Tag Archives: quantum bits

Graphene can be used in quantum components

A November 3, 2022 news item on phys.org provides a brief history of graphene before announcing the latest work from ETH Zurich,

Less than 20 years ago, Konstantin Novoselov and Andre Geim first created two-dimensional crystals consisting of just one layer of carbon atoms. Known as graphene, this material has had quite a career since then.

Due to its exceptional strength, graphene is used today to reinforce products such as tennis rackets, car tires or aircraft wings. But it is also an interesting subject for fundamental research, as physicists keep discovering new, astonishing phenomena that have not been observed in other materials.

The right twist

Bilayer graphene crystals, in which the two atomic layers are slightly rotated relative to each other, are particularly interesting for researchers. About one year ago, a team of researchers led by Klaus Ensslin and Thomas Ihn at ETH Zurich’s Laboratory for Solid State Physics was able to demonstrate that twisted graphene could be used to create Josephson junctions, the fundamental building blocks of superconducting devices.

Based on this work, researchers were now able to produce the first superconducting quantum interference device, or SQUID, from twisted graphene for the purpose of demonstrating the interference of superconducting quasiparticles. Conventional SQUIDs are already being used, for instance in medicine, geology and archaeology. Their sensitive sensors are capable of measuring even the smallest changes in magnetic fields. However, SQUIDs work only in conjunction with superconducting materials, so they require cooling with liquid helium or nitrogen when in operation.

In quantum technology, SQUIDs can host quantum bits (qubits); that is, as elements for carrying out quantum operations. “SQUIDs are to superconductivity what transistors are to semiconductor technology—the fundamental building blocks for more complex circuits,” Ensslin explains.

A November 3, 2022 ETH Zurich news release by Felix Würsten, which originated the news item, delves further into the work,

The spectrum is widening

The graphene SQUIDs created by doctoral student Elías Portolés are not more sensitive than their conventional counterparts made from aluminium and also have to be cooled down to temperatures lower than 2 degrees above absolute zero. “So it’s not a breakthrough for SQUID technology as such,” Ensslin says. However, it does broaden graphene’s application spectrum significantly. “Five years ago, we were already able to show that graphene could be used to build single-electron transistors. Now we’ve added superconductivity,” Ensslin says.

What is remarkable is that the graphene’s behaviour can be controlled in a targeted manner by biasing an electrode. Depending on the voltage applied, the material can be insulating, conducting or superconducting. “The rich spectrum of opportunities offered by solid-state physics is at our disposal,” Ensslin says.

Also interesting is that the two fundamental building blocks of a semiconductor (transistor) and a superconductor (SQUID) can now be combined in a single material. This makes it possible to build novel control operations. “Normally, the transistor is made from silicon and the SQUID from aluminium,” Ensslin says. “These are different materials requiring different processing technologies.”

An extremely challenging production process

Superconductivity in graphene was discovered by an MIT [Massachusetts Institute of Technology] research group five years ago, yet there are only a dozen or so experimental groups worldwide that look at this phenomenon. Even fewer are capable of converting superconducting graphene into a functioning component.

The challenge is that scientists have to carry out several delicate work steps one after the other: First, they have to align the graphene sheets at the exact right angle relative to each other. The next steps then include connecting electrodes and etching holes. If the graphene were to be heated up, as happens often during cleanroom processing, the two layers re-align the twist angle vanishes. “The entire standard semiconductor technology has to be readjusted, making this an extremely challenging job,” Portolés says.

The vision of hybrid systems

Ensslin is thinking one step ahead. Quite a variety of different qubit technologies are currently being assessed, each with its own advantages and disadvantages. For the most part, this is being done by various research groups within the National Center of Competence in Quantum Science and Technology (QSIT). If scientists succeed in coupling two of these systems using graphene, it might be possible to combine their benefits as well. “The result would be two different quantum systems on the same crystal,” Ensslin says.

This would also generate new possibilities for research on superconductivity. “With these components, we might be better able to understand how superconductivity in graphene comes about in the first place,” he adds. “All we know today is that there are different phases of superconductivity in this material, but we do not yet have a theoretical model to explain them.”

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

A tunable monolithic SQUID in twisted bilayer graphene by Elías Portolés, Shuichi Iwakiri, Giulia Zheng, Peter Rickhaus, Takashi Taniguchi, Kenji Watanabe, Thomas Ihn, Klaus Ensslin & Folkert K. de Vries. Nature Nanotechnology volume 17, pages 1159–1164 (2022) Issue Date: November 2022 DOI: https://doi.org/10.1038/s41565-022-01222-0 Published online: 24 October 2022

This paper is behind a paywall.

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.

For first time: high-dimensional quantum encryption performed in real world city conditions

Having congratulated China on the world’s first quantum communication network a few weeks ago (August 22, 2017 posting), this quantum encryption story seems timely. From an August 24, 2017 news item on phys.org,

For the first time, researchers have sent a quantum-secured message containing more than one bit of information per photon through the air above a city. The demonstration showed that it could one day be practical to use high-capacity, free-space quantum communication to create a highly secure link between ground-based networks and satellites, a requirement for creating a global quantum encryption network.

Quantum encryption uses photons to encode information in the form of quantum bits. In its simplest form, known as 2D encryption, each photon encodes one bit: either a one or a zero. Scientists have shown that a single photon can encode even more information—a concept known as high-dimensional quantum encryption—but until now this has never been demonstrated with free-space optical communication in real-world conditions. With eight bits necessary to encode just one letter, for example, packing more information into each photon would significantly speed up data transmission.

This looks like donuts on a stick to me,

For the first time, researchers have demonstrated sending messages in a secure manner using high dimensional quantum cryptography in realistic city conditions. Image Credit: SQO team, University of Ottawa.

An Aug. 24, 2017 Optical Society news release (also on EurekAlert), which originated the news item, describes the work done by a team in Ottawa, Canada, (Note: The ‘Congratulate China’ piece (August 22, 2017 posting) includes excerpts from an article that gave a brief survey of various national teams [including Canada] working on quantum communication networks; Links have been removed),

“Our work is the first to send messages in a secure manner using high-dimensional quantum encryption in realistic city conditions, including turbulence,” said research team lead, Ebrahim Karimi, University of Ottawa, Canada. “The secure, free-space communication scheme we demonstrated could potentially link Earth with satellites, securely connect places where it is too expensive to install fiber, or be used for encrypted communication with a moving object, such as an airplane.”

For the first time, researchers have demonstrated sending messages in a secure manner using high dimensional quantum cryptography in realistic city conditions. Image Credit: SQO team, University of Ottawa.

As detailed in Optica, The Optical Society’s journal for high impact research, the researchers demonstrated 4D quantum encryption over a free-space optical network spanning two buildings 0.3 kilometers apart at the University of Ottawa. This high-dimensional encryption scheme is referred to as 4D because each photon encodes two bits of information, which provides the four possibilities of 01, 10, 00 or 11.

In addition to sending more information per photon, high-dimensional quantum encryption can also tolerate more signal-obscuring noise before the transmission becomes unsecure. Noise can arise from turbulent air, failed electronics, detectors that don’t work properly and from attempts to intercept the data. “This higher noise threshold means that when 2D quantum encryption fails, you can try to implement 4D because it, in principle, is more secure and more noise resistant,” said Karimi.

Using light for encryption

Today, mathematical algorithms are used to encrypt text messages, banking transactions and health information. Intercepting these encrypted messages requires figuring out the exact algorithm used to encrypt a given piece of data, a feat that is difficult now but that is expected to become easier in the next decade or so as computers become more powerful.

Given the expectation that current algorithms may not work as well in the future, more attention is being given to stronger encryption techniques such as quantum key distribution, which uses properties of light particles known as quantum states to encode and send the key needed to decrypt encoded data.

Although wired and free-space quantum encryption has been deployed on some small, local networks, implementing it globally will require sending encrypted messages between ground-based stations and the satellite-based quantum communication networks that would link cities and countries. Horizontal tests through the air can be used to simulate sending signals to satellites, with about three horizontal kilometers being roughly equal to sending the signal through the Earth’s atmosphere to a satellite.

Before trying a three-kilometer test, the researchers wanted to see if it was even possible to perform 4D quantum encryption outside. This was thought to be so challenging that some other scientists in the field said that the experiment would not work. One of the primary problems faced during any free-space experiment is dealing with air turbulence, which distorts the optical signal.

Real-world testing

For the tests, the researchers brought their laboratory optical setups to two different rooftops and covered them with wooden boxes to provide some protection from the elements. After much trial and error, they successfully sent messages secured with 4D quantum encryption over their intracity link. The messages exhibited an error rate of 11 percent, below the 19 percent threshold needed to maintain a secure connection. They also compared 4D encryption with 2D, finding that, after error correction, they could transmit 1.6 times more information per photon with 4D quantum encryption, even with turbulence.

“After bringing equipment that would normally be used in a clean, isolated lab environment to a rooftop that is exposed to the elements and has no vibration isolation, it was very rewarding to see results showing that we could transmit secure data,” said Alicia Sit, an undergraduate student in Karimi’s lab.

As a next step, the researchers are planning to implement their scheme into a network that includes three links that are about 5.6 kilometers apart and that uses a technology known as adaptive optics to compensate for the turbulence. Eventually, they want to link this network to one that exists now in the city. “Our long-term goal is to implement a quantum communication network with multiple links but using more than four dimensions while trying to get around the turbulence,” said Sit.

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

High-dimensional intracity quantum cryptography with structured photons by Alicia Sit, Frédéric Bouchard, Robert Fickler, Jérémie Gagnon-Bischoff, Hugo Larocque, Khabat Heshami, Dominique Elser, Christian Peuntinger, Kevin Günthner, Bettina Heim, Christoph Marquardt, Gerd Leuchs, Robert W. Boyd, and Ebrahim Karimi. Optica Vol. 4, Issue 9, pp. 1006-1010 (2017) •https://doi.org/10.1364/OPTICA.4.001006

This is an open access paper.

Seeing the future with quantum computing

Researchers at the University of Sydney (Australia) have demonstrated the ability to see the ‘quantum future’ according to a Jan. 16, 2017 news item on ScienceDaily,

Scientists at the University of Sydney have demonstrated the ability to “see” the future of quantum systems, and used that knowledge to preempt their demise, in a major achievement that could help bring the strange and powerful world of quantum technology closer to reality.

The applications of quantum-enabled technologies are compelling and already demonstrating significant impacts — especially in the realm of sensing and metrology. And the potential to build exceptionally powerful quantum computers using quantum bits, or qubits, is driving investment from the world’s largest companies.

However a significant obstacle to building reliable quantum technologies has been the randomisation of quantum systems by their environments, or decoherence, which effectively destroys the useful quantum character.

The physicists have taken a technical quantum leap in addressing this, using techniques from big data to predict how quantum systems will change and then preventing the system’s breakdown from occurring.

A Jan. 14, 2017 University of Sydney press release (also on EurekAlert), which originated the news item, expands on the theme,

“Much the way the individual components in mobile phones will eventually fail, so too do quantum systems,” said the paper’s senior author Professor Michael J.  Biercuk.

“But in quantum technology the lifetime is generally measured in fractions of a second, rather than years.”

Professor Biercuk, from the University of Sydney’s School of Physics and a chief investigator at the Australian Research Council’s Centre of Excellence for Engineered Quantum Systems, said his group had demonstrated it was possible to suppress decoherence in a preventive manner. The key was to develop a technique to predict how the system would disintegrate.

Professor Biercuk highlighted the challenges of making predictions in a quantum world: “Humans routinely employ predictive techniques in our daily experience; for instance, when we play tennis we predict where the ball will end up based on observations of the airborne ball,” he said.

“This works because the rules that govern how the ball will move, like gravity, are regular and known.  But what if the rules changed randomly while the ball was on its way to you?  In that case it’s next to impossible to predict the future behavior of that ball.

“And yet this situation is exactly what we had to deal with because the disintegration of quantum systems is random. Moreover, in the quantum realm observation erases quantumness, so our team needed to be able to guess how and when the system would randomly break.

“We effectively needed to swing at the randomly moving tennis ball while blindfolded.”

The team turned to machine learning for help in keeping their quantum systems – qubits realised in trapped atoms – from breaking.

What might look like random behavior actually contained enough information for a computer program to guess how the system would change in the future. It could then predict the future without direct observation, which would otherwise erase the system’s useful characteristics.

The predictions were remarkably accurate, allowing the team to use their guesses preemptively to compensate for the anticipated changes.

Doing this in real time allowed the team to prevent the disintegration of the quantum character, extending the useful lifetime of the qubits.

“We know that building real quantum technologies will require major advances in our ability to control and stabilise qubits – to make them useful in applications,” Professor Biercuk said.

Our techniques apply to any qubit, built in any technology, including the special superconducting circuits being used by major corporations.

“We’re excited to be developing new capabilities that turn quantum systems from novelties into useful technologies. The quantum future is looking better all the time,” Professor Biercuk said.

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

Prediction and real-time compensation of qubit decoherence via machine learning by Sandeep Mavadia, Virginia Frey, Jarrah Sastrawan, Stephen Dona, & Michael J. Biercuk. Nature Communications 8, Article number: 14106 (2017) doi:10.1038/ncomms14106 Published online: 16 January 2017

This paper is open access.