40 research outputs found
Tunneling out of metastable vacuum in a system consisting of two capacitively coupled phase qubits
Using a powerful combination of Coleman's instanton technique and the method
of Banks and Bender, the exponential factor for the zero temperature rate of
tunneling out of metastable vacuum in a system of two identical capacitively
coupled phase qubits is calculated in closed form to second order in asymmetry
parameter for a special case of intermediate coupling C=C_J/2.Comment: 10 pages, 5 figures (select PostScript to download Fig. 1). Corrected
version, to appear in PR
Controlled-NOT logic gate for phase qubits based on conditional spectroscopy
A controlled-NOT logic gate based on conditional spectroscopy has been
demonstrated recently for a pair of superconducting flux qubits [Plantenberg et
al., Nature 447, 836 (2007)]. Here we study the fidelity of this type of gate
applied to a phase qubit coupled to a resonator (or a pair of capacitively
coupled phase qubits). Our results show that an intrinsic fidelity of more than
99% is achievable in 45ns.Comment: 5 pages, 5 figures, To appear in Quantum Inf. Pro
Cliffordons
At higher energies the present complex quantum theory with its unitary group
might expand into a real quantum theory with an orthogonal group, broken by an
approximate operator at lower energies. Implementing this possibility
requires a real quantum double-valued statistics. A Clifford statistics,
representing a swap (12) by a difference of Clifford units,
is uniquely appropriate. Unlike the Maxwell-Boltzmann, Fermi-Dirac,
Bose-Einstein, and para- statistics, which are tensorial and single-valued, and
unlike anyons, which are confined to two dimensions, Clifford statistics are
multivalued and work for any dimensionality. Nayak and Wilczek proposed a
Clifford statistics for the fractional quantum Hall effect. We apply them to
toy quanta here. A complex-Clifford example has the energy spectrum of a system
of spin-1/2 particles in an external magnetic field. This supports the proposal
that the double-valued rotations --- spin --- seen at current energies might
arise from double-valued permutations --- swap --- to be seen at higher
energies. Another toy with real Clifford statistics illustrates how an
effective imaginary unit can arise naturally within a real quantum theory.Comment: 15 pages, no figures; original title ("Clifford statistics") changed;
to appear in J. Math. Phys., 42, 2001. Key words: Clifford statistics,
cliffordons, double-valued representations of permutation groups, spin, swap,
imaginary unit , applications to quantum space-time and the Standard
Model. Some of these results were presented at the American Physical Society
Centennial Meeting, Atlanta, March 25, 199
Resonator/zero-Qubit architecture for superconducting qubits
We analyze the performance of the Resonator/zero-Qubit (RezQu) architecture
in which the qubits are complemented with memory resonators and coupled via a
resonator bus. Separating the stored information from the rest of the
processing circuit by at least two coupling steps and the zero qubit state
results in a significant increase of the ON/OFF ratio and the reduction of the
idling error. Assuming no decoherence, we calculate such idling error, as well
as the errors for the MOVE operation and tunneling measurement, and show that
the RezQu architecture can provide high fidelity performance required for
medium-scale quantum information processing.Comment: 11 pages, 5 figure
Quantum logic with weakly coupled qubits
There are well-known protocols for performing CNOT quantum logic with qubits
coupled by particular high-symmetry (Ising or Heisenberg) interactions.
However, many architectures being considered for quantum computation involve
qubits or qubits and resonators coupled by more complicated and less symmetric
interactions. Here we consider a widely applicable model of weakly but
otherwise arbitrarily coupled two-level systems, and use quantum gate design
techniques to derive a simple and intuitive CNOT construction. Useful
variations and extensions of the solution are given for common special cases.Comment: 4 pages, Revte
Greenberger-Horne-Zeilinger state protocols for fully connected qubit networks
We generalize the recently proposed Greenberger-Horne-Zeilinger (GHZ)
tripartite protocol [A. Galiautdinov, J. M. Martinis, Phys. Rev. A 78,
010305(R) (2008)] to fully connected networks of weakly coupled qubits
interacting by way of anisotropic Heisenberg exchange g(XX+YY)+g1*ZZ. Our model
adopted here differs from the more familiar Ising-Heisenberg chain in that here
every qubit interacts with every other qubit in the circuit. The assumption of
identical couplings on all qubit pairs allows an elegant proof of the protocol
for arbitrary N. In order to further make contact with experiment, we study
fidelity degradation due to coupling imperfections by numerically simulating
the N=3 and N=4 cases. Our simulations indicate that the best fidelity at
unequal couplings is achieved when (a) the system is initially prepared in the
uniform superposition state (similarly to how it is done in the ideal case),
and (b) the entangling time and the final rotations on each of the qubits are
appropriately adjusted.Comment: 11 pages, 1 figur
Isolated Eigenvalues of the Ferromagnetic Spin-J XXZ Chain with Kink Boundary Conditions
We investigate the low-lying excited states of the spin J ferromagnetic XXZ
chain with Ising anisotropy Delta and kink boundary conditions. Since the third
component of the total magnetization, M, is conserved, it is meaningful to
study the spectrum for each fixed value of M. We prove that for J>= 3/2 the
lowest excited eigenvalues are separated by a gap from the rest of the
spectrum, uniformly in the length of the chain. In the thermodynamic limit,
this means that there are a positive number of excitations above the ground
state and below the essential spectrum