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Sandwiched between superconductors, graphene adopts exotic electronic states




In normal conductive materials such as silver and copper, electric current flows with varying degrees of resistance, in the form of individual electrons that ping-pong off defects, dissipating energy as they go. Superconductors, by contrast, are so named for their remarkable ability to conduct electricity without resistance, by means of electrons that pair up and move through a material as one, generating no friction.

Now MIT physicists have found that a flake of graphene, when brought in close proximity with two superconducting materials, can inherit some of those materials’ superconducting qualities. As graphene is sandwiched between superconductors, its electronic state changes dramatically, even at its center.

The researchers found that graphene’s electrons, formerly behaving as individual, scattering particles, instead pair up in “Andreev states” — a fundamental electronic configuration that allows a conventional, nonsuperconducting material to carry a “supercurrent,” an electric current that flows without dissipating energy.

Their findings, published this week in Nature Physics, are the first investigation of Andreev states due to superconductivity’s “proximity effect” in a two-dimensional material such as graphene.

Down the road, the researchers’ graphene platform may be used to explore exotic particles, such as Majorana fermions, which are thought to arise from Andreev states and may be key particles for building powerful, error-proof quantum computers.

“There is a huge effort in the condensed physics community to look for exotic quantum electronic states,” says lead author Landry Bretheau, a postdoc in MIT’s Department of Physics. “In particular, new particles called Majorana fermions are predicted to emerge in graphene that is connected to superconducting electrodes and exposed to large magnetic fields. Our experiment is promising, as we are unifying some of these ingredients.”

Landry’s MIT co-authors are postdoc Joel I-Jan Wang, visiting student Riccardo Pisoni, and associate professor of physics Pablo Jarillo-Herrero, along with Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science, in Japan.

The superconducting proximity effect

In 1962, the British physicist Brian David Josephson predicted that two superconductors sandwiching a nonsuperconducting layer between them could sustain a supercurrent of electron pairs, without any external voltage.

As a whole, the supercurrent associated with the Josephson effect has been measured in numerous experiments. But Andreev states — considered the microscopic building blocks of a supercurrent — have been observed only in a handful of systems, such as silver wires, and never in a two-dimensional material.  

Bretheau, Wang, and Jarillo-Herrero tackled this issue by using graphene — an ultrathin sheet of interlinked carbon atoms — as the nonsuperconducting material. Graphene, as Bretheau explains, is an extremely “clean” system, exhibiting very little scattering of electrons. Graphene’s extended, atomic configuration also enables scientists to measure graphene’s electronic Andreev states as the material comes in contact with superconductors. Scientists can also control the density of electrons in graphene and investigate how it affects the superconducting proximity effect.

The researchers exfoliated a very thin flake of graphene, just a few hundred nanometers wide, from a larger chunk of graphite, and placed the flake on a small platform made from a crystal of boron nitride overlaying a sheet of graphite. On either end of the graphene flake, they placed an electrode made from aluminum, which behaves as a superconductor at low temperatures. They then placed the entire structure in a dilution refrigerator and lowered the temperature to 20 millikelvin — well within aluminum’s superconducting range.

“Frustrated” states

In their experiments, the researchers varied the magnitude of the supercurrent flowing between the superconductors by applying a changing magnetic field to the entire structure. They also applied an external voltage directly to graphene, to vary the number of electrons in the material.

Under these changing conditions, the team measured the graphene’s density of electronic states while the flake was in contact with both aluminum superconductors. Using tunneling spectroscopy, a common technique that measures the density of electronic states in a conductive sample, the researchers were able to probe the graphene’s central region to see whether the superconductors had any effect, even in areas where they weren’t physically touching the graphene.

The measurements indicated that graphene’s electrons, which normally act as individual particles, were pairing up, though in “frustrated” configurations, with energies dependent on magnetic field.

“Electrons in a superconductor dance harmoniously in pairs, like a ballet, but the choreography in the left and right superconductors can be different,” Bretheau says. “Pairs in the central graphene are frustrated as they try to satisfy both ways of dancing. These frustrated pairs are what physicists know as Andreev states; they are carrying the supercurrent.”

Bretheau and Wang found Andreev states vary their energy in response to a changing magnetic field. Andreev states are more pronounced when graphene has a higher density of electrons and there is a stronger supercurrent running between electrodes.

“[The superconductors] are actually giving graphene some superconducting qualities,” Bretheau says. “We found these electrons can be dramatically affected by superconductors.”

While the researchers carried out their experiments under low magnetic fields, they say their platform may be a starting point for exploring the more exotic Majorana fermions that should appear under high magnetic fields.

“There are proposals for how to use Majorana fermions to build powerful quantum computers,” Bretheau says. “These particles could be the elementary brick of topological quantum computers, with very strong protection against errors. Our work is an initial step in this direction.”

This work was supported, in part, by the U.S. Department of Energy and the Gordon and Betty Moore Foundation.



Seven reasons why I chose to do science in the government




When I was in college, people asked me what I wanted to do with my life. I’d answer, “I want to be of use and to learn always.” The question resurfaced in grad school and at the beginning of my postdoc. I answered that I wanted to do extraordinary science that I’d steer. Academia attracted me most, but I wouldn’t discount alternatives.

Last spring, I accepted an offer to build my research group as a member of NIST, the National Institute for Standards and Technology in the U.S. government. My group will be headquartered on the University of Maryland campus, nestled amongst quantum and interdisciplinary institutes. I’m grateful to be joining NIST, and I’m surprised. I never envisioned myself working for the government. I could have accepted an assistant professorship (and I was extremely grateful for the offers), but NIST swept me off my feet. Here are seven reasons why, for other early-career researchers contemplating possibilities.

1) The science. One event illustrates this reason: The notice of my job offer came from NIST Maryland’s friendly neighborhood Nobel laureate. NIST and the university invested in quantum science years before everyone and her uncle began scrambling to create a quantum institute. That investment has flowered, including in reason (2).

2) The research environment. I wouldn’t say that I have a love affair with the University of Maryland. But I’ve found myself visiting every few years (sometimes blogging about the experience). Why? Much of the quantum community passes through Maryland. Seminars fill the week, visitors fill many offices, and conferences happen once or twice a year. Theorists and experimentalists mingle over lunch and collaborate. 

The university shares two quantum institutes with NIST: QuICS (the Joint Center for Quantum Information and Computer Science) and the JQI (the Joint Quantum Institute). My group will be based at the former and affiliated with the latter. We’ll also belong to IPST (the university’s Institute for Physical Science and Technology), a hub for interdisciplinarity and thermodynamics. When visiting a university, I ask how much researchers collaborate across department lines. I usually hear an answer along the lines of “We value interdisciplinarity, and we wish that we had more of it, but we don’t have much.” Few universities ingrain interdisciplinarity into their bones by dedicating institutes to it.

Maryland’s quantum community and thermodynamics communities bustle and produce. They grant NIST researchers an academic environment, independence to shape their research paths, and the freedom to participate in the broader scientific community. If weary of the three institutes mentioned above, one can explore the university’s Quantum Technology Center and Condensed-Matter-Theory Center

3) The people. The first Maryland quantum researcher I met was the friendly neighborhood Nobel laureate, Bill Phillips. Bill was presenting a keynote address at Dartmouth College’s physics department, where I’d earned my Bachelors. Bill said that he’d attended a small liberal-arts college before pursuing his PhD at MIT. During the question-and-answer session, I welcomed him back to a small liberal-arts college. How, I asked, had he benefited from the liberal arts? Juniata College, Bill said, had made him a good person. MIT had helped make him a good scientist. Since then, I’ve kept in occasional contact with Bill, we’ve attended talks of each other’s, and I’ve watched him exhibit the most curiosity I’ve seen in almost anyone. What more could one wish for in a colleague?

An equality used across thermodynamics bears Chris Jarzynski’s last name, but he never calls the equality what everyone else does. I benefited from Chris’s mentorship during my PhD, despite our working on opposite sides of the country. His awards include not only membership in the National Academy of Sciences, but also an Outstanding Referee designation, for reviewing so many journal submissions in service to the scientific community. Chris calls IPST, the university’s interdisciplinary and thermodynamic institute, his intellectual home. That recommendation suffices for me.

I’ve looked up to Alexey Gorshkov since beginning my PhD. I keep an eye out for Mohammad Hafezi’s and Pratyush Tiwari’s papers. A quantum researcher couldn’t ignore Chris Monroe’s papers if she tried. Postdoctoral and graduate fellowships stock the community with energetic young researchers. Three energetic researchers are joining QuICS as senior Fellows around the time I am. I’ll spare you the rest of my sources of inspiration.

4) The teaching. Most faculty members at R1 research universities teach two to three courses per year. NIST members can teach once every other year. I value teaching and appreciate how teaching benefits not only students, but also instructors. I respect teachers and remain grateful for their influence. I’m grateful to have received reports that I teach well. Because I’ve acquired some skill at communicating, people tend to assume that I adore teaching. I adore presenting talks, but I don’t feel a calling to teach. Mentors have exhorted me to pursue what excites me most and what only I can accomplish. I feel called to do research and to mentor younger researchers. 

Furthermore, if I had to teach much, I wouldn’t have time for writing anything other than papers or grants, such as blog posts. Some of you readers have astonished me with accounts of what my writing means to you. You’ve approached me at conferences, buttonholed me after seminars, and emailed. I’m grateful (as I keep saying, but I mean what I say) for the opportunity to touch lives across the world. I hope to inspire students to take quantum, information-theory, and thermodynamics courses (including the quantum-thermodynamics course that I’d like to teach occasionally). Instructors teach quantum courses throughout the world. No one else writes about Egyptian sarcophagi and the second law of thermodynamics, to my knowledge, or the Russian writer Alexander Pushkin and reproductive science. Perhaps no one should. But, since no one else does, I have to.1

5) The funding. Faculty members complain that they do little apart from applying for grants. Grants fund students, postdocs, travel, summer salaries, equipment, visitors, and workshops. NIST provides primary investigators with research funding every year. Not all the funding that some groups need, but enough to free up time to undertake the research that primary investigators love.

6) The lack of tenure stress. Many junior faculty members fear that they won’t achieve tenure. The fear pushes them away from taking risks in their research programs. This month, I embarked upon a risk that I know I should take but that, had I been facing an assistant professorship, would have given me pause.

7) The acronyms. Above, I introduced NIST (the National Institute of Standards and Technology), UMD (the University of Maryland), QuICS (the Joint Center for Quantum Information and Computer Science), the JQI (the Joint Quantum Institute), and IPST (the Institute for Physical Science and Technology). I’ll also have an affiliation with UMIACS (the University of Maryland Institute for Advanced Computer Science). Where else can one acquire six acronyms? I adore collecting affiliations, which force me to cross intellectual borders. I also enjoy the opportunity to laugh at my CV.

I’ve deferred joining NIST until summer 2021, to complete my postdoctoral fellowship at the Harvard-Smithsonian Institute for Theoretical Atomic, Molecular, and Optical Physics (an organization that needs its acronym, ITAMP, as much as “the Joint Center for Quantum Information and Computer Science” does). After then, please stop by. If you’d like to join my group, please email: I’m accepting applications for PhD and postdoctoral positions this fall. See you in Maryland next year.

1Also, blogging benefits my research. I’ll leave the explanation for another post.

I credit my husband with the Nesquick-NIST/QuICS parallel.


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Transforming graph states to Bell-pairs is NP-Complete




Axel Dahlberg, Jonas Helsen, and Stephanie Wehner

QuTech – TU Delft, Lorentzweg 1, 2628CJ Delft, The Netherlands

Find this paper interesting or want to discuss? Scite or leave a comment on SciRate.


Critical to the construction of large scale quantum networks, i.e. a quantum internet, is the development of fast algorithms for managing entanglement present in the network. One fundamental building block for a quantum internet is the distribution of Bell pairs between distant nodes in the network. Here we focus on the problem of transforming multipartite entangled states into the tensor product of bipartite Bell pairs between specific nodes using only a certain class of local operations and classical communication. In particular we study the problem of deciding whether a given graph state, and in general a stabilizer state, can be transformed into a set of Bell pairs on specific vertices using only single-qubit Clifford operations, single-qubit Pauli measurements and classical communication. We prove that this problem is ${mathbb{NP}}$-Complete.

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► References

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Cited by

[1] Axel Dahlberg, Jonas Helsen, and Stephanie Wehner, “Counting single-qubit Clifford equivalent graph states is #P -complete”, Journal of Mathematical Physics 61 2, 022202 (2020).

The above citations are from SAO/NASA ADS (last updated successfully 2020-10-26 03:05:50). The list may be incomplete as not all publishers provide suitable and complete citation data.

On Crossref’s cited-by service no data on citing works was found (last attempt 2020-10-26 03:05:49).


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Environmentally Induced Entanglement – Anomalous Behavior in the Adiabatic Regime




Richard Hartmann and Walter T. Strunz

Institut für Theoretische Physik, Technische Universität Dresden, D-01062 Dresden, Germany

Find this paper interesting or want to discuss? Scite or leave a comment on SciRate.


Considering two non-interacting qubits in the context of open quantum systems, it is well known that their common environment may act as an entangling agent. In a perturbative regime the influence of the environment on the system dynamics can effectively be described by a unitary and a dissipative contribution. For the two-spin Boson model with (sub-) Ohmic spectral density considered here, the particular unitary contribution (Lamb shift) easily explains the buildup of entanglement between the two qubits. Furthermore it has been argued that in the adiabatic limit, adding the so-called counterterm to the microscopic model compensates the unitary influence of the environment and, thus, inhibits the generation of entanglement. Investigating this assertion is one of the main objectives of the work presented here. Using the hierarchy of pure states (HOPS) method to numerically calculate the exact reduced dynamics, we find and explain that the degree of inhibition crucially depends on the parameter $s$ determining the low frequency power law behavior of the spectral density $J(omega) sim omega^s e^{-omega/omega_c}$. Remarkably, we find that for resonant qubits, even in the adiabatic regime (arbitrarily large $omega_c$), the entanglement dynamics is still influenced by an environmentally induced Hamiltonian interaction. Further, we study the model in detail and present the exact entanglement dynamics for a wide range of coupling strengths, distinguish between resonant and detuned qubits, as well as Ohmic and deep sub-Ohmic environments. Notably, we find that in all cases the asymptotic entanglement does not vanish and conjecture a linear relation between the coupling strength and the asymptotic entanglement measured by means of concurrence. Further we discuss the suitability of various perturbative master equations for obtaining approximate entanglement dynamics.

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