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
Okt
2019
Cryo-CMOS Band-gap Reference Circuits for Quantum Computing
The control interface of a large-scale quantum computer will likely require electronic sub-systems that operate in close proximity to the qubits, at deep cryogenic temperatures. Here,
we report the low-temperature performance of custom cryo-CMOS band-gap reference circuits designed to provide stable voltages and currents on-chip, independent of local temperature fluctuations. Our circuits are fabricated in 0.35 um silicon Germanium (SiGe) BiCMOS and 28 nm Fully Depleted Silicon On Insulator (FDSOI) CMOS processes, and we compare the performance of each. Beyond their specific application as low-power references, these circuits are ideal test-vehicles for developing design approaches that mitigate the adverse effects of cryogenic temperatures on circuit performance.
Radiative cooling of a superconducting resonator
Cooling microwave resonators to near the quantum ground state, crucial for their operation in the quantum regime, is typically achieved by direct device refrigeration to a few tens
of millikelvin. However, in quantum experiments that require high operation power such as microwave-to-optics quantum transduction, it is desirable to operate at higher temperatures with non-negligible environmental thermal excitations, where larger cooling power is available. In this Letter, we present a radiative cooling protocol to prepare a superconducting microwave mode near its quantum ground state in spite of warm environment temperatures for the resonator. In this proof-of-concept experiment, the mode occupancy of a 10-GHz superconducting resonator thermally anchored at 1.02~K is reduced to 0.44±0.05 by radiatively coupling to a 70-mK cold load. This radiative cooling scheme allows high-operation-power microwave experiments to work in the quantum regime, and opens possibilities for routing microwave quantum states to elevated temperatures.
Superradiant phase transition with flat bands in a circuit QED lattice
We investigate the interplay of superradiant phase transition (SPT) and energy band physics in an extended Dicke-Hubbard lattice whose unit cell consists of a Dicke model coupled to
an atomless cavity. We found in such a periodic lattice the critical point that occurs in a single Dicke model becomes a critical region that is periodically changing with the wavenumber k. In the weak-coupling normal phase of the system we observed a flat band and its corresponding localization that can be controlled by the ground-state SPT. Our work builds the connection between flat band physics and SPT, which may fundamentally broaden the regimes of many-body theory and quantum optics.
27
Sep
2019
Microwave measurement beyond the quantum limit with a nonreciprocal amplifier
The measurement of a quantum system is often performed by encoding its state in a single observable of a light field. The measurement efficiency of this observable can be reduced by
loss or excess noise on the way to the detector. Even a \textit{quantum-limited} detector that simultaneously measures a second non-commuting observable would double the output noise, therefore limiting the efficiency to 50%. At microwave frequencies, an ideal measurement efficiency can be achieved by noiselessly amplifying the information-carrying quadrature of the light field, but this has remained an experimental challenge. Indeed, while state-of-the-art Josephson-junction based parametric amplifiers can perform an ideal single-quadrature measurement, they require lossy ferrite circulators in the signal path, drastically decreasing the overall efficiency. In this paper, we present a nonreciprocal parametric amplifier that combines single-quadrature measurement and directionality without the use of strong external magnetic fields. We extract a measurement efficiency of 62+17−9% that exceeds the quantum limit and that is not limited by fundamental factors. The amplifier can be readily integrated with superconducting devices, creating a path for ideal measurements of quantum bits and mechanical oscillators.
26
Sep
2019
Quantum Microwave Radiometry with a Superconducting Qubit
The interaction of photons and coherent quantum systems can be employed to detect electromagnetic radiation with remarkable sensitivity. We introduce a quantum radiometer based on the
photon-induced-dephasing process of a superconducting qubit for sensing microwave radiation at the sub-unit-photon level. Using this radiometer, we demonstrated the radiative cooling of a 1-K microwave resonator and measured its mode temperature with an uncertainty ~0.01 K. We have thus developed a precise tool for studying the thermodynamics of quantum microwave circuits, which provides new solutions for calibrating hybrid quantum systems and detecting candidate particles for dark matter.
24
Sep
2019
Mixed algorithmic-analog simulation of many body dynamics using interaction of fixed-frequency superconducting qubits
In recent years there was a huge experimental progress towards the development of prototypes of algorithmic quantum processors. These quantum machines are not free from imperfections
and various technological and scientific problems remain to be solved in the following years. Until that moment computational schemes different from the digital approach can be used in order to perform calculations using state-of-the-art quantum hardware. A prospective idea is to combine positive aspects of both digital and analog computation. Particularly, it is possible to use qubit-qubit interaction embedded in architecture in order to replace those parts of algorithms which are responsible for the quantum entanglement. In this paper, we provide an example of such an approach based on unwanted couplings between fixed-frequency superconducting qubits (crosstalks). These couplings are normally considered as a source of errors in standard digital quantum computation, but we argue that they can be utilized instead of two-qubit gates in some quantum algorithms thus avoiding an accumulation of errors associated with these gates. We illustrate our ideas with quantum processors of IBM Quantum Experience, which are used by us for simulating the dynamics of spin clusters through the Trotterized evolution. We demonstrate a significant improvement in the quality of results compared to the conventional digital approach with the same processor. We also show that crosstalks result in a highly non-markovian dynamics of qubits. This fact must be taken into account while developing error-correction strategies with qubits of this type.
22
Sep
2019
Random walk on the Bloch sphere realized by a simultaneous feedback and feed-forward control in a superconducting Xmon qubit system
Measurement-based feedback control is central in quantum computing and precise quantum control. Here we realize a fast and flexible field-programmable-gate-array-based feedback control
in a superconducting Xmon qubit system. The latency of room-temperature electronics is custom optimized to be as short as 140 ns. Projective measurement of a signal qubit produces a feedback tag to actuate a conditional pulse gate to the qubit. In a feed-forward process, the measurement-based feedback tag is brought to a different target qubit for a conditional control. In a two-qubit experiment, the feedback and feed-forward controls are simultaneously actuated in consecutive steps. A quantum number is then generated by the signal qubit, and a random walk of the target qubit is correspondingly triggered and realized on the Bloch sphere. Our experiment provides a conceptually simple and intuitive benchmark for the feedback control in a multi-qubit system. The feedback control can also be further explored to study complex stochastic quantum control.
Realizing modular quadrature measurements via a tunable photon-pressure coupling in circuit-QED
One of the most direct preparations of a Gottesman-Kitaev-Preskill qubit in an oscillator uses a tunable photon-pressure (also called optomechanical) coupling of the form gq^a†a,
enabling to imprint the modular value of the position q^ of one oscillator onto the state of an ancilla oscillator. We analyze the practical feasibility of executing such modular quadrature measurements in a parametric circuit-QED realization of this coupling. We provide estimates for the expected GKP squeezing induced by the protocol and discuss the effect of photon loss and other errors on the resulting squeezing.
Experimental realization of nonadiabatic geometric gates with a superconducting Xmon qubit
Geometric phases are only dependent on evolution paths but independent of evolution details so that they own some intrinsic noise-resilience features. Based on different geometric phases,
various quantum gates have been proposed, such as nonadiabatic geometric gates based on nonadiabatic Abelian geometric phases and nonadiabatic holonomic gates based on nonadiabatic non-Abelian geometric phases. Up to now, nonadiabatic holonomic one-qubit gates have been experimentally demonstrated with the supercondunting transmon, where three lowest levels with cascaded configuration are all applied in the operation. However, the second excited states of transmons have relatively short coherence time, which results in a lessened fidelity of quantum gates. Here, we experimentally realize Abelian-geometric-phase-based nonadiabatic geometric one-qubit gates with a superconducting Xmon qubit. The realization is performed on two lowest levels of an Xmon qubit and thus avoids the influence from the short coherence time of the second excited state. The experimental result indicates that the average fidelities of single-qubit gates can be up to 99.6% and 99.7% characterized by quantum process tomography and randomized benchmarking, respectively.
21
Sep
2019
Electric field spectroscopy of material defects in transmon qubits
Superconducting integrated circuits have demonstrated a tremendous potential to realize integrated quantum computing processors. However, the downside of the solid-state approach is
that superconducting qubits suffer strongly from energy dissipation and environmental fluctuations caused by atomic-scale defects in device materials. Further progress towards upscaled quantum processors will require improvements in device fabrication techniques which need to be guided by novel analysis methods to understand and prevent mechanisms of defect formation. Here, we present a new technique to analyse individual defects in superconducting qubits by tuning them with applied electric fields. This provides a new spectroscopy method to extract the defects‘ energy distribution, electric dipole moments, and coherence times. Moreover, it enables one to distinguish defects residing in Josephson junction tunnel barriers from those at circuit interfaces. We find that defects at circuit interfaces are responsible for about 60% of the dielectric loss in the investigated transmon qubit sample. About 40% of all detected defects are contained in the tunnel barriers of the large-area parasitic Josephson junctions that occur collaterally in shadow evaporation, and only about 3% are identified as strongly coupled defects which presumably reside in the small-area qubit tunnel junctions. The demonstrated technique provides a valuable tool to assess the decoherence sources related to circuit interfaces and to tunnel junctions that is readily applicable to standard qubit samples.