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
08
Jan
2013
High-fidelity CZ gate for resonator-based superconducting quantum computers
. This architecture
consists of superconducting"]qubits capacitively coupled both to individual
memory resonators as well as a common bus. In this work we study a natural
primitive entangling gate for this and related resonator-based architectures,
which consists of a CZ operation between a qubit and the bus. The CZ gate is
implemented with the aid of the non-computational qubit |2> state [F. W.
Strauch et al., Phys. Rev. Lett. 91, 167005 (2003)]. Assuming phase or transmon
qubits with 300 MHz anharmonicity, we show that by using only low frequency
qubit-bias control it is possible to implement the qubit-bus CZ gate with 99.9%
(99.99%) fidelity in about 17ns (23ns) with a realistic two-parameter pulse
profile, plus two auxiliary z rotations. The fidelity measure we refer to here
is a state-averaged intrinsic process fidelity, which does not include any
effects of noise or decoherence. These results apply to a multi-qubit device
that includes strongly coupled memory resonators. We investigate the
performance of the qubit-bus CZ gate as a function of qubit anharmonicity,
indentify the dominant intrinsic error mechanism and derive an associated
fidelity estimator, quantify the pulse shape sensitivity and precision
requirements, simulate qubit-qubit CZ gates that are mediated by the bus
resonator, and also attempt a global optimization of system parameters
including resonator frequencies and couplings. Our results are relevant for a
wide range of superconducting hardware designs that incorporate resonators and
suggest that it should be possible to demonstrate a 99.9% CZ gate with existing
transmon qubits, which would constitute an important step towards the
development of an error-corrected superconducting quantum computer.
Quantum memory for microwave photons in an inhomogeneously broadened spin ensemble
We propose a multi-mode quantum memory protocol able to store the quantum
state of the field in a microwave resonator into an ensemble of electronic
spins. The stored information is
protected against inhomogeneous broadening of
the spin ensemble by spin-echo techniques resulting in memory times orders of
magnitude longer than previously achieved. By calculating the evolution of the
first and second moments of the spin-cavity system variables for realistic
experimental parameters, we show that a memory based on NV center spins in
diamond can store a qubit encoded on the |0> and |1> Fock states of the field
with 80% fidelity.
Quantum memory using a hybrid circuit with flux qubits and NV centers
We propose how to realize high-fidelity quantum storage using a hybrid
quantum architecture including two coupled flux qubits and a nitrogen-vacancy
center ensemble (NVE). One of the
flux qubits is considered as the quantum
computing processor and the NVE serves as the quantum memory. By separating the
computing and memory units, the influence of the quantum computing process on
the quantum memory can be effectively eliminated, and hence the quantum storage
of an arbitrary quantum state of the computing qubit could be achieved with
high fidelity. Furthermore the present proposal is robust with respect to
fluctuations of the system parameters, and it is experimentally feasibile with
currently available technology.
06
Jan
2013
Discrete time quantum walk with nitrogen-vacancy centers in diamond coupled to a superconducting flux qubit
We propose a quantum-electrodynamics scheme for implementing the
discrete-time, coined quantum walk with the walker corresponding to the phase
degree of freedom for a quasi-magnon field
realized in an ensemble of
nitrogen-vacancy centres in diamond. The coin is realized as a superconducting
flux qubit. Our scheme improves on an existing proposal for implementing
quantum walks in cavity quantum electrodynamics by removing the cumbersome
requirement of varying drive-pulse durations according to mean quasiparticle
number. Our improvement is relevant to all indirect-coin-flip cavity
quantum-electrodynamics realizations of quantum walks. Our numerical analysis
shows that this scheme can realize a discrete quantum walk under realistic
conditions.
03
Jan
2013
First-order sideband transitions with flux-driven asymmetric transmon qubits
We demonstrate rapid, first-order sideband transitions between a
superconducting resonator and a frequency-modulated transmon qubit. The qubit
contains a substantial asymmetry between
its Josephson junctions leading to a
linear portion of the energy band near the resonator frequency. The sideband
transitions are driven with a magnetic flux signal of a few hundred MHz coupled
to the qubit. This modulates the qubit splitting at a frequency near the
detuning between the dressed qubit and resonator frequencies, leading to rates
up to 85 MHz for exchanging quanta between the qubit and resonator.
Feedback-controlled adiabatic quantum computation
We propose a simple feedback-control scheme for adiabatic quantum computation
with superconducting flux qubits. The proposed method makes use of existing
on-chip hardware to monitor
the ground-state curvature, which is then used to
control the computation speed to maximize the success probability. We show that
this scheme can provide a polynomial speed-up in performance and that it is
possible to choose a suitable set of feedback-control parameters for an
arbitrary problem Hamiltonian.
26
Dez
2012
A circuit QED architecture with current-biased flux qubits
We theoretically study a circuit quantum electrodynamics (QED) architecture
with current-biased flux qubits. The qubit is coupled to the transmission line
resonator by a bias current
originating from the current mode of the resonator.
Ultrastrong coupling regime can be obtained by varying the capacitance between
the qubit and the resonator. We propose a scalable design for the circuit QED
with current-biased flux qubits, where the dc-SQUIDs take the role of switching
the qubit-resonator coupling. An exact calculation on two-qubit coupling
strength in the scalable design shows the transition from ferromagnetic to
antiferromagnetic xy-type interaction.
17
Dez
2012
Stabilizer quantum error correction toolbox for superconducting qubits
We present a general protocol for stabilizer measurements and pumping in a
system of N superconducting qubits. We assume always-on, equal dispersive
couplings to a single mode of a
high-Q microwave resonator in the ultra-strong
dispersive limit where the dispersive shifts largely exceed the spectral
linewidth. In this limit, we show how to map the two eigenvalues of an
arbitrary weight M < N Pauli operator, onto two quasi-orthogonal coherent
states of the cavity. Together with a fast cavity readout, this enables the
efficient measurement of stabilizer operators.
14
Dez
2012
Circuit QED bright source for chiral entangled light based on dissipation
Based on a circuit QED qubit-cavity array a source of two-mode entangled
microwave radiation is designed. Our scheme is rooted in the combination of
external driving, collective phenomena
and dissipation. On top of that the
reflexion symmetry is broken via external driving permitting the appearance of
chiral emission. Our findings go beyond the applications and are relevant for
fundamental physics, since we show how to implement quantum lattice models
exhibiting criticality driven by dissipation.
13
Dez
2012
Quantum information processing using quasiclassical electromagnetic interactions between qubits and electrical resonators
Electrical resonators are widely used in quantum information processing with
any qubits that are manipulated via electromagnetic interactions. In nearly all
examples to date they are
engineered to interact with qubits via real or
virtual exchange of (typically microwave) photons, and the resonator must
therefore have both a high quality factor and strong quantum fluctuations,
corresponding to the strong-coupling limit of cavity QED. Although great
strides in the control of quantum information have been made using this
so-called „circuit QED“ architecture, it also comes with some characteristic
limitations. In this paper, we discuss a new paradigm for coupling qubits
electromagnetically via resonators, in which the qubits do not exchange photons
with the resonator, but instead where the qubits exert quasi-classical,
effective „forces“ on it. We show how this type of interaction is similar to
that induced between the internal state of a trapped atomic ion and its
center-of-mass motion by the photon recoil momentum, and that the resulting
multiqubit entangling operations are insensitive textit{both to the state of
the resonator and to its quality factor}. The method we describe is potentially
applicable to a variety of qubit modalities, including superconducting and
semiconducting solid-state qubits, trapped molecular ions, and possibly even
electron spins in solids.