60 research outputs found
The Nanotechnology R(evolution)
Nanotechnology as a social concept and investment focal point has drawn much
attention. Here we consider the place of nanotechnology in the second great
technological revolution of mankind that began some 200 years ago. The
so-called nanotechnology revolution represents both a continuation of prior
science and technology trends and a re-awakening to the benefits of significant
investment in fundamental research. We consider the role the military might
play in the development of nanotechnology innovations, nanotechnology's context
in the history of technology, and the global competition to lead the next
technological revolution.Comment: Preprint of chapter to appear in Nanoethics: Examining the Societal
Impact of Nanotechnology, Fritz Allhoff, Patrick Lin, James Moor, and John
Weckert, eds., (2007). Visit http://www.tahan.com/charlie/nanosociety/ for
more informatio
Identifying Nanotechnology in Society
Manufacturing materials and systems with components thousands of times
smaller than the width of a human hair promises vast and sometimes unimaginable
advances in technology. Yet the term nanotechnology has formed as much from
people's expectations as from scientific reality. Understanding the creation
and context of this social construction can help us appreciate and guide what
may be a burgeoning revolution. This chapter considers what different groups
are referring to when they say nanotechnology, how this relates to the science
involved, and how the various definitions of this broad field of endeavor might
be improved. The ramifications and implications of these seemingly innocuous
naming choices are also discussed. Although in many respects nanotechnology
serves as cover justification for increased spending in the physical sciences,
at present it is the most hopeful route to solving some of the planet's
greatest problems.Comment: Preprint of article to appear in Advances in Computers, Marvin
Zelkowitz, ed. (2007). Visit http://www.tahan.com/charlie/nanosociety/ for
more informatio
Quantum-limited measurement of spin qubits via curvature coupling to a cavity
We investigate coupling an encoded spin qubit to a microwave resonator via
qubit energy level curvature versus gate voltage. This approach enables quantum
non-demolition readout with strength of tens to hundred MHz all while the qubit
stays at its full sweet-spot to charge noise, with zero dipole moment. A
"dispersive-like" spin readout approach similar to circuit-QED but avoiding the
Purcell effect is proposed. With the addition of gate voltage modulation,
selective longitudinal readout and n-qubit entanglement-by-measurement are
possible.Comment: 5 pages, 1 figur
Relaxation of excited spin, orbital, and valley qubit states in single electron silicon quantum dots
We expand on previous work that treats relaxation physics of low-lying
excited states in ideal, single electron, silicon quantum dots in the context
of quantum computing. These states are of three types: orbital, valley, and
spin. The relaxation times depend sensitively on system parameters such as the
dot size and the external magnetic field. Generally, however, orbital
relaxation times are short in strained silicon (from a tenth of a microsecond
to picoseconds), spin relaxation times are long (microseconds to greater than
seconds), while valley relaxation times are expected to lie in between. The
focus is on relaxation due to emission or absorption of phonons, but for spin
relaxation we also consider competing mechanisms such as charge noise. Where
appropriate, comparison is made to reference systems such as quantum dots in
III-V materials and silicon donor states. The phonon bottleneck effect is shown
to be rather small in the silicon dots of interest. We compare the theoretical
predictions to some recent spin relaxation experiments and comment on the
possible effects of non-ideal dots.Comment: Previously unpublished as well as new results for spin relaxation in
ideal silicon quantum dots. Minor update: fixed Fig
Semiconductor-inspired design principles for superconducting quantum computing
Superconducting circuits offer tremendous design flexibility in the quantum
regime culminating most recently in the demonstration of few qubit systems
supposedly approaching the threshold for fault-tolerant quantum information
processing. Competition in the solid-state comes from semiconductor qubits,
where nature has bestowed some very useful properties which can be utilized for
spin qubit based quantum computing. Here we begin to explore how selective
design principles deduced from spin-based systems could be used to advance
superconducting qubit science. We take an initial step along this path
proposing an encoded qubit approach realizable with state-of-the-art tunable
Josephson junction qubits. Our results show that this design philosophy holds
promise, enables microwave-free control, and offers a pathway to future qubit
designs with new capabilities such as with higher fidelity or, perhaps,
operation at higher temperature. The approach is also especially suited to
qubits based on variable super-semi junctions.Comment: 10 pages, 6 figures, accepted versio
Toward engineered quantum many-body phonon systems
Arrays of coupled phonon cavities each including an impurity qubit in silicon
are considered. We study experimentally feasible architectures that can exhibit
quantum many-body phase transitions of phonons, e.g. Mott insulator and
superfluid states, due to a strong phonon-phonon interaction (which is mediated
by the impurity qubit-cavity phonon coupling). We investigate closed
equilibrium systems as well as driven dissipative non-equilibrium systems at
zero and non-zero temperatures. Our results indicate that quantum many-body
phonon systems are achievable both in on-chip nanomechanical systems in silicon
and distributed Bragg reflector phonon cavity heterostructures in
silicon-germanium. Temperature and driving field are shown to play a critical
role in achieving these phonon superfluid and insulator states, results that
are also applicable to polariton systems. Experimental procedures to detect
these states are also given. Cavity-phoniton systems enable strong
phonon-phonon interactions as well as offering long wavelengths for forming
extended quantum states; they may have some advantage in forming truly quantum
many-body mechanical states as compared to other optomechanical systems.Comment: 7 pages, 5 figure
Superconducting-semiconductor quantum devices: from qubits to particle detectors
Recent improvements in materials growth and fabrication techniques may
finally allow for superconducting semiconductors to realize their potential.
Here we build on a recent proposal to construct superconducting devices such as
wires, Josephson junctions, and qubits inside and out-of single crystal silicon
or germanium. Using atomistic fabrication techniques such as STM hydrogen
lithography, heavily-doped superconducting regions within a single crystal
could be constructed. We describe the characteristic parameters of basic
superconducting elements---a 1D wire and a tunneling Josephson junction---and
estimate the values for boron-doped silicon. The epitaxial, single-crystal
nature of these devices, along with the extreme flexibility in device design
down to the single-atom scale, may enable lower-noise or new types of devices
and physics. We consider applications for such super-silicon devices, showing
that the state-of-the-art transmon qubit and the sought-after phase-slip qubit
can both be realized. The latter qubit leverages the natural high kinetic
inductance of these materials. Building on this, we explore how kinetic
inductance based particle detectors (e.g., photon or phonon) could be realized
with potential application in astronomy or nanomechanics. We discuss super-semi
devices (such as in silicon, germanium, or diamond) which would not require
atomistic fabrication approaches and could be realized today.Comment: 8 pages, 6 figures; (v2) accepted version, to appear in IEEE Journal
of Selected Topics in Quantum Electronic
Theory of barrier vs tilt exchange gate operations in spin-based quantum computing
We present a theory for understanding the exchange interaction between
electron spins in neighboring quantum dots, either by changing the detuning of
the two quantum dots or independently tuning the tunneling barrier between
quantum dots. The Hubbard model and a more realistic confining-potential model
are used to investigate how the tilting and barrier control affect the
effective exchange coupling and thus the gate fidelity in both the detuning and
symmetric regimes. We show that the exchange coupling is less sensitive to the
charge noise through tunnel barrier control (while allowing for exchange
coupling operations on a sweet spot where the exchange interaction has zero
derivative with respect to the detuning). Both GaAs and Si quantum dots are
considered and we compare our results with experimental data showing
qualitative agreements. Our results answer the open question of why barrier
gates are preferable to tilt gates for exchange-base gate operations
Bottom-up superconducting and Josephson junction devices inside a group-IV semiconductor
Superconducting circuits are exceptionally flexible, enabling many different
devices from sensors to quantum computers. Separately, epitaxial semiconductor
devices such as spin qubits in silicon offer more limited device variation but
extraordinary quantum properties for a solid-state system. It might be possible
to merge the two approaches, making single-crystal superconducting devices out
of a semiconductor by utilizing the latest atomistic fabrication techniques.
Here we propose superconducting devices made from precision hole-doped regions
within a silicon (or germanium) single crystal. We analyze the properties of
this superconducting semiconductor and show that practical superconducting
wires, Josephson tunnel junctions or weak links, superconducting quantum
interference devices (SQUIDs), and qubits are feasible. This work motivates the
pursuit of "bottom-up" superconductivity for improved or fundamentally
different technology and physics.Comment: 9 pages, 4 figures; (v2) Fixed math error in estimate of the hole
density and critical temperature for one doped atomic layer (all other
numbers and figures are unchanged); even a single doped layer may be
sufficient to observe superconductivity; (v3) accepted versio
Silicon in the Quantum Limit: Quantum Computing and Decoherence in Silicon Architectures
Semiconductor architectures hold promise for quantum information processing
(QIP) applications due to their large industrial base and perceived scalability
potential. Electron spins in silicon in particular may be an excellent
architecture for QIP and also for spin electronics (spintronics) applications.
While the charge of an electron is easily manipulated by charged gates, the
spin degree of freedom is well isolated from charge fluctuations. Inherently
small spin-orbit coupling and the existence of a spin-zero Si isotope
facilitate long single spin qubit coherence times. Here we consider the
relaxation properties of localized electronic states in silicon due to donors,
quantum wells, and quantum dots, including effects due to phonons and Rashba
spin-orbit coupling. Our analysis is impeded by the complicated, many-valley
band structure of silicon and previously unaddressed physics in silicon quantum
wells. We find that electron spins in silicon and especially strained silicon
have excellent decoherence properties. Where possible we compare with
experiment to test our theories. We go beyond issues of coherence in a quantum
computer to problems of control and measurement. Precisely what makes spin
relaxation so long in semiconductor architectures makes spin measurement so
difficult. To address this, we propose a new scheme for spin readout which has
the added benefit of automatic spin initialization, a vital component of
quantum computing and quantum error correction. Our results represent important
practical milestones on the way to the design and construction of a
silicon-based quantum computer.Comment: 2005 Thesis. Per a request to post, bit out-dated but includes some
relevant calculations (not completely sure if this is the published version
or a late draft
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