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
16
Okt
2015
Measurement of geometric dephasing using a superconducting qubit
A quantum system interacting with its environment is subject to dephasing which ultimately destroys the information it holds. Using a superconducting qubit, we experimentally show that
this dephasing has both dynamic and geometric origins. It is found that geometric dephasing, which is present even in the adiabatic limit and when no geometric phase is acquired, can either reduce or restore coherence depending on the orientation of the path the qubit traces out in its projective Hilbert space. It accompanies the evolution of any system in Hilbert space subjected to noise.
15
Okt
2015
Building logical qubits in a superconducting quantum computing system
The technological world is in the midst of a quantum computing and quantum information revolution. Since Richard Feynman’s famous „plenty of room at the bottom“ lecture,
hinting at the notion of novel devices employing quantum mechanics, the quantum information community has taken gigantic strides in understanding the potential applications of a quantum computer and laid the foundational requirements for building one. We believe that the next significant step will be to demonstrate a quantum memory, in which a system of interacting qubits stores an encoded logical qubit state longer than the incorporated parts. Here, we describe the important route towards a logical memory with superconducting qubits, employing a rotated version of the surface code. The current status of technology with regards to interconnected superconducting-qubit networks will be described and near-term areas of focus to improve devices will be identified. Overall, the progress in this exciting field has been astounding, but we are at an important turning point where it will be critical to incorporate engineering solutions with quantum architectural considerations, laying the foundation towards scalable fault-tolerant quantum computers in the near future.
Artificial Quantum Thermal Bath
Temperature determines the relative probability of observing a physical system in an energy state when that system is energetically in equilibrium with its environment. In this manuscript,
we present a theory for engineering the temperature of a quantum system different from its ambient temperature, that is basically an analog version of the quantum metropolis algorithm. We define criteria for an engineered quantum bath that, when couples to a quantum system with Hamiltonian H, drives the system to the equilibrium state e−H/TTr(e−H/T) with a tunable parameter T. For a system of superconducting qubits, we propose a circuit-QED approximate realization of such an engineered thermal bath consisting of driven lossy resonators. We consider an artificial thermal bath as a simulator for many-body physics or a controllable temperature knob for a hybrid quantum-thermal annealer.
13
Okt
2015
Discrete photon statistics from continuous microwave measurements
Photocount statistics are an important tool for the characterization of electromagnetic fields, especially for fields with an irrelevant phase. In the microwave domain, continuous rather
than discrete measurements are the norm. Using a novel approach, we recover discrete photon statistics from the cumulants of a continuous distribution of field quadrature measurements. The use of cumulants allows the separation between the signal of interest and experimental noise. Using a parametric amplifier as the first stage of the amplification chain, we extract useful data from up to the sixth cumulant of the continuous distribution of a coherent field, hence recovering up to the third moment of the discrete statistics associated with a signal with much less than one average photon.
12
Okt
2015
Multi-Qubit Joint Measurements in Circuit QED: Stochastic Master Equation Analysis
We derive a family of stochastic master equations describing homodyne measurement of multi-qubit diagonal observables in circuit quantum electrodynamics. Our approach replaces the polaron-like
transformation of previous work, which required a lengthy calculation for the physically interesting case of three qubits and two resonator modes. The technique introduced here makes this calculation straightforward and manifestly correct. Using this technique, we are able to show that registers larger than one qubit evolve under a non-Markovian master equation. We perform numerical simulations of the three-qubit, two-mode case from previous work, obtaining an average post-measurement state fidelity near 94%.
Broken selection rule in the quantum Rabi model
We report the spectroscopic observation of a resonant transition that breaks a selection rule in the quantum Rabi model, implemented using an LC resonator and a superconducting qubit.
The eigenstates of the system consist of a superposition of bare qubit-oscillator states with a relative sign. In the limit of low qubit-oscillator coupling strength, the matrix element between excited eigenstates of different sign is very small in presence of an oscillator drive, establishing a sign-preserving selection rule. Here, our qubit-resonator system operates in the ultrastrong coupling regime, where the coupling strength is 10% of the resonator frequency, allowing sign-changing transitions to be activated and, therefore, detected. This work shows that sign-changing transitions are an unambiguous, distinctive signature of systems operating in the ultrastrong coupling regime of the quantum Rabi model. These results pave the way to further studies of sign-preserving selection rules in multiqubit and multiphoton models.
11
Okt
2015
Broadband parametric amplification with impedance engineering: Beyond the gain-bandwidth product
We present an impedance engineered Josephson parametric amplifier capable of providing bandwidth beyond the traditional gain-bandwidth product. We achieve this by introducing a positive
linear slope in the imaginary component of the input impedance seen by the Josephson oscillator using a λ/2 transformer. Our theoretical model predicts an extremely flat gain profile with a bandwidth enhancement proportional to the square root of amplitude gain. We experimentally demonstrate a nearly flat 20 dB gain over a 640 MHz band, along with a mean 1-dB compression point of -110 dBm and near quantum-limited noise. The results are in good agreement with our theoretical model.
Methodology for bus layout for topological quantum error correcting codes
Most quantum computing architectures can be realized as two-dimensional lattices of qubits that interact with each other. We take transmon qubits and transmission line resonators as
promising candidates for qubits and couplers; we use them as basic building elements of a quantum code. We then propose a simple framework to determine the optimal experimental layout to realize quantum codes. We show that this engineering optimization problem can be reduced to the solution of standard binary linear programs. While solving such programs is a NP-hard problem, we propose a way to find scalable optimal architectures that require solving the linear program for a restricted number of qubits and couplers. We apply our methods to two celebrated quantum codes, namely the surface code and the Fibonacci code.
06
Okt
2015
Qubit metrology for building a fault-tolerant quantum computer
Recent progress in quantum information has led to the start of several large national and industrial efforts to build a quantum computer. Researchers are now working to overcome many
scientific and technological challenges. The program’s biggest obstacle, a potential showstopper for the entire effort, is the need for high-fidelity qubit operations in a scalable architecture. This challenge arises from the fundamental fragility of quantum information, which can only be overcome with quantum error correction. In a fault-tolerant quantum computer the qubits and their logic interactions must have errors below a threshold: scaling up with more and more qubits then brings the net error probability down to appropriate levels ~ 10−18 needed for running complex algorithms. Reducing error requires solving problems in physics, control, materials and fabrication, which differ for every implementation. I explain here the common key driver for continued improvement – the metrology of qubit errors.
02
Okt
2015
Generation and efficient measurement of single photons from fixed frequency superconducting qubits
We demonstrate and evaluate an on-demand source of single itinerant microwave photons. Photons are generated using a highly coherent, fixed-frequency qubit-cavity system, and a protocol
where the microwave control field is far detuned from the photon emission frequency. By using a Josephson parametric amplifier (JPA), we perform efficient single-quadrature detection of the state emerging from the cavity. We characterize the imperfections of the photon generation and detection, including detection inefficiency and state infidelity caused by measurement backaction over a range of JPA gains from 17 to 33 dB. We observe that both detection efficiency and undesirable backaction increase with JPA gain. We find that the density matrix has its maximum single photon component ρ11=0.36±0.01 at 29 dB JPA gain. At this gain, backaction of the JPA creates cavity photon number fluctuations that we model as a thermal distribution with an average photon number n¯=0.041±0.003.