56 research outputs found
The Power of Unentanglement
The class QMA(k). introduced by Kobayashi et al., consists of all languages that can be verified using k unentangled quantum proofs. Many of the simplest questions about this class have remained embarrassingly open: for example, can we give any evidence that k quantum proofs are more powerful than one? Does QMA(k) = QMA(2) for k ≥ 2? Can QMA(k) protocols be amplified to exponentially small error?
In this paper, we make progress on all of the above questions.
* We give a protocol by which a verifier can be convinced that a 3SAT formula of size m is satisfiable, with constant soundness, given Õ (√m) unentangled quantum witnesses with O(log m) qubits each. Our protocol relies on the existence of very short PCPs.
* We show that assuming a weak version of the Additivity Conjecture from quantum information theory, any QMA(2) protocol can be amplified to exponentially small error, and QMA(k) = QMA(2) for all k ≥ 2.
* We prove the nonexistence of "perfect disentanglers" for simulating multiple Merlins with one
Critical dislocation speed in helium-4 crystals
Our experiments show that in He crystals, the binding of He
impurities to dislocations does not necessarily imply their pinning. Indeed, in
these crystals, there are two different regimes of the motion of dislocations
when impurities bind to them. At lowdriving strain and frequency
, where the dislocation speed is less than a critical value (45
m/s), dislocations and impurities apparently move together. Impurities
really pin the dislocations only at higher values of . The critical
speed separating the two regimes is two orders of magnitude smaller than the
average speed of free He impurities in the bulk crystal lattice.We obtained
this result by studying the dissipation of dislocation motion as a function of
the frequency and amplitude of a driving strain applied to a crystal at low
temperature. Our results solve an apparent contradiction between some
experiments, which showed a frequency-dependent transition temperature from a
soft to a stiff state, and other experiments or models where this temperature
was assumed to be independent of frequency. The impurity pinning mechanism for
dislocations appears to be more complicated than previously assumed
Strong gate coupling of high-Q nanomechanical resonators
The detection of mechanical vibrations near the quantum limit is a formidable
challenge since the displacement becomes vanishingly small when the number of
phonon quanta tends towards zero. An interesting setup for on-chip
nanomechanical resonators is that of coupling them to electrical microwave
cavities for detection and manipulation. Here we show how to achieve a large
cavity coupling energy of up to (2 \pi) 1 MHz/nm for metallic beam resonators
at tens of MHz. We used focused ion beam (FIB) cutting to produce uniform slits
down to 10 nm, separating patterned resonators from their gate electrodes, in
suspended aluminum films. We measured the thermomechanical vibrations down to a
temperature of 25 mK, and we obtained a low number of about twenty phonons at
the equilibrium bath temperature. The mechanical properties of Al were
excellent after FIB cutting and we recorded a quality factor of Q ~ 3 x 10^5
for a 67 MHz resonator at a temperature of 25 mK. Between 0.2K and 2K we find
that the dissipation is linearly proportional to the temperature.Comment: 6 page
Classical decoherence in a nanomechanical resonator
SI not providedInternational audienceDecoherence is an essential mechanism that defines the boundary between classical and quantum behaviours, while imposing technological bounds for quantum devices. Little is known about quantum coherence of mechanical systems, as opposed to electromagnetic degrees of freedom. But decoherence can also be thought of in a purely classical context, as the loss of phase coherence in the classical phase space. Indeed the bridge between quantum and classical physics is under intense investigation, using classical nanomechanical analogues of quantum phenomena. In the present work, by separating pure dephasing from dissipation, we quantitatively model the classical decoherence of a mechanical resonator: through the experimental control of frequency fluctuations, we engineer artificial dephasing. We report on the methods available to define pure dephasing in these systems, which are prerequisite in the understanding of decoherence processes in mechanical devices, both classical and quantum
Fully suspended nano-beams for quantum fluids
Non-invasive probes are keystones of fundamental research. Their size, and
maneuverability (in terms of e.g. speed, dissipated power) define their
applicability range for a specific use. As such, solid state physics possesses
e.g. Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM), or
Scanning SQUID Microscopy. In comparison, quantum fluids (superfluid He,
He) are still lacking probes able to sense them (in a fully controllable
manner) down to their smallest relevant lengthscales, namely the coherence
length . In this work we report on the fabrication and cryogenic
characterization of fully suspended (hanging over an open window, with no
substrate underneath) SiN nano-beams, of width down to 50 nm and
quality factor up to . As a benchmark experiment we used them to
investigate the Knudsen boundary layer of a rarefied gas: He at very low
pressures. The absence of the rarefaction effect due to the nearby chip surface
discussed in Gazizulin et al. [1] is attested, while we report on the effect of
the probe size itself
The thermal conductivity of silicon nitride membranes is not sensitive to stress
We have measured the thermal properties of suspended membranes from 10 K to
300 K for two amplitudes of internal stress (about 0.1 GPa and 1 GPa) and for
two different thicknesses (50 nm and 100 nm). The use of the original 3 \omega
-Volklein method has allowed the extraction of both the specific heat and the
thermal conductivity of each SiN membrane over a wide temperature range. The
mechanical properties of the same substrates have been measured at helium
temperatures using nanomechanical techniques. Our measurements show that the
thermal transport in freestanding SiN membranes is not affected by the presence
of internal stress. Consistently, mechanical dissipation is also unaffected
even though Qs increase with increasing tensile stress. We thus demonstrate
that the theory developed by Wu and Yu [Phys. Rev. B 84, 174109 (2011)] does
not apply to this amorphous material in this stress range. On the other hand,
our results can be viewed as a natural consequence of the "dissipation
dilution" argument [Y. L. Huang and P. R. Saulson, Rev. Sci. Instrum. 69, 544
(1998)] which has been introduced in the context of mechanical damping.Comment: 15 pages, 6 figures. Submitted to PR
Nano-beam clamping revisited
Within recent years, the field of nano-mechanics has diversified in a variety
of applications, ranging from quantum information processing to biological
molecules recognition. Among the diversity of devices produced these days, the
simplest (but versatile) element remains the doubly-clamped beam: it can store
very large tensile stresses (producing high resonance frequencies and
quality factors ), is interfaceable with electric setups (by means of
conductive layers), and can be produced easily in clean rooms (with scalable
designs including multiplexing). Besides, its mechanical properties are the
simplest to describe. Resonance frequencies and s are being modeled, with as
specific achievement the ultra-high quality resonances based on ``soft
clamping'' and ``phonon shields''. Here, we demonstrate that the fabrication
undercut of the clamping regions of basic nano-beams produces a ``natural soft
clamping'', given for free. We present the analytic theory that enables to fit
experimental data, which can be used for design.Comment: With Supplementary Informatio
Progress toward detection of individual TLS in nanomechanical resonators
The low temperature properties of amorphous solids are usually explained in
terms of atomic-scale tunneling two level systems (TLS). For almost 20 years,
individual TLS have been probed in insulating layers of superconducting quantum
circuits. Detecting individual TLS in mechanical systems has been proposed but
not definitively demonstrated. We describe an optomechanical system that is
appropriate for this goal and describe our progress toward achieving it. In
particular, we show that the expected coupling between the mechanical mode and
a resonant TLS is strong enough for high visibility of a TLS given the
linewidth of the mechanical mode. Furthermore, the electronic noise level of
our measurement system is low enough and the anomalous force noise observed in
other nanomechanical devices is absent
Measuring frequency fluctuations in nonlinear nanomechanical resonators
Advances in nanomechanics within recent years have demonstrated an always
expanding range of devices, from top-down structures to appealing bottom-up
MoS and graphene membranes, used for both sensing and component-oriented
applications. One of the main concerns in all of these devices is frequency
noise, which ultimately limits their applicability. This issue has attracted a
lot of attention recently, and the origin of this noise remains elusive up to
date. In this Letter we present a very simple technique to measure frequency
noise in nonlinear mechanical devices, based on the presence of bistability. It
is illustrated on silicon-nitride high-stress doubly-clamped beams, in a
cryogenic environment. We report on the same dependence of the frequency
noise power spectra as reported in the literature. But we also find unexpected
{\it damping fluctuations}, amplified in the vicinity of the bifurcation
points; this effect is clearly distinct from already reported nonlinear
dephasing, and poses a fundamental limit on the measurement of bifurcation
frequencies. The technique is further applied to the measurement of frequency
noise as a function of mode number, within the same device. The relative
frequency noise for the fundamental flexure lies in the range
ppm (consistent with literature for cryogenic MHz devices), and
decreases with mode number in the range studied. The technique can be applied
to {\it any types} of nano-mechanical structures, enabling progresses towards
the understanding of intrinsic sources of noise in these devices.Comment: Published 7 may 201
Very low resistance Al/Cu joints for use at cryogenic temperatures
We present two different techniques for achieving low resistance (20 n) contacts between copper and aluminium at cryogenic temperatures. The
best method is based on gold plating of the surfaces in an e-beam evaporator
immediately after Ar plasma etching in the same apparatus, yielding resistances
as low as 3 n that are stable over time. The second approach
involves inserting indium in the Al/Cu joint. For both methods, we believe key
elements are surface polishing, total removal of the aluminum oxide surface
layer, and temporary application of large (typ. 11 kN) compression forces. We
believe the values for gold plated contacts are the lowest ever reported for a
Cu/Al joint of a few . This technology could simplify the
construction of thermal links for advanced cryogenics applications, in
particular that of extremely low resistance heat switches for nuclear
demagnetization refrigerators.Comment: Accepted by Journal of Low Temperature Physic
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