I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
02
Mä
2022
Energetic cost of measurements using quantum, coherent, and thermal light
Quantum measurements are basic operations that play a critical role in the study and application of quantum information. We study how the use of quantum, coherent, and classical thermal
states of light in a circuit quantum electrodynamics setup impacts the performance of quantum measurements, by comparing their respective measurement backaction and measurement signal to noise ratio per photon. In the strong dispersive limit, we find that thermal light is capable of performing quantum measurements with comparable efficiency to coherent light, both being outperformed by single-photon light. We then analyze the thermodynamic cost of each measurement scheme. We show that single-photon light shows an advantage in terms of energy cost per information gain, reaching the fundamental thermodynamic cost.
On-Demand Directional Photon Emission using Waveguide Quantum Electrodynamics
Routing quantum information between non-local computational nodes is a foundation for extensible networks of quantum processors. Quantum information can be transferred between arbitrary
nodes by photons that propagate between them, or by resonantly coupling nearby nodes. Notably, conventional approaches involving propagating photons have limited fidelity due to photon loss and are often unidirectional, whereas architectures that use direct resonant coupling are bidirectional in principle, but can generally accommodate only a few local nodes. Here, we demonstrate high-fidelity, on-demand, bidirectional photon emission using an artificial molecule comprising two superconducting qubits strongly coupled to a waveguide. Quantum interference between the photon emission pathways from the molecule generate single photons that selectively propagate in a chosen direction. This architecture is capable of both photon emission and capture, and can be tiled in series to form an extensible network of quantum processors with all-to-all connectivity.
28
Feb
2022
A superconducting qubit with noise-insensitive plasmon levels and decay-protected fluxon states
The inductively shunted transmon (IST) is a superconducting qubit with exponentially suppressed fluxon transitions and a plasmon spectrum approximating that of the transmon. It shares
many characteristics with the transmon but offers charge offset insensitivity for all levels and precise flux tunability with quadratic flux noise suppression. In this work we propose and realize IST qubits deep in the transmon limit where the large geometric inductance acts as a mere perturbation. With a flux dispersion of only 5.1 MHz we reach the ’sweet-spot everywhere‘ regime of a SQUID device with a stable coherence time over a full flux quantum. Close to the flux degeneracy point the device reveals tunneling physics between the two quasi-degenerate ground states with typical observed lifetimes on the order of minutes. In the future, this qubit regime could be used to avoid leakage to unconfined transmon states in high-power read-out or driven-dissipative bosonic qubit realizations. Moreover, the combination of well controllable plasmon transitions together with stable fluxon states in a single device might offer a way forward towards improved qubit encoding schemes.
25
Feb
2022
Singlet-doublet transitions of a quantum dot Josephson junction detected in a transmon circuit
We realize a hybrid superconductor-semiconductor transmon device in which the Josephson effect is controlled by a gate-defined quantum dot in an InAs/Al nanowire. Microwave spectroscopy
of the transmon’s transition spectrum allows us to probe the ground state parity of the quantum dot as a function of gate voltages, external magnetic flux, and magnetic field applied parallel to the nanowire. The measured parity phase diagram is in agreement with that predicted by a single-impurity Anderson model with superconducting leads. Through continuous time monitoring of the circuit we furthermore resolve the quasiparticle dynamics of the quantum dot Josephson junction across the phase boundaries. Our results can facilitate the realization of semiconductor-based 0−π qubits and Andreev qubits.
24
Feb
2022
Engineering symmetry-selective couplings of a superconducting artificial molecule to microwave waveguides
Tailoring the decay rate of structured quantum emitters into their environment opens new avenues for nonlinear quantum optics, collective phenomena, and quantum communications. Here
we demonstrate a novel coupling scheme between an artificial molecule comprising two identical, strongly coupled transmon qubits, and two microwave waveguides. In our scheme, the coupling is engineered so that transitions between states of the same (opposite) symmetry, with respect to the permutation operator, are predominantly coupled to one (the other) waveguide. The symmetry-based coupling selectivity, as quantified by the ratio of the coupling strengths, exceeds a factor of 30 for both the waveguides in our device. In addition, we implement a two-photon Raman process activated by simultaneously driving both waveguides, and show that it can be used to coherently couple states of different symmetry in the single-excitation manifold of the molecule. Using that process, we implement frequency conversion across the waveguides, mediated by the molecule, with efficiency of about 95%. Finally, we show that this coupling arrangement makes it possible to straightforwardly generate spatially-separated Bell states propagating across the waveguides. We envisage further applications to quantum thermodynamics, microwave photodetection, and photon-photon gates.
23
Feb
2022
Quarter-wave Resonator Based Tunable Coupler for Xmon Qubits
We propose a scheme of tunable coupler based on quarter-wave resonator for scalable quantum integrated circuits. The open end of the T-type resonator is capacitively coupled to two
Xmon qubits, while another end is an asymmetric DC-Squid which dominates the inductive energy of coupler resonator. The DC current applied through the bias line can change the magnetic flux inside the DC-Squid, so the frequency of coupler resonator can be effectively tuned and the qubit-qubit coupling can be totally switched off at a certain frequency. As the increase of junction asymmetry for the DC-Squid, the coupling of Squid’s effective phase difference and cavity modes become smaller at required working frequency regime of coupler resonator, and this could reduce the descent of the resonators quality factor. The separation between two cross-capacitor can be larger with help of transverses width of the T-shape resonator, and then the ZZ crosstalk coupling can be effectively suppressed. The asymmetric DC squid is about 5 millimeters away from the Xmon qubits and only needs a small current on the flux bias line, which in principle creates less flux noises to superconducting Xmon qubits.
Nonlinear mechanisms in Al and Ti superconducting travelling-wave parametric amplifiers
The underlying nonlinear mechanisms behind the operation of travelling-wave parametric amplifiers (TWPAs) are important in determining their performance in terms of added noise, maximum
gain, and bandwidth. We describe a method of characterising the underlying nonlinearity of a superconducting material in terms of its dissipative-reactive ratio and the response time of the underlying microscopic processes. We describe and calculate the different behaviour arising from the equilibrium supercurrent nonlinearity, which has low dissipation and fast response time, and the non-equilibrium heating nonlinearity, which has high dissipation and slow response time. We have fabricated TWPAs based on Al and Ti, and characterised their nonlinearities using our analysis. For both Al and Ti, the measured dissipative-reactive ratios and response times are quantitatively similar to predictions for the non-equilibrium heating nonlinearity. In line with this, we were able to obtain more than 20 dB of peak power gain, although only over a narrow bandwidth of a few kilohertz.
21
Feb
2022
Path toward manufacturable superconducting qubits with relaxation times exceeding 0.1 ms
As the superconducting qubit platform matures towards ever-larger scales in the race towards a practical quantum computer, limitations due to qubit inhomogeneity through lack of process
control become apparent. To benefit from the advanced process control in industry-scale CMOS fabrication facilities, different processing methods will be required. In particular, the double-angle evaporation and lift-off techniques used for current, state-of-the art superconducting qubits are generally incompatible with modern day manufacturable processes. Here, we demonstrate a fully CMOS compatible qubit fabrication method, and show results from overlap Josephson junction devices with long coherence and relaxation times, on par with the state-of-the-art. We experimentally verify that Argon milling – the critical step during junction fabrication – and a subtractive etch process nevertheless result in qubits with average qubit energy relaxation times T1 reaching 70 μs, with maximum values exceeding 100 μs. Furthermore, we show that our results are still limited by surface losses and not, crucially, by junction losses. The presented fabrication process therefore heralds an important milestone towards a manufacturable 300 mm CMOS process for high-coherence superconducting qubits and has the potential to advance the scaling of superconducting device architectures.
17
Feb
2022
Gate-tunable kinetic inductance in proximitized nanowires
We report the detection of a gate-tunable kinetic inductance in a hybrid InAs/Al nanowire. For this purpose, we have embedded the nanowire into a quarter-wave coplanar waveguide resonator
and measured the resonance frequency of the circuit. We find that the resonance frequency can be changed via the gate voltage that controls the electron density of the proximitized semiconductor and thus the nanowire inductance. Applying Mattis-Bardeen theory, we extract the gate dependence of the normal state conductivity of the nanowire, as well as its superconducting gap. Our measurements complement existing characterization methods for hybrid nanowires and provide a new and useful tool for gate-controlled superconducting electronics.
15
Feb
2022
Saturable Purcell filter for circuit QED
We consider a typical circuit QED setup where a data artificial atom encodes a qubit and is dispersively coupled to a measurement resonator that in turn is coupled to a transmission
line. We show theoretically that by placing a filter artificial atom in this transmission line, the effective decay of the data artificial atom into the transmission line (the Purcell decay) is suppressed. When strong control fields are present in the transmission line, the filter artificial atom is saturated and hence effectively switched off. This permits both control and measurement of the data artificial atom using a single transmission line, while maintaining the Purcell filtering capability and therefore a long coherence time of the data artificial atom in the absence of the control pulses. We also apply open systems optimal control techniques to optimize the π-pulse on the data artificial atom, achieving high fidelity. The considered setup is compatible with the frequency multiplexing, potentially enabling both control and measurement of several qubits using a single Purcell-filtered transmission line. Our work helps with the scalability of the superconducting quantum computers by decreasing the ratio between the number of qubits and the number of the required transmission lines. For the setups that already use one transmission line both for measurement and control, our work provides a way to add Purcell filtering without removing the possibility of controlling the system.