267 research outputs found
Concatenating dynamical decoupling with decoherence-free subspaces for quantum computation
A scheme to implement a quantum computer subjected to decoherence and
governed by an untunable qubit-qubit interaction is presented. By concatenating
dynamical decoupling through bang-bang (BB) pulse with decoherence-free
subspaces (DFSs) encoding, we protect the quantum computer from
environment-induced decoherence that results in quantum information dissipating
into the environment. For the inherent qubit-qubit interaction that is
untunable in the quantum system, BB control plus DFSs encoding will eliminate
its undesired effect which spoils quantum information in qubits. We show how
this quantum system can be used to implement universal quantum computation.Comment: 6 pages,2 figures, 1 tabl
Error Avoiding Quantum Codes and Dynamical Stabilization of Grover's Algorithm
An error avoiding quantum code is presented which is capable of stabilizing
Grover's quantum search algorithm against a particular class of coherent
errors. This error avoiding code consists of states only which are factorizable
in the computational basis. Furthermore, its redundancy is smaller than the one
which is achievable with a general error correcting quantum code saturating the
quantum Hamming bound. The fact that this code consists of factorizable states
only may offer advantages for the implementation of quantum gates in the error
free subspace
Preparation of GHZ states via Grover's quantum searching algorithm
In this paper we propose an approach to prepare GHZ states of an arbitrary
multi-particle system in terms of Grover's fast quantum searching algorithm.
This approach can be regarded as an extension of the Grover's algorithm to
find one or more items in an unsorted database.Comment: 9 pages, Email address: [email protected]
Information Erasure and Recover in Quantum Memory
We show that information in quantum memory can be erased and recovered
perfectly if it is necessary. That the final states of environment are
completely determined by the initial states of the system allows that an easure
operation can be realized by a swap operation between system and an ancilla.
Therefore, the erased information can be recoverd. When there is an
irreversible process, e.g. an irreversible operation or a decoherence process,
in the erasure process, the information would be erased perpetually. We present
that quantum erasure will also give heat dissipation in environment. And a
classical limit of quantum erasure is given which coincides with Landauer's
erasure principle.Comment: PACS: 0365.Bz. 03.67.Hk;3page
A proposal for implementing an n-qubit controlled-rotation gate with three-level superconducting qubit systems in cavity QED
We present a way for implementing an n-qubit controlled-rotation gate with
three-level superconducting qubit systems in cavity QED. The two logical states
of a qubit are represented by the two lowest levels of each system while a
higher-energy level is used for the gate implementation. The method operates
essentially by preparing a state conditioned on the states of the control
qubits, creating a single photon in the cavity mode, and then performing an
arbitrary rotation on the states of the target qubit with assistance of the
cavity photon. It is interesting to note that the basic operational steps for
implementing the proposed gate do not increase with the number of qubits,
and the gate operation time decreases as the number of qubits increases. This
proposal is quite general, which can be applied to various types of
superconducting devices in a cavity or coupled to a resonator.Comment: Six figures, accepted by Journal of Physics: Condensed Matte
Quantum Measurements and Gates by Code Deformation
The usual scenario in fault tolerant quantum computation involves certain
amount of qubits encoded in each code block, transversal operations between
them and destructive measurements of ancillary code blocks. We introduce a new
approach in which a single code layer is used for the entire computation, in
particular a surface code. Qubits can be created, manipulated and
non-destructively measured by code deformations that amount to `cut and paste'
operations in the surface. All the interactions between qubits remain purely
local in a two-dimensional setting.Comment: Revtex4, 6 figure
Internal Consistency of Fault-Tolerant Quantum Error Correction in Light of Rigorous Derivations of the Quantum Markovian Limit
We critically examine the internal consistency of a set of minimal
assumptions entering the theory of fault-tolerant quantum error correction for
Markovian noise. These assumptions are: fast gates, a constant supply of fresh
and cold ancillas, and a Markovian bath. We point out that these assumptions
may not be mutually consistent in light of rigorous formulations of the
Markovian approximation. Namely, Markovian dynamics requires either the
singular coupling limit (high temperature), or the weak coupling limit (weak
system-bath interaction). The former is incompatible with the assumption of a
constant and fresh supply of cold ancillas, while the latter is inconsistent
with fast gates. We discuss ways to resolve these inconsistencies. As part of
our discussion we derive, in the weak coupling limit, a new master equation for
a system subject to periodic driving.Comment: 19 pages. v2: Significantly expanded version. New title. Includes a
debate section in response to comments on the previous version, many of which
appeared here http://dabacon.org/pontiff/?p=959 and here
http://dabacon.org/pontiff/?p=1028. Contains a new derivation of the
Markovian master equation with periodic drivin
A quantum search for zeros of polynomials
A quantum mechanical search procedure to determine the real zeros of a polynomial is introduced. It is based on the construction of a spin observable whose eigenvalues coincide with the zeros of the polynomial. Subsequent quantum mechanical measurements of the observable output directly the numerical values of the zeros. Performing the measurements is the only computational resource involved
Remote information concentration by GHZ state and by bound entangled state
We compare remote information concentration by a maximally entangled GHZ
state with by an unlockable bound entangled state. We find that the bound
entangled state is as useful as the GHZ state, even do better than the GHZ
state in the context of communication security.Comment: 4 pages,1 figur
A Quantum to Classical Phase Transition in Noisy Quantum Computers
The fundamental problem of the transition from quantum to classical physics
is usually explained by decoherence, and viewed as a gradual process. The study
of entanglement, or quantum correlations, in noisy quantum computers implies
that in some cases the transition from quantum to classical is actually a phase
transition. We define the notion of entanglement length in -dimensional
noisy quantum computers, and show that a phase transition in entanglement
occurs at a critical noise rate, where the entanglement length transforms from
infinite to finite. Above the critical noise rate, macroscopic classical
behavior is expected, whereas below the critical noise rate, subsystems which
are macroscopically distant one from another can be entangled.
The macroscopic classical behavior in the super-critical phase is shown to
hold not only for quantum computers, but for any quantum system composed of
macroscopically many finite state particles, with local interactions and local
decoherence, subjected to some additional conditions.
This phenomenon provides a possible explanation to the emergence of classical
behavior in such systems. A simple formula for an upper bound on the
entanglement length of any such system in the super-critical phase is given,
which can be tested experimentally.Comment: 15 pages. Latex2e plus one figure in eps fil
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