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Researchers were able to detect a "needle" of highly fragile quantum information in a "haystack" of nuclei.

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Researchers have found a way to use light and a single electron to communicate with a cloud of quantum bits and sense their behaviour, making it possible to detect a single quantum bit in a dense cloud.

The researchers, from the University of Cambridge, were able to inject a needle of highly fragile quantum information in a haystack of 100,000 nuclei. Using lasers to control an electron, the researchers could then use that electron to control the behaviour of the haystack, making it easier to find the needle. They were able to detect the needle with a precision of 1.9 parts per million: high enough to detect a single quantum bit in this large ensemble.

The technique makes it possible to send highly fragile quantum information optically to a nuclear system for storage, and to verify its imprint with minimal disturbance, an important step in the development of a quantum internet based on quantum light sources. The results are reported in the journal Nature Physics.

The first quantum computers which will harness the strange behaviour of subatomic particles to far outperform even the most powerful supercomputers are on the horizon. However, leveraging their full potential will require a way to network them: a quantum internet. Channels of light that transmit quantum information are promising candidates for a quantum internet, and currently there is no better quantum light source than the semiconductor quantum dot: tiny crystals that are essentially artificial atoms.

However, one thing stands in the way of quantum dots and a quantum internet: the ability to store quantum information temporarily at staging posts along the network.

The solution to this problem is to store the fragile quantum information by hiding it in the cloud of 100,000 atomic nuclei that each quantum dot contains, like a needle in a haystack, said Professor Mete Atatre from Cambridges Cavendish Laboratory, who led the research. But if we try to communicate with these nuclei like we communicate with bits, they tend to flip randomly, creating a noisy system.

The cloud of quantum bits contained in a quantum dot dont normally act in a collective state, making it a challenge to get information in or out of them. However, Atatre and his colleagues showed in 2019 that when cooled to ultra-low temperatures also using light, these nuclei can be made to do quantum dances in unison, significantly reducing the amount of noise in the system.

Now, they have shown another fundamental step towards storing and retrieving quantum information in the nuclei. By controlling the collective state of the 100,000 nuclei, they were able to detect the existence of the quantum information as a flipped quantum bit at an ultra-high precision of 1.9 parts per million: enough to see a single bit flip in the cloud of nuclei.

Technically this is extremely demanding, said Atatre, who is also a Fellow of St Johns College. We dont have a way of talking to the cloud and the cloud doesnt have a way of talking to us. But what we can talk to is an electron: we can communicate with it sort of like a dog that herds sheep.

Using the light from a laser, the researchers are able to communicate with an electron, which then communicates with the spins, or inherent angular momentum, of the nuclei.

By talking to the electron, the chaotic ensemble of spins starts to cool down and rally around the shepherding electron; out of this more ordered state, the electron can create spin waves in the nuclei.

If we imagine our cloud of spins as a herd of 100,000 sheep moving randomly, one sheep suddenly changing direction is hard to see, said Atatre. But if the entire herd is moving as a well-defined wave, then a single sheep changing direction becomes highly noticeable.

In other words, injecting a spin wave made of a single nuclear spin flip into the ensemble makes it easier to detect a single nuclear spin flip among 100,000 nuclear spins.

Using this technique, the researchers are able to send information to the quantum bit and listen in on what the spins are saying with minimal disturbance, down to the fundamental limit set by quantum mechanics.

Having harnessed this control and sensing capability over this large ensemble of nuclei, our next step will be to demonstrate the storage and retrieval of an arbitrary quantum bit from the nuclear spin register, said co-first author Daniel Jackson, a PhD student at the Cavendish Laboratory.

This step will complete a quantum memory connected to light a major building block on the road to realizing the quantum internet, said co-first author Dorian Gangloff, a Research Fellow at St Johns College.

Besides its potential usage for a future quantum internet, the technique could also be useful in the development of solid-state quantum computing.

Reference: D. M. Jackson, et al, Quantum sensing of a coherent single spin excitation in a nuclear ensemble, Nature Physics (2021). DOI: 10.1038/s41567-020-01161-4

Excerpt from:

Extracting information stored in 100,000 nuclear quantum bits - Advanced Science News

February 17, 2021• Physics 14, 25

A newly proposed superconducting circuit architecture employs a synthetic magnetic field to create a qubit that is intrinsically protected from noise.

Today, noise poses one of the biggest challenges for quantum computation efforts. Be it in the form of dissipated heat, electromagnetic radiation, or something else, noise can disrupt fragile quantum superpositions and lead to errors. The jury is still out on which approach will be most successful in protecting quantum information against noise, but the hope clearly lies in quantum error correction (QEC) protocols. Now, Martin Rymarz of RWTH Aachen University in Germany and colleagues have proposed a novel superconducting circuit implementation that realizes a QEC strategy in which robustness against noise is an intrinsic feature of the hardware [1]. This strategy, known as the Gottesman-Kitaev-Preskill (GKP) code, was proposed in 2001 [2]. However, implementing it with superconducting circuits has so far been impossible because it requires a large magnetic field. The newly proposed architecture circumvents this obstacle by employing a synthetic magnetic field, pushing the GKP protocol closer to a possible realization.

The noise processes that threaten quantum computing are assumed to be local, meaning that they act on specific parts of circuits, such as individual physical qubits. In the scaling-up approach to QEC, quantum information is encoded into multiple physical qubits that form each logical qubit used for the actual computation tasks. So even if one physical qubit is disrupted by noise, the information carried by the logical qubit will not be corrupted. In the exotic-state approach to QEC, each computational unit is a single oscillator, and the logical bits are represented by two special states of the oscillator, called nontrivial states, that are robust against local noise. The exotic-state technique employs continuous-variable systems, such as electromagnetic modes, which are initialized in states that are either robust by themselves (passive QEC) or can be stabilized via operations that do not affect the logical qubit (active QEC).

The GKP strategy is one example of the exotic-state approach [2]. In the GKP code, the exotic states are called grid states, which are superpositions of an oscillators position eigenstates [2]. The robustness to noise in an active GKP protocol stems from the fact that small shifts in the momentum and position of the oscillator can be identified and corrected before they can corrupt the logical information. An experimental demonstration of grid states was recently realized in a superconducting circuit architecture with an active QEC protocol [3]. A GKP code with passive QEC, however, has not yet been demonstrated. Compared to active QEC, which requires complicated operations for error recovery, a passive QEC approach promises to be more efficient and could be advantageous for scaling up to larger computing architectures, as it requires fewer physical units.

A prototypical implementation of a passive GKP code involves an electron confined to two dimensions in a large magnetic field. Realizing such a passive GKP-code design with superconducting circuit architectures is not straightforward. The design would require a magnetic field to interact with microwave photons, which are the oscillations of the electromagnetic field in the superconducting circuit. But photons are neutral particles and do not interact with magnetic fields in the same way that charged particles, such as electrons, do. Strategies for creating artificial magnetic fields that can interact with photons have been discussed and demonstrated in some superconducting systems [46]. The role of magnetic fields, whether real or artificial, in these systems is to break time-reversal symmetry, creating nonreciprocal circuits with multiple ports. The nonreciprocity means that the circuits process photons in a different way depending on which port they are injected into. This asymmetry can be exploited to build nonreciprocal devices that transmit microwave signals in one direction while blocking them in the reverse direction [6].

Rymarz and colleagues have proposed a way of utilizing synthetic magnetic fields, allowing for a superconducting qubit realization of the GKP code. They propose a system in which two superconducting anharmonic oscillators, called fluxonium circuits, are coupled via a gyrator, a device that can invert the current-voltage characteristics of a circuit element (Fig. 1). The asymmetric response of the gyrator implies a breaking of time-reversal symmetry like that produced by a magnetic field. The team shows that the ground states of the system correspond to the GKP code wordsthe grid states that are used to encode the logical information. The huge advantage here is that the logical qubit is constructed from the ground states of the systemin which the system will reside if no external energy is supplied. Leaving the ground state would corrupt the logical qubit, but it comes with an energy penalty, so the protection is naturally built in.

The researchers show that the proposed superconducting circuit simulates the model of an electron confined to a two-dimensional plane and subjected to a magnetic field. As such, the circuits energies resemble those of a quantum oscillator with discrete energy levels. For a given magnetic flux, the lowest-energy states can be used to encode the GKP code words.

A qualitative analysis of the circuit predicts a robustness against common noise sources, such as charge and flux noise, making it a promising passive-QEC candidate. Clearly, the characteristics of the circuit needed to implement the new scheme require improvements of existing technology. For example, the fluxonium circuit should have a very large inductance, which isnt currently attainable but will hopefully be possible in next generation designs. The proposed implementation of the hardware-encoded grid states represents a novel utilization of synthetic magnetism and a new application for gyrators based on the anomalous quantum Hall effect [7, 8]. It remains to be seen, however, whether these gyrators can successfully be married with two fluxonium circuits on-chip. Another question is whether an actively driven nonreciprocal on-chip device [5] could be a better alternative than a gyrator based on the anomalous quantum Hall effect.

This hardware-encoded GKP code implementation complements other ongoing efforts in designing intrinsically error-protected, superconducting circuit qubits, such as the realization of the 0 qubit [9] and the proposal of the doubly nonlinear qubit, or dualmon [10]. All designs come with challenging demands on the parameters of the employed materials and of the circuit elements. Encouragingly, the implementation proposed by Rymarz and colleagues comes within feasible reach of near-future technology. Realizing GKP code words using superconducting circuits is especially promising, as it makes it relatively straightforward to implement a subset of logic gates called Clifford gates, which are required for fault-tolerant computation [2, 3]. The realization of an intrinsically robust computation unit is only the first step on the complex path towards fault-tolerant quantum computation. But every new design pushes the field of superconducting circuits towards new horizons.

Anja Metelmann is an Emmy Noether research group leader in the Department of Theoretical Physics at the Free University Berlin in Germany. In 2012, she received her Ph.D. in physics from the Technical University Berlin in Germany. She spent her postdoctoral time in the Physics Department of McGill University in Montreal, and in the Department of Electrical Engineering at Princeton University.Her research interests lie in the fundamental aspects and applications of superconducting circuits and mechanical systems in the quantum regime. Part of her current research focuses on nonreciprocity as a resource for quantum information processing.

A small prototype of a drone-based quantum network has successfully relayed a quantum signal over a kilometer of free space. Read More

See the rest here:

Physics - A Superconducting Qubit that Protects Itself - Physics

Pat Stuart

By Pat Stuart

The time has come and passed for New Years resolutions. But this doesnt preclude resolving to read more.

The possibilities are endless. In 2018, 1.6 million books were published. That figure includes self-published volumes and e-books. No way you cant find something that appeals.

This came to mind because CBS Sunday Morning ran a piece about an old fictional friend and a famous teen-aged heroine: Nancy Drew. Shes been with us for 90 years! Who knew. Various versions of her adventures have sold over 80 million copies, and the books are still coming. Some are sitting on my bookshelves.

Yikes. To think that my mother read Nancy Drew as a pre-teen, as did I, as did my daughter and as my grandsons would have done if theyd been girls. We all probably picked up tips on how to deal with the world of adults, how to be independent thinkers, how to be adventurous and risk-takers.

Midnight Library

Which brings me to one of this seasons acclaimed books, Midnight Library. A great title. Michael Haigs new novel has won rave reviews from the critics, but Im not quite sure why. The writing is mostly good and the plot line tantalizing in the abstract but, in sum, I found it far from the brilliant some critics proclaimed.

The almost middle-aged heroine herself is dull ... running, not walking, from every opportunity life offers until shes bricked herself into a lonely cell. No one wants to enter while theres nowhere she wants to go.

No surprise. She commits suicide ... and finds herself at a nexus between an infinite number of realities (think quantum physics) where every possible permutation of her life comes together. Here, she can sift through her regrets and live the experiences she might have had if only shed had the courage. Think purgatory, atonement and redemption.

I suspect were supposed to cheer when she learns what should be a teen years coming-of-age lesson: You take from this life what you put into it. Nancy Drew knew that.

D-Day Girls

Much, much better is a 2019 book that almost fell off a library shelf and into my hands. D-Day Girls by Sarah Rose is an extraordinarily well-written and researched non-fiction book about the women Britain sent into France to run spy rings, train saboteurs, blow up rail lines and power plants, and pave the way for an invasion.

The book treats us to belly-shaking wit along with a full range of other emotions as the author expertly draws us through the complexities and dangers of war-time, occupied France. Nor does she spare the spies and partisans while leaving their courage and heroism untouched. Ego trips, self-delusion, wishful thinking, laziness, romance its all there, too often leading to torture and death.

Its no secret that Im a fan of World War IIs espionage heroines who, if they survived, were rusticated to file rooms as what the men of the CIA dismissively called little old ladies in tennis shoes, and our hereditary memory.

Ive mentioned before that some of them were still around when I entered on duty. Now I wish Id visited the file rooms where they spent their last working days. In my own defense, I had no idea then of what theyd done or how brilliantly they had performed. They (and I) were victims of the post-WWII effort to put the war-time genie back in the bottle and women back in the kitchen.

(As an aside and as an example, John Stein a really superb operations officer and a generally supportive one who became Deputy Director for Operations strongly advised me for my own good early in our careers that Id be happier if I devoted myself to cooking for my husband, the way his wife Charlie did. No joke.)

As you can see, the acceptance of the D-Day womens accomplishments and the less spectacular and meaningful ones of my generation came only slowly. But a number of well-researched books this one by Sarah Rose as a superb example is changing that.

Life really is stranger and much more interesting than fiction.

The Night Watchman

Finally, Id like to mention Louise Erdrichs award-winning and much-praised 2020 book of fictionalized history, The Night Watchman. She, too, has a female character in a coming-of-age situation. But, what a difference from Michael Haigs colorless heroine or from the real women of D-Day!

Probably like most of you, I consider a book worth reading when I learn from it. And, learn I did.

For the first time in a long life, years of it lived in proximity to Indian reservations, I found myself walking in the footsteps of our first citizens. Erdrich brings her many characters to life in ways that allow us to come as close as possible to understanding them and to feel their joys and pains, their frustrations, ambitions and resignations. I loved every minute of the page-turning, superbly written read.

Maybe these arent the books for you. We all have our tastes in reading whether in digital or hard copy. Whatever yours might be, one of the easiest and most enjoyable resolutions you can make is to visit your local independent bookseller or your library online or on foot.

Read more:

Resolving to read more in 2021 - Powell Tribune

G. Wayne Miller|The Providence Journal

PROVIDENCE Pretty much everyone has a ghost story. Some are comforting. Others amuse. Still others terrify. But not everyone has made a flourishing career ofthem.

Rhode Island resident Amy Bruni has.

You may have watched paranormal investigator Bruni, formerly of the popular TV show Ghost Hunters, and now as co-star of the Travel Channels hit show Kindred Spirits.

You may have read her USA TODAY best-selling book, Life with the Afterlife: 13 Truths I Learned about Ghosts, which chronicles her adventures, including her investigations of ghosts inside the old Biltmore Hotel and the house in Burrillville that was the basis for the 2013 supernatural movie The Conjuring.

More: Would you dare to stay at The Conjuring house in Burrillville?

And now you can watch Bruni discuss her life and work this week on Story in the Public Square,a Providence Journal and Pell Center partnership that airs on SiriusXM and public television stations across the country.

Bruni began by discussing her childhood in California in a house that was haunted with a mother and father who both were interested in, and did not fear, the supernatural.

The way they talked about it was so matter of fact, Bruni said. They were just, like, sometimes there's ghosts. And, I never thought that it was a scary thing. Obviously, since then I've had many scary ghost experiences, butbeing raised in that environment it just really piquedmy curiosity and that never went away.

Surprisingly, for someone who has experienced so many hauntings, Bruni does not proclaim dogmatic belief.

From my interaction with what I believe could be spirits, I do think that they are some bit of us that's left behind after we pass, she said. I do think that classic idea of 'unfinished business' sometimes holds a piece of someone's consciousness here. That is my best guess, but I will never claim to say that that's absolutely what a ghost is.

And you never know if it's going to be explained one day, with quantum physics or other possibilities. The more that we do this and the more kind of strange experiments we do, the more questions we have. And so that's why I make a really big point in the book to never speak in absolutes.

Historical research of the homes, institutions and other places Bruni investigates and of the deceased people who lived in them and the people alive today who occupy these spaces is a critical element of her work.

History is a huge part of what we do, Bruni said. I can sit all day and try to prove the existence of ghosts. And people can be skeptical about it or not. But I will absolutely make sure my history is right. That is one thing that I can prove and I can get records of.

Bruni broke into the national spotlight during her time, from 2008 to 2014, as a star on Ghost Hunters, investigating cases for episodes including Dead Presidents, Family Plot, Nine Mens Misery and An Officer and an Apparition.

Kindred Spirits launched in 2016 and among the most-watched episodes are Ghost Train, set on Cape Cod; Hell House, about the Burrillville home; Fire Starter, which also happens to be the title of a Stephen King novel; Keeper of the Light, filmed on Rose Island, off Newport; and The Legacy of Lizzie Borden, taped in Fall River. Bruni has served as executive producer of all episodes of Kindred Spirits, which remains in production.

Bruni said the well-being of the inhabitants of a haunted house are a critical concern for her and her Kindred Spirits co-hosts, Adam Berry and Chip Coffey.

Some of these families have been dealing with this paranormal activity for years, and they've either been too afraid to say something or they feel like people don't believe them, Bruni said. We've actually encountered people who have legitimate PTSD from some of their experiences. And anxiety issues. Its so much more than just my house is haunted.

And there are so many people with stories than fans of horror movies orKing novels.

I meet people who don't even believe in ghosts, but they'll say but there was this one time and then they'll tell me this really crazy story. I think people feel comfortable talking to me about it. And you know, I live in one of the most haunted places in the country, in New England. And everyone's got a great ghost story here.

Story in the Public Square broadcasts each week on public television stations across the United States. In Rhode Island and southeastern New England, the show is broadcast on Rhode Island PBS on Sundays at 11 a.m. and is rebroadcast Thursdays at 7:30 p.m. An audio version of the program airs Saturdays at 8:30 a.m. & 6:30 p.m. ET, Sundays at 3:30 a.m. & 11:30 p.m. ET on SiriusXMs popular P.O.T.U.S. (Politics of the United States), channel 124. Story in the Public Square is a partnership between the Pell Center at Salve Regina Universityand The Providence Journal/USA TODAY Network.

Read more:

RI local and star of 'Ghost Hunters' + 'Kindred Spirits' on her search for the paranormal - The Providence Journal