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.
It is customary to use arrays of superconducting quantum interference devices (SQUIDs) for implementing magnetic field-tunable inductors. Here, we demonstrate an equivalent tunabilityin a (SQUID-free) array of single Al/AlOx/Al Josephson tunnel junctions. With the proper choice of junction geometry, a perpendicularly applied magnetic field bends along the plane of the superconductor and focuses into the tunnel barrier region due to a demagnetization effect. Consequently, the Josephson inductance can be efficiently modulated by the Fraunhoffer-type supercurrent interference. The elimination of SQUIDs not only simplifies the device design and fabrication, but also facilitates a denser packing of junctions and, hence, a higher inductance per unit length. As an example, we demonstrate a transmission line, the wave impedance of which is field-tuned in the range of 4−8 kΩ, centered around the important value of the resistance quantum h/(2e)2≈6.5 kΩ.
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.
We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T2 > 100 microseconds (T2 > 300 microseconds in onedevice). Moreover, given the state of the art values of the surface loss tangent and the 1/f flux noise amplitude, coherence can be further improved beyond 1 millisecond. Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit — over 100 here — must be minimized for best qubit coherence. We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.
Vacuum fluctuations fundamentally affect an atom by inducing a fnite excited state lifetime along with a Lamb shift of its transition frequency. Here we report the reverse effect: modifcationof vacuum by a single atom in circuit quantum electrodynamics. Our one-dimensional vacuum is a long section of a high wave impedance (comparable to resistance quantum) superconducting transmission line. It is directly wired to a transmon qubit circuit. Owing to the combination of high impedance and galvanic connection, the transmon’s spontaneous emission linewidth can greatly exceed the discrete transmission line modes spacing. This condition defines a previously unexplored superstrong coupling regime of quantum electrodynamics where many vacuum modes hybridize with each other through interactions with a single atom. We explore this regime by spectroscopically measuring the positions of over 100 consecutive transmission line resonances. The atom reveals itself as a broad peak in the vacuum’s density of states (DOS) together with the Kerr and cross-Kerr interaction of photons at frequencies within the DOS peak. Both dispersive effects are well described by a dissipative Caldeira-Leggett model of our circuit, with the transmon’s quartic anharmonicity treated as a perturbation. Non-perturbative modifications of such a vacuum, including inelastic scattering of single photons, are expected upon replacing the transmon by more anharmonic circuits, with broad implications for simulating critical dynamics of quantum impurity models.
Quantum control of atomic systems is largely enabled by the rich structure of selection rules in the spectra of most real atoms. Their macroscopic superconducting counterparts havebeen lacking this feature, being limited to a single transition type with a large dipole. Here we report a superconducting artificial atom with tunable transition dipoles, designed such that its forbidden (qubit) transition can dispersively interact with microwave photons due to the virtual excitations of allowed transitions. Owing to this effect, we have demonstrated an in-situ tuning of qubit’s energy decay lifetime by over two orders of magnitude, exceeding a value of 2 ms, while keeping the transition frequency fixed around 3,5 GHz