
photon beam intensity, leads toramifications for the standard input-output
circuit theory. We consider an extreme case of anharmonicity in which photons
scatter off a small set of weak links within a Josephson junction array. We
show that this quantum impurity displays Kondo physics and evaluate the elastic
and inelastic photon scattering cross sections. These cross sections reveal
many-body properties of the Kondo problem that are hard to access in its
traditional fermionic version.