Quantum-Information Processing Using Trapped Ions:
to develop quantumlogic components and
quantum-information systems based on trapped ions, in support of new atomic
frequency standards and a national program aimed at advancing computation and
communication.
INTENDED OUTCOME AND
BACKGROUND
The intended outcome of this program is the development and
demonstration of prototype logic circuits that can serve as the basis for
quantum computation, quantum measurement, and noise reduction in atomic clocks.
The current program grew out of a NIST competence project to reduce fundamental,
quantum-projection noise in multi-ion atomic clocks. The realization that the
same trapped-ion systems could also be used for quantum information processing
generated substantial outside interest. As a result, the ion program expanded
and its success stimulated the development of other quantum-computation and
quantum-communication programs at NIST. Today, the ion quantum-information
project represents only a portion of a NIST centrally coordinated effort that
now has fairly broad objectives.
There are several strong motivations for this work. From a national perspective,
developing a quantum computer, while an extremely challenging undertaking,
would have a major impact. This arises from the fact that a quantum computer
should be able to perform certain functions exponentially faster than even the
largest array of conventional computers. As examples, a quantum computer could
much more efficiently factor large numbers, a process that is at the heart of
decryption, and it could greatly speed database searches.
The second broad motivation is the expected impact on measurement science. The
ability to produce and manipulate arbitrary states of single atoms and
collections of atoms has clear implications for measurement science and for
atomic clocks in particular. Last year, we demonstrated noise reduction below
the standard quantum limit for two entangled ions, and implied that these
methods can be extended to many ions, providing for increased performance in
atomic clocks. More generally, the ability to engineer quantum states at the
single- and few-atom level has implications for metrology in nanoscale
systems.
Accomplishments
Geometrical-Phase Quantum-Logic Gate
The recent demonstration of a novel, two-qubit, geometrical-phase quantum-logic
gate significantly advances prospects for quantum computation. This new gate
appears to have features that overcome a number of difficulties identified
earlier.
In this device, rather than manipulating the direction of the spin states, we
control the phases of the spin states using optical dipole forces. The greatest
advantage of this change is that the requirements on control of the phase of
the probe laser are dramatically reduced. For gates of the sort we demonstrated
and reported two years ago in Nature, the laser phase had to be kept
constant throughout the full, coherent sequence of operations on the entire set
of qubits involved in the system. For the new gate, the laser phase has to be
well controlled only during a single-gate operation. This is dramatically
easier to do.
Figure 7. David Wineland adjusting one of the systems used for studying
quantum-logic gates. |
The geometrical-phase gate also shares many of the positive attributes of the
earlier gate. It is a one-step gate. All of the qubits involved can be placed
into proper states at the same time. Also, individual-ion addressing is not
required. In fact, the ions are best illuminated simultaneously. This means
that careful laser focusing is not needed. Finally, motional eigenstates are
not required as long as the ions are tightly contained and within the
Lamb-Dicke limit.
The new gate is particularly well suited to application in multiplexed trap
systems, a design approach that will provide for scaling to larger logic
systems. For example, there is no requirement for maintaining equal laser
couplings to each ion as in the previous, two-qubit gate. This means that
sympathetic cooling can be used in more complex systems without introducing
difficulties in maintaining specific laser-beam couplings to the ions. In
addition, in contrast with the practical realization of other gates, the phases
imposed on qubits can be generated with co-propagating Raman laser beams. This
means that the qubit phases will be highly immune from displacement between the
ions and the laser beams. Thus, motions (vibrations) of the trap relative to
the laser beams will cause fewer problems.
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2002" - Table of Contents |
