Tag Archives: Michael J. Hartmann

Quantum supremacy

This supremacy, refers to an engineering milestone and a October 23, 2019 news item on ScienceDaily announces the milestone has been reached,

Researchers in UC [University of California] Santa Barbara/Google scientist John Martinis’ group have made good on their claim to quantum supremacy. Using 53 entangled quantum bits (“qubits”), their Sycamore computer has taken on — and solved — a problem considered intractable for classical computers.

An October 23, 2019 UC Santa Barbara news release (also on EurekAlert) by Sonia Fernandez, which originated the news item, delves further into the work,

“A computation that would take 10,000 years on a classical supercomputer took 200 seconds on our quantum computer,” said Brooks Foxen, a graduate student researcher in the Martinis Group. “It is likely that the classical simulation time, currently estimated at 10,000 years, will be reduced by improved classical hardware and algorithms, but, since we are currently 1.5 trillion times faster, we feel comfortable laying claim to this achievement.”

The feat is outlined in a paper in the journal Nature.

The milestone comes after roughly two decades of quantum computing research conducted by Martinis and his group, from the development of a single superconducting qubit to systems including architectures of 72 and, with Sycamore, 54 qubits (one didn’t perform) that take advantage of the both awe-inspiring and bizarre properties of quantum mechanics.

“The algorithm was chosen to emphasize the strengths of the quantum computer by leveraging the natural dynamics of the device,” said Ben Chiaro, another graduate student researcher in the Martinis Group. That is, the researchers wanted to test the computer’s ability to hold and rapidly manipulate a vast amount of complex, unstructured data.

“We basically wanted to produce an entangled state involving all of our qubits as quickly as we can,” Foxen said, “and so we settled on a sequence of operations that produced a complicated superposition state that, when measured, returns bitstring with a probability determined by the specific sequence of operations used to prepare that particular superposition. The exercise, which was to verify that the circuit’s output correspond to the equence used to prepare the state, sampled the quantum circuit a million times in just a few minutes, exploring all possibilities — before the system could lose its quantum coherence.

‘A complex superposition state’

“We performed a fixed set of operations that entangles 53 qubits into a complex superposition state,” Chiaro explained. “This superposition state encodes the probability distribution. For the quantum computer, preparing this superposition state is accomplished by applying a sequence of tens of control pulses to each qubit in a matter of microseconds. We can prepare and then sample from this distribution by measuring the qubits a million times in 200 seconds.”

“For classical computers, it is much more difficult to compute the outcome of these operations because it requires computing the probability of being in any one of the 2^53 possible states, where the 53 comes from the number of qubits — the exponential scaling is why people are interested in quantum computing to begin with,” Foxen said. “This is done by matrix multiplication, which is expensive for classical computers as the matrices become large.”

According to the new paper, the researchers used a method called cross-entropy benchmarking to compare the quantum circuit’s output (a “bitstring”) to its “corresponding ideal probability computed via simulation on a classical computer” to ascertain that the quantum computer was working correctly.

“We made a lot of design choices in the development of our processor that are really advantageous,” said Chiaro. Among these advantages, he said, are the ability to experimentally tune the parameters of the individual qubits as well as their interactions.

While the experiment was chosen as a proof-of-concept for the computer, the research has resulted in a very real and valuable tool: a certified random number generator. Useful in a variety of fields, random numbers can ensure that encrypted keys can’t be guessed, or that a sample from a larger population is truly representative, leading to optimal solutions for complex problems and more robust machine learning applications. The speed with which the quantum circuit can produce its randomized bit string is so great that there is no time to analyze and “cheat” the system.

“Quantum mechanical states do things that go beyond our day-to-day experience and so have the potential to provide capabilities and application that would otherwise be unattainable,” commented Joe Incandela, UC Santa Barbara’s vice chancellor for research. “The team has demonstrated the ability to reliably create and repeatedly sample complicated quantum states involving 53 entangled elements to carry out an exercise that would take millennia to do with a classical supercomputer. This is a major accomplishment. We are at the threshold of a new era of knowledge acquisition.”

Looking ahead

With an achievement like “quantum supremacy,” it’s tempting to think that the UC Santa Barbara/Google researchers will plant their flag and rest easy. But for Foxen, Chiaro, Martinis and the rest of the UCSB/Google AI Quantum group, this is just the beginning.

“It’s kind of a continuous improvement mindset,” Foxen said. “There are always projects in the works.” In the near term, further improvements to these “noisy” qubits may enable the simulation of interesting phenomena in quantum mechanics, such as thermalization, or the vast amount of possibility in the realms of materials and chemistry.

In the long term, however, the scientists are always looking to improve coherence times, or, at the other end, to detect and fix errors, which would take many additional qubits per qubit being checked. These efforts have been running parallel to the design and build of the quantum computer itself, and ensure the researchers have a lot of work before hitting their next milestone.

“It’s been an honor and a pleasure to be associated with this team,” Chiaro said. “It’s a great collection of strong technical contributors with great leadership and the whole team really synergizes well.”

Here’s a link to and a citation for the paper,

Quantum supremacy using a programmable superconducting processor by Frank Arute, Kunal Arya, Ryan Babbush, Dave Bacon, Joseph C. Bardin, Rami Barends, Rupak Biswas, Sergio Boixo, Fernando G. S. L. Brandao, David A. Buell, Brian Burkett, Yu Chen, Zijun Chen, Ben Chiaro, Roberto Collins, William Courtney, Andrew Dunsworth, Edward Farhi, Brooks Foxen, Austin Fowler, Craig Gidney, Marissa Giustina, Rob Graff, Keith Guerin, Steve Habegger, Matthew P. Harrigan, Michael J. Hartmann, Alan Ho, Markus Hoffmann, Trent Huang, Travis S. Humble, Sergei V. Isakov, Evan Jeffrey, Zhang Jiang, Dvir Kafri, Kostyantyn Kechedzhi, Julian Kelly, Paul V. Klimov, Sergey Knysh, Alexander Korotkov, Fedor Kostritsa, David Landhuis, Mike Lindmark, Erik Lucero, Dmitry Lyakh, Salvatore Mandrà, Jarrod R. McClean, Matthew McEwen, Anthony Megrant, Xiao Mi, Kristel Michielsen, Masoud Mohseni, Josh Mutus, Ofer Naaman, Matthew Neeley, Charles Neill, Murphy Yuezhen Niu, Eric Ostby, Andre Petukhov, John C. Platt, Chris Quintana, Eleanor G. Rieffel, Pedram Roushan, Nicholas C. Rubin, Daniel Sank, Kevin J. Satzinger, Vadim Smelyanskiy, Kevin J. Sung, Matthew D. Trevithick, Amit Vainsencher, Benjamin Villalonga, Theodore White, Z. Jamie Yao, Ping Yeh, Adam Zalcman, Hartmut Neven & John M. Martinis. Nature volume 574, pages505–510 (2019) DOI: https://doi.org/10.1038/s41586-019-1666-5 Issue Date 24 October 2019

This paper appears to be open access.

Gold atoms: sometimes they’re a metal and sometimes they’re a molecule

Fascinating work out of Finland shows that a minor change in the number of gold atoms in your gold nanoparticle can mean the difference between a metal and a molecule (coincidentally, this phenomenon is alluded to in my April 14, 2015 post (Nature’s patterns reflected in gold nanoparticles); more about that at the end of this piece. Getting back to Finland and when gold is metal and when it’s a molecule, here’s more from an April 10, 2015 news item on ScienceDaily,

Researchers at the Nanoscience Center at the University of Jyväskylä, Finland, have shown that dramatic changes in the electronic properties of nanometre-sized chunks of gold occur in well-defined size range. Small gold nanoclusters could be used, for instance, in short-term storage of energy or electric charge in the field of molecular electronics. Funded by the Academy of Finland, the researchers have been able to obtain new information which is important, among other things, in developing bioimaging and sensing based on metal-like clusters.

An April 10, 2015 news release (also on EurekAlert) on the Academy of Finland (Suomen Akatemia) website, which originated the news item, describes the work in more detail,

Two recent papers by the researchers at Jyväskylä (1, 2) demonstrate that the electronic properties of two different but still quite similar gold nanoclusters can be drastically different. The clusters were synthesised by chemical methods incorporating a stabilising ligand layer on their surface. The researchers found that the smaller cluster, with up to 102 gold atoms, behaves like a giant molecule while the larger one, with at least 144 gold atoms, already behaves, in principle, like a macroscopic chunk of metal, but in nanosize.

The fundamentally different behaviour of these two differently sized gold nanoclusters was demonstrated by shining a laser light onto solution samples containing the clusters and by monitoring how energy dissipates from the clusters into the surrounding solvent.

“Molecules behave drastically different from metals,” said Professor Mika Pettersson, the principal investigator of the team conducting the experiments. “The additional energy from light, absorbed by the metal-like clusters, transfers to the environment extremely rapidly, in about one hundred billionth of a second, while a molecule-like cluster is excited to a higher energy state and dissipates the energy into the environment with a rate that is at least 100 times slower. This is exactly what we saw: the 102-gold atom cluster is a giant molecule showing even a transient magnetic state while the 144-gold atom cluster is already a metal. We’ve thus managed to bracket an important size region where this fundamentally interesting change in the behaviour takes place.”

“These experimental results go together very well with what our team has seen from computational simulations on these systems,” said Professor Hannu Häkkinen, a co-author of the studies and the scientific director of the nanoscience centre. “My team predicted this kind of behaviour back in 2008-2009 when we saw big differences in the electronic structure of exactly these nanoclusters. It’s wonderful that robust spectroscopic experiments have now proved these phenomena. In fact, the metal-like 144-atom cluster is even more interesting, since we just published a theoretical paper where we saw a big enhancement of the metallic properties of just a few copper atoms mixed with gold.” (3)

Here are links to and citation for the papers,

Ultrafast Electronic Relaxation and Vibrational Cooling Dynamics of Au144(SC2H4Ph)60 Nanocluster Probed by Transient Mid-IR Spectroscopy by Satu Mustalahti, Pasi Myllyperkiö, Tanja Lahtinen, Kirsi Salorinne, Sami Malola, Jaakko Koivisto, Hannu Häkkinen, and Mika Pettersson. J. Phys. Chem. C, 2014, 118 (31), pp 18233–18239 DOI: 10.1021/jp505464z Publication Date (Web): July 3, 2014

Copyright © 2014 American Chemical Society

Copper Induces a Core Plasmon in Intermetallic Au(144,145)–xCux(SR)60 Nanoclusters by Sami Malola, Michael J. Hartmann, and Hannu Häkkinen. J. Phys. Chem. Lett., 2015, 6 (3), pp 515–520 DOI: 10.1021/jz502637b Publication Date (Web): January 22, 2015

Copyright © 2015 American Chemical Society

Molecule-like Photodynamics of Au102(pMBA)44 Nanocluster by Satu Mustalahti, Pasi Myllyperkiö, Sami Malola, Tanja Lahtinen, Kirsi Salorinne, Jaakko Koivisto, Hannu Häkkinen, and Mika Pettersson. ACS Nano, 2015, 9 (3), pp 2328–2335 DOI: 10.1021/nn506711a Publication Date (Web): February 22, 2015

Copyright © 2015 American Chemical Society

These papers are behind paywalls.

As for my April 14, 2015 post (Nature’s patterns reflected in gold nanoparticles), researchers at Carnegie Mellon University were researching patterns in different sized gold nanoparticles when this was noted in passing,

… Normally, gold is one of the best conductors of electrical current, but the size of Au133 is so small that the particle hasn’t yet become metallic. …

Carbon nanotubes, good vibrations, and quantum computing

Apparently carbon nanotubes can store information within their vibrations and this could have implications for quantum computing, from the Mar. 21, 2013 news release on EurekAlert,

A carbon nanotube that is clamped at both ends can be excited to oscillate. Like a guitar string, it vibrates for an amazingly long time. “One would expect that such a system would be strongly damped, and that the vibration would subside quickly,” says Simon Rips, first author of the publication. “In fact, the string vibrates more than a million times. The information is thus retained up to one second. That is long enough to work with.”

Since such a string oscillates among many physically equivalent states, the physicists resorted to a trick: an electric field in the vicinity of the nanotube ensures that two of these states can be selectively addressed. The information can then be written and read optoelectronically. “Our concept is based on available technology,” says Michael Hartmann, head of the Emmy Noether research group Quantum Optics and Quantum Dynamics at the TU Muenchen. “It could take us a step closer to the realization of a quantum computer.”

The research paper can be found here,

Quantum Information Processing with Nanomechanical Qubits
Simon Rips and Michael J. Hartmann,
Physical Review Letters, 110, 120503 (2013) DOI: 10.1103/PhysRevLett.110.120503
Link: http://prl.aps.org/abstract/PRL/v110/i12/e120503

The paper is behind a paywall.

There are two Good Vibrations songs on YouTube, one by the Beach Boys and one by Marky Mark. I decided to go with this Beach Boys version in part due to its technical description at http://youtu.be/NwrKKbaClME,

FIRST TRUE STEREO version with lead vocals properly placed in the mix. I also restored the original full length of the bridge that was edited out of the released version. An official true stereo mix of the vocal version was not made back in 1967. While there are other “stereo” versions posted, for the most part they are “fake” or poor stereo versions. I tried to make the best judicious decision on sound quality, stereo imaging and mastering while maintaining TRUE STEREO integrity given the source parts available. I hope you enjoy it!

The video,