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
18
Feb
2016
Low-noise amplification and frequency conversion with a multiport microwave optomechanical device
High-gain amplifiers of electromagnetic signals operating near the quantum limit are crucial for quantum information systems and ultrasensitive quantum measurements. However, the existing
techniques have a limited gain-bandwidth product and only operate with weak input signals. Here we demonstrate a two-port optomechanical scheme for amplification and routing of microwave signals, a system that simultaneously performs high-gain amplification and frequency conversion in the quantum regime. Our amplifier, implemented in a two-cavity microwave optomechanical device, shows 41 dB of gain and has a high dynamic range, handling input signals up to 1013 photons per second, three orders of magnitude more than corresponding Josephson parametric amplifiers. We show that although the active medium, the mechanical resonator, is at a high temperature far from the quantum limit, only 4.6 quanta of noise is added to the input signal. Our method can be readily applied to a wide variety of optomechanical systems, including hybrid optical-microwave systems, creating a universal hub for signals at the quantum level.
A Reconfigurable Cryogenic Platform for the Classical Control of Scalable Quantum Computers
Recent advances in solid-state qubit technology are paving the way to fault-tolerant quantum computing systems. However, qubit technology is limited by qubit coherence time and by the
complexity of coupling the quantum system with a classical electronic infrastructure.
We propose an infrastructure, enabling to read and control qubits, that is implemented on a field-programmable gate array (FPGA). The FPGA platform supports functionality required by several qubit technologies and can operate physically close to the qubits over a temperature range from 4K to 300K. Extensive characterization of the platform over this temperature range revealed all major components (such as LUTs, MMCM, PLL, BRAM, IDELAY2) operate correctly and the logic speed is very stable. The stability is finally concretized by operating an integrated ADC with relatively stable performance over temperature.
17
Feb
2016
Emulating the one-dimensional Fermi-Hubbard model by a double chain of qubits
The Jordan-Wigner transformation maps a one-dimensional spin-1/2 system onto a Fermionic model without spin degree of freedom. Here we show that a double chain of qubits with XX and
ZZ couplings of neighboring qubits along and between the chains, respectively, can be mapped on a spin-full 1D Fermi-Hubbard model. The qubit system can thus be used to emulate the quantum properties of this model. We analyze physical implementations of such analog quantum simulators, including one based on transmon qubits, where the ZZ interaction arises due to an inductive coupling and the XX interaction due to a capacitive interaction. We propose protocols to gain confidence in the results of the simulation through measurements of local operators.
Using Spontaneous Emission of a Qubit as a Resource for Feedback Control
Persistent control of a transmon qubit is performed by a feedback protocol based on continuous weak measurement of its fluorescence. By driving the qubit and cavity with microwave signals
whose amplitudes depend linearly on the instantaneous values of the quadratures of the measured fluorescence field, we demonstrate the permanent stabilization of the qubit in any direction of the Bloch sphere. Using a Josephson mixer as a phase-preserving amplifier, it was possible to reach a total measurement efficiency η=35%, leading to a maximum of 59% of excitation and 44% of coherence for the stabilized states. The experiment demonstrates multiple-input multiple-output (MIMO) analog markovian feedback in the quantum regime.
16
Feb
2016
Engineered dissipative reservoir for microwave light using circuit optomechanics
Dissipation can significantly affect the quantum behaviour of a system and even completely suppress it. Counterintuitively, engineered dissipation enables the preparation of quantum
states as well as their stabilization. In cavity electro- and optomechanics, the control over mechanical oscillators relies on a dissipation hierarchy in which the electromagnetic energy decay rate significantly exceeds that of the mechanical oscillator. In contrast, recent theoretical work has considered the opposite regime in which the mechanical oscillator dissipation dominates and provides a cold dissipative reservoir to the electromagnetic degree of freedom. This novel regime allows to manipulate the electromagnetic mode and enables a new class of dissipative interactions. Here, we report on the experimental realization of this reversed dissipation regime in a microwave cavity optomechanical system. We directly evidence the preparation of a quasi-instantaneous, cold reservoir for a microwave field by on-demand decreasing or increasing the damping rate of the microwave mode, that corresponds to amplification and de-amplification of the microwave field. Moreover, we observe the onset of parametric instability, i.e. stimulated emission of microwaves (masing). The dissipative interaction additionally enables to operate the system as a low-noise, large-gain phase-preserving amplifier. Realizing a dissipative reservoir for microwave light is a requirement for the dissipative coupling of multiple cavity modes, which in turn forms the basis of dissipative quantum phase transitions, microwave entanglement schemes, and electromechanical quantum-limited amplifiers. Equally importantly, this interaction underpins recently predicted non-reciprocal devices, which would extend the available toolbox of quantum-limited microwave manipulation techniques.
Hamiltonian engineering for robust quantum state transfer and qubit readout in cavity QED
Quantum state transfer into a memory, state shuttling over long distances via a quantum bus, and high-fidelity readout are important tasks for quantum technology. Realizing these tasks
is challenging in the presence of realistic couplings to an environment. Here, we introduce and assess protocols that can be used in cavity QED to perform high-fidelity quantum state transfer and fast quantum nondemolition qubit readout through Hamiltonian engineering. We show that high-fidelity state transfer between a cavity and a single qubit or between a cavity and the collective mode of a qubit ensemble can be performed, even in the limit of strong dephasing due to inhomogeneous broadening. Moreover, we show that large signal-to-noise and high single-shot fidelity can be achieved in a cavity-based qubit readout, even in the weak-coupling limit. These ideas may be important for novel systems coupling single spins to a microwave cavity.
15
Feb
2016
Demonstrating Quantum Error Correction that Extends the Lifetime of Quantum Information
The remarkable discovery of Quantum Error Correction (QEC), which can overcome the errors experienced by a bit of quantum information (qubit), was a critical advance that gives hope
for eventually realizing practical quantum computers. In principle, a system that implements QEC can actually pass a „break-even“ point and preserve quantum information for longer than the lifetime of its constituent parts. Reaching the break-even point, however, has thus far remained an outstanding and challenging goal. Several previous works have demonstrated elements of QEC in NMR, ions, nitrogen vacancy (NV) centers, photons, and superconducting transmons. However, these works primarily illustrate the signatures or scaling properties of QEC codes rather than test the capacity of the system to extend the lifetime of quantum information over time. Here we demonstrate a QEC system that reaches the break-even point by suppressing the natural errors due to energy loss for a qubit logically encoded in superpositions of coherent states, or cat states of a superconducting resonator. Moreover, the experiment implements a full QEC protocol by using real-time feedback to encode, monitor naturally occurring errors, decode, and correct. As measured by full process tomography, the enhanced lifetime of the encoded information is 320 microseconds without any post-selection. This is 20 times greater than that of the system’s transmon, over twice as long as an uncorrected logical encoding, and 10% longer than the highest quality element of the system (the resonator’s 0, 1 Fock states). Our results illustrate the power of novel, hardware efficient qubit encodings over traditional QEC schemes. Furthermore, they advance the field of experimental error correction from confirming the basic concepts to exploring the metrics that drive system performance and the challenges in implementing a fault-tolerant system.
10
Feb
2016
Resonance fluorescence from an artificial atom in squeezed vacuum
We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect
the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. \textbf{58}, 2539-2542 (1987)], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments.
Landau-Zener-Stuckelberg-Majorana interference in a 3D transmon driven by a chirped microwave
By driving a 3D transmon with microwave fields, we generate an effective avoided energy-level crossing. Then we chirp microwave frequency, which is equivalent to driving the system
through the avoided energy-level crossing by sweeping the avoided crossing. A double-passage chirp produces Landau-Zener-St\“uckelberg-Majorana interference that agree well with the numerical results. Our method is fully applicable to other quantum systems that contain no intrinsic avoided level crossing, providing an alternative approach for quantum control and quantum simulation.
08
Feb
2016
Traveling-wave parametric amplifier with three-wave mixing in superconducting transmission line with embedded rf-SQUIDs
We present an experimental realization of resonance fluorescence in squeezed vacuum. We strongly couple microwave-frequency squeezed light to a superconducting artificial atom and detect
the resulting fluorescence with high resolution enabled by a broadband traveling-wave parametric amplifier. We investigate the fluorescence spectra in the weak and strong driving regimes, observing up to 3.1 dB of reduction of the fluorescence linewidth below the ordinary vacuum level and a dramatic dependence of the Mollow triplet spectrum on the relative phase of the driving and squeezed vacuum fields. Our results are in excellent agreement with predictions for spectra produced by a two-level atom in squeezed vacuum [Phys. Rev. Lett. \textbf{58}, 2539-2542 (1987)], demonstrating that resonance fluorescence offers a resource-efficient means to characterize squeezing in cryogenic environments.