16 research outputs found
Observation of the Presuperfluid Regime in a Two-Dimensional Bose Gas
In complementary images of coordinate-space and momentum-space density in a
trapped 2D Bose gas, we observe the emergence of pre-superfluid behavior. As
phase-space density increases toward degenerate values, we observe a
gradual divergence of the compressibility from the value predicted by
a bare-atom model, . grows to 1.7 before
reaches the value for which we observe the sudden emergence of a spike
at in momentum space. Momentum-space images are acquired by means of a 2D
focusing technique. Our data represent the first observation of non-meanfield
physics in the pre-superfluid but degenerate 2D Bose gas.Comment: Replace with the version appeared in PR
High-energy-resolution molecular beams for cold collision studies
Stark deceleration allows for precise control over the velocity of a pulsed
molecular beam and, by the nature of its limited phase-space acceptance,
reduces the energy width of the decelerated packet. We describe an alternate
method of operating a Stark decelerator that further reduces the energy spread
over the standard method of operation. In this alternate mode of operation, we
aggressively decelerate the molecular packet using a high phase angle. This
technique brings the molecular packet to the desired velocity before it reaches
the end of the decelerator; the remaining stages are then used to
longitudinally and transversely guide the packet to the detection/interaction
region. The result of the initial aggressive slowing is a reduction in the
phase-space acceptance of the decelerator and thus a narrowing of the velocity
spread of the molecular packet. In addition to the narrower energy spread, this
method also results in a velocity spread that is nearly independent of the
final velocity. Using the alternate deceleration technique, the energy
resolution of molecular collision measurements can be improved considerably.Comment: 12 pages, 9 figure
Collapse and revival of the monopole mode of a degenerate Bose gas in an isotropic harmonic trap
We study the monopole (breathing) mode of a finite temperature Bose-Einstein condensate in an isotropic harmonic trap recently developed by Lobser et al. [Nat. Phys. 11, 1009 (2015)]. We observe a nonexponential collapse of the amplitude of the condensate oscillation followed by a partial revival. This behavior is identified as being due to beating between two eigenmodes of the system, corresponding to in-phase and out-of-phase oscillations of the condensed and noncondensed fractions of the gas. We perform finite temperature simulations of the system dynamics using the Zaremba-Nikuni-Griffin methodology [J. Low Temp. Phys. 116, 277 (1999)], and find good agreement with the data, thus confirming the two mode description
Quantum Computation of Hydrogen Bond Dynamics and Vibrational Spectra
Calculating the observable properties of chemical systems is often
classically intractable, and is widely viewed as a promising application of
quantum information processing. This is because a full description of chemical
behavior relies upon the complex interplay of quantum-mechanical electrons and
nuclei, demanding an exponential scaling of computational resources with system
size. While considerable progress has been made in mapping electronic-structure
calculations to quantum hardware, these approaches are unsuitable for
describing the quantum dynamics of nuclei, proton- and hydrogen-transfer
processes, or the vibrational spectra of molecules. Here, we use the QSCOUT
ion-trap quantum computer to determine the quantum dynamics and vibrational
properties of a shared proton within a short-strong hydrogen-bonded system. For
a range of initial states, we experimentally drive the ion-trap system to
emulate the quantum trajectory of the shared proton wavepacket as it evolves
along the potential surface generated by the nuclear frameworks and electronic
structure. We then extract the characteristic vibrational frequencies for the
shared proton motion to spectroscopic accuracy and determine all energy
eigenvalues of the system Hamiltonian to > 99.9% fidelity. Our approach offers
a new paradigm for studying the quantum chemical dynamics and vibrational
spectra of molecules, and when combined with quantum algorithms for electronic
structure, opens the possibility to describe the complete behavior of molecules
using exclusively quantum computation techniques.Comment: 10 pages, 4 figure
Error mitigation, optimization, and extrapolation on a trapped ion testbed
Current noisy intermediate-scale quantum (NISQ) trapped-ion devices are
subject to errors around 1% per gate for two-qubit gates. These errors
significantly impact the accuracy of calculations if left unchecked. A form of
error mitigation called Richardson extrapolation can reduce these errors
without incurring a qubit overhead. We demonstrate and optimize this method on
the Quantum Scientific Computing Open User Testbed (QSCOUT) trapped-ion device
to solve an electronic structure problem. We explore different methods for
integrating this error mitigation technique into the Variational Quantum
Eigensolver (VQE) optimization algorithm for calculating the ground state of
the HeH+ molecule at 0.8 Angstrom. We test two methods of scaling noise for
extrapolation: time-stretching the two-qubit gates and inserting two-qubit gate
identity operations into the ansatz circuit. We find the former fails to scale
the noise on our particular hardware. Scaling our noise with global gate
identity insertions and extrapolating only after a variational optimization
routine, we achieve an absolute relative error of 0.363% +- 1.06 compared to
the true ground state energy of HeH+. This corresponds to an absolute error of
0.01 +- 0.02 Hartree; outside chemical accuracy, but greatly improved over our
non error mitigated estimate. We ultimately find that the efficacy of this
error mitigation technique depends on choosing the right implementation for a
given device architecture and sampling budget.Comment: 16 pages, 11 figure
Sample-efficient verification of continuously-parameterized quantum gates for small quantum processors
Most near-term quantum information processing devices will not be capable of implementing quantum error correction and the associated logical quantum gate set. Instead, quantum circuits will be implemented directly using the physical native gate set of the device. These native gates often have a parameterization (e.g., rotation angles) which provide the ability to perform a continuous range of operations. Verification of the correct operation of these gates across the allowable range of parameters is important for gaining confidence in the reliability of these devices. In this work, we demonstrate a procedure for sample-efficient verification of continuously-parameterized quantum gates for small quantum processors of up to approximately 10 qubits. This procedure involves generating random sequences of randomly-parameterized layers of gates chosen from the native gate set of the device, and then stochastically compiling an approximate inverse to this sequence such that executing the full sequence on the device should leave the system near its initial state. We show that fidelity estimates made via this technique have a lower variance than fidelity estimates made via cross-entropy benchmarking. This provides an experimentally-relevant advantage in sample efficiency when estimating the fidelity loss to some desired precision. We describe the experimental realization of this technique using continuously-parameterized quantum gate sets on a trapped-ion quantum processor from Sandia QSCOUT and a superconducting quantum processor from IBM Q, and we demonstrate the sample efficiency advantage of this technique both numerically and experimentally
Characterizing and mitigating coherent errors in a trapped ion quantum processor using hidden inverses
Quantum computing testbeds exhibit high-fidelity quantum control over small
collections of qubits, enabling performance of precise, repeatable operations
followed by measurements. Currently, these noisy intermediate-scale devices can
support a sufficient number of sequential operations prior to decoherence such
that small algorithms can be performed reliably. While the results of these
algorithms are imperfect, these imperfections can help bootstrap quantum
computer testbed development. Demonstrations of these small algorithms over the
past few years, coupled with the idea that imperfect algorithm performance can
be caused by several dominant noise sources in the quantum processor, which can
be measured and calibrated during algorithm execution or in post-processing,
has led to the use of noise mitigation to improve typical computational
results. Conversely, small benchmark algorithms coupled with noise mitigation
can help diagnose the nature of the noise, whether systematic or purely random.
Here, we outline the use of coherent noise mitigation techniques as a
characterization tool in trapped-ion testbeds. We perform model-fitting of the
noisy data to determine the noise source based on realistic physics focused
noise models and demonstrate that systematic noise amplification coupled with
error mitigation schemes provides useful data for noise model deduction.
Further, in order to connect lower level noise model details with application
specific performance of near term algorithms, we experimentally construct the
loss landscape of a variational algorithm under various injected noise sources
coupled with error mitigation techniques. This type of connection enables
application-aware hardware codesign, in which the most important noise sources
in specific applications, like quantum chemistry, become foci of improvement in
subsequent hardware generations.Comment: 9 pages, 5 figure