258 research outputs found
Efficient quantum gates for individual nuclear spin qubits by indirect control
Hybrid quantum registers, such as electron-nuclear spin systems, have emerged
as promising hardware for implementing quantum information and computing
protocols in scalable systems. Nevertheless, the coherent control of such
systems still faces challenges. Particularly, the lower gyromagnetic ratios of
the nuclear spins cause them to respond slowly to control fields, resulting in
gate times that are generally longer than the coherence time of the electron
spin. Here, we demonstrate a scheme for circumventing this problem by indirect
control: We apply a small number of short pulses only to the electron spin and
let the full system undergo free evolution under the hyperfine coupling between
the pulses. Using this scheme, we realize robust quantum gates in an
electron-nuclear spin system, including a Hadamard gate on the nuclear spin and
a controlled-NOT gate with the nuclear spin as the target qubit. The durations
of these gates are shorter than the electron spin coherence time, and thus
additional operations to extend the system coherence time are not needed. Our
demonstration serves as a proof of concept for achieving efficient coherent
control of electron-nuclear spin systems, such as NV centers in diamond. Our
scheme is still applicable when the nuclear spins are only weakly coupled to
the electron spin.Comment: Supplementary material added; Accepted for publication in PR
Iterative quantum state transfer along a chain of nuclear spin qubits
Transferring quantum information between two qubits is a basic requirement
for many applications in quantum communication and quantum information
processing. In the iterative quantum state transfer (IQST) proposed by D.
Burgarth et al. [Phys. Rev. A 75, 062327 (2007)], this is achieved by a static
spin chain and a sequence of gate operations applied only to the receiving end
of the chain. The only requirement on the spin chain is that it transfers a
finite part of the input amplitude to the end of the chain, where the gate
operations accumulate the information. For an appropriate sequence of
evolutions and gate operations, the fidelity of the transfer can asymptotically
approach unity. We demonstrate the principle of operation of this transfer
scheme by implementing it in a nuclear magnetic resonance quantum information
processor.Comment: Version for submission. Comments are welcom
Effect of system level structure and spectral distribution of the environment on the decoherence rate
Minimizing the effect of decoherence on a quantum register must be a central
part of any strategy to realize scalable quantum information processing. Apart
from the strength of the coupling to the environment, the decoherence rate is
determined by the the system level structure and by the spectral composition of
the noise trace that the environment generates. Here, we discuss a relatively
simple model that allows us to study these different effects quantitatively in
detail. We evaluate the effect that the perturbation has on an NMR system while
it performs a Grover search algorithm.Comment: Generalizations are added. Comments are welcom
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