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
21
Dez
2015
Scalable Quantum Simulation of Molecular Energies
We report the first electronic structure calculation performed on a quantum computer without exponentially costly precompilation. We use a programmable array of superconducting qubits
to compute the energy surface of molecular hydrogen using two distinct quantum algorithms. First, we experimentally execute the unitary coupled cluster method using the variational quantum eigensolver. Our efficient implementation predicts the correct dissociation energy to within chemical accuracy of the numerically exact result. Next, we experimentally demonstrate the canonical quantum algorithm for chemistry, which consists of Trotterization and quantum phase estimation. We compare the experimental performance of these approaches to show clear evidence that the variational quantum eigensolver is robust to certain errors, inspiring hope that quantum simulation of classically intractable molecules may be viable in the near future.
Loops and strings in a superconducting lattice gauge simulator
We propose a quantum simulation of electromagnetism in (2+1) dimensions using an array of superconducting fluxonium devices. The encoding is in the integer (S=1) representation of the
quantum link model formulation of compact U(1) lattice gauge theory. We show how to engineer the Gauss constraint via an ancilla mediated gadget construction and how to tune between the strongly coupled and intermediately coupled regimes. The witnesses to the existence of the predicted confining phase of the model are provided by non-local order parameters from Wilson loops and disorder parameters from ‚t Hooft strings and we show how to measure these operators non-destructively via dispersive coupling of the fluxonium islands to a microwave cavity mode. Evidence for existence of the confined phase in the ground state of the simulation Hamiltonian is found by DMRG calculations on a ladder geometry.
20
Dez
2015
On-chip microwave-to-optical quantum coherent converter based on a superconducting resonator coupled to an electro-optic microresonator
We propose a device architecture capable of direct quantum electro-optical conversion of microwave to optical photons. The hybrid system consists of a planar superconducting microwave
circuit coupled to an integrated whispering-gallery-mode microresonator made from an electro-optical material. We show that electro-optical (vacuum) coupling rates g0 as large as∼2π(10−100) kHz are achievable with currently available technology, due to the small mode volume of the planar microwave resonator. Operating at millikelvin temperatures, such a converter would enable high-efficiency conversion of microwave to optical photons. We analyze the added noise, and show that maximum conversion efficiency is achieved for a multi-photon cooperativity of unity which can be reached with optical power as low as (1)mW.
18
Dez
2015
Demonstration of quantum advantage in machine learning
The main promise of quantum computing is to efficiently solve certain problems that are prohibitively expensive for a classical computer. Most problems with a proven quantum advantage
involve the repeated use of a black box, or oracle, whose structure encodes the solution. One measure of the algorithmic performance is the query complexity, i.e., the scaling of the number of oracle calls needed to find the solution with a given probability. Few-qubit demonstrations of quantum algorithms, such as Deutsch-Jozsa and Grover, have been implemented across diverse physical systems such as nuclear magnetic resonance, trapped ions, optical systems, and superconducting circuits. However, at the small scale, these problems can already be solved classically with a few oracle queries, and the attainable quantum advantage is modest. Here we solve an oracle-based problem, known as learning parity with noise, using a five-qubit superconducting processor. Running classical and quantum algorithms on the same oracle, we observe a large gap in query count in favor of quantum processing. We find that this gap grows by orders of magnitude as a function of the error rates and the problem size. This result demonstrates that, while complex fault-tolerant architectures will be required for universal quantum computing, a quantum advantage already emerges in existing noisy systems
17
Dez
2015
Coherence from vacuum fluctuations
The existence of vacuum fluctuations is one of the most important predictions of modern quantum field theory. In the vacuum state, fluctuations occurring at different frequencies are
uncorrelated. However, if a parameter in the Lagrangian of the field is modulated by an external pump, vacuum fluctuations stimulate spontaneous downconversion processes, creating squeezing between modes symmetric with respect to half of the frequency of the pump. Here we show that by double parametric pumping of a superconducting microwave cavity in the ground state, it is possible to generate another fundamental type of correlation, namely coherence between photons in separate frequency modes that are not directly connected through a single downconversion process. The coherence is tunable by the phases of the pumps and it is established by a quantum fluctuation that takes simultaneously part in creation of two photon pairs. Our analysis indicates that the origin of this vacuum-induced coherence is the absence of „which-way“ information in the frequency space.
Optimal qubit control using single flux quantum pulses
Single flux quantum (SFQ) pulses are a natural candidate for on-chip control of superconducting qubits. We perform single qubit gates at a constant gate time using trains of single
flux quantum pulses with fixed amplitudes. The pulse sequence is optimized by applying genetic algorithms, which decreases the gate error by two orders of magnitude compared to an evenly spaced pulse train. Hereby, we consider leakage transitions into a third energy level as well. Timing jitter of the pulses is taken into account, exploring the robustness of our optimized sequence. This takes us one step further to on-chip qubit controls.
14
Dez
2015
Dynamical polaron ansatz: a theoretical tool for the ultra-strong coupling regime of circuit QED
In this work we develop a semi-analytical variational ansatz to study the properties of few photon excitations interacting with a collection of quantum emitters in regimes that go beyond
the rotating wave approximation. This method can be used to approximate both the static and dynamical properties of a superconducting qubit in an open transmission line, including the spontaneous emission spectrum and the resonances in scattering experiments. The approximations are quantitatively accurate for rather strong couplings, as shown by a direct comparison to Matrix-Product-State numerical methods, and provide also a good qualitative description for stronger couplings well beyond the Markovian regime.
13
Dez
2015
Quantum Zeno effect in the strong measurement regime of circuit quantum electrodynamics
We observe the quantum Zeno effect — where the act of measurement slows the rate of quantum state transitions — in a superconducting qubit using linear circuit quantum electrodynamics
readout and a near-quantum-limited following amplifier. Under simultaneous strong measurement and qubit drive, the qubit undergoes a series of quantum jumps between states. These jumps are visible in the experimental measurement record and are analyzed using maximum likelihood estimation to determine qubit transition rates. The observed rates agree with both analytical predictions and numerical simulations. The analysis methods are suitable for processing general noisy random telegraph signals
07
Dez
2015
What is the Computational Value of Finite Range Tunneling?
Quantum annealing (QA) has been proposed as a quantum enhanced optimization heuristic exploiting tunneling. Here, we demonstrate how finite range tunneling can provide considerable
computational advantage. For a crafted problem designed to have tall and narrow energy barriers separating local minima, the D-Wave 2X quantum annealer achieves significant runtime advantages relative to Simulated Annealing (SA). For instances with 945 variables this results in a time-to-99\%-success-probability that is ∼108 times faster than SA running on a single processor core. We also compared physical QA with Quantum Monte Carlo (QMC), an algorithm that emulates quantum tunneling on classical processors. We observe a substantial constant overhead against physical QA: D-Wave 2X runs up to ∼108 times faster than an optimized implementation of QMC on a single core. To investigate whether finite range tunneling will also confer an advantage for problems of practical interest, we conduct numerical studies on binary optimization problems that cannot yet be represented on quantum hardware. For random instances of the number partitioning problem, we find numerically that QMC, as well as other algorithms designed to simulate QA, scale better than SA and better than the best known classical algorithms for this problem. We discuss the implications of these findings for the design of next generation quantum annealers.
02
Dez
2015
Measurement-Induced Long-Distance Entanglement of Superconducting Qubits using Optomechanical Transducers
While superconducting systems provide a promising platform for quantum computing, their networking poses a considerable challenge as they cannot be interfaced directly to light–the
natural carrier for transmission of quantum signals through channels at room temperature. Here, we show that remote superconducting qubits can be prepared in entangled states by coupling them to mechanical oscillators whose positions are monitored with optical fields. Continuous homodyne detection of light provides information on the total spin of the two qubits such that entangled qubit states can be post-selected. Entanglement generation is possible without ground state cooling of the mechanical oscillators for systems with an optomechanical cooperativity moderately larger than unity; in addition, our setup tolerates a substantial loss of photons in transmission. The approach is scalable to generation of multipartite entanglement and represents a crucial step towards quantum networks with nodes using superconducting circuits.