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Strong words on Weak Measurements

Even the greatest ships can get it wrong.Recently news made the round that the Heisenberg Uncertainty principle was supposedly violated (apparently BBC online news tries to build on its bad track track record for news related to quantum mechanics).

Of course this is utter and complete nonsense, and the underlying papers are quite mundane. But all caveats get stripped out in the reporting until only the wrong sensational twist remains.

Yes, Heisenberg did at some point speculate that the uncertainty relationship may be due to the measurements disturbing the system that is probed, but this idea has long been relegated to the dust bin of science history, and Robert R. Tucci deservedly demolishes it.

I am now breathlessly awaiting the results of this crack team revisiting the Inquisition’s test for witchcraft by attempted drowning.  I bet they’ll find this doesn’t hold up either. Cutting edge science.



Order from Quantum Discord

Have they no shame?
Even our children are already indoctrinated by popular Disney cartoons to think that Discord is bad. Borivoje Dakić et al. beg to differ.

Conventional hardware thrives on our ability to create precision structures in the micro domain.  Computers are highly ordered and usually (for good reason) regarded as perfectly deterministic in the way that they process information. After all, the error rate of the modern computer is astronomically low.

Even before the advent of quantum computing, it was discovered that this defining feature of our hardware can sometimes be a disadvantage i.e. randomized algorithms can sometimes outperform deterministic ones. Computers actually gain functionality by being able to use randomness as an information processing resource.

Due to their fundamentally probabilistic nature, randomness is always inherent in quantum computing designs. On the other hand, most quantum computing algorithms exploit one of the most fragile, ordered physical states: Entanglement, the peculiar quantum mechanical phenomenon that two systems can be entwined in a common quantum state. It is characterized by perfect correlation of spatially or temporally separated measurements.  The simplest protocol to exploit this feature is the quantum information channel, and it results in some quite surprising and, as is so often the case with quantum mechanics, counter-intuitive results. For instance, if two parties are connected via two very noisy directional channels with zero quantum information capacity, the participants will still be able to establish a qubit flow via entanglement distillation.

It has often been argued that entanglement is at the heart of quantum computing.  This credo has caused quite a bit of grief for the company D-Wave, that lays claim on shipping the first commercially available quantum computer. Although their erstwhile fiercest critic Scot Aaronson has made peace with them, he expressed that he still would like to see a measure for the degree of entanglement that they achieve on their chip.

Is it therefore quite surprising to see papers like this one recently published in Nature Physics that describe Quantum Discord as an optimal resource for quantum information processing. On first glance, some of this seems to be due to semantics.  For instance, John Preskill refers in his overview paper to all non-classical correlations as entanglement, but strictly speaking the term entanglement would never be applied to separable states. However, the paper demonstrates, theoretically as well as experimentally, that separable two qubit states with non-vanishing quantum discord can be found that offer better performance for their test case of quantum teleportation than a fully entangled state:

Experimentally achieved Remote State Preparation payoff for 58 distinct states of a Bloch sphere. Shown are the respective values for the two resource states $ \tilde{p}_w $(red) and $ \tilde{p}_B $ (blue). The dashed lines represent the theoretical expectations. There is a clear separation between the two resource states, which indicates that the separable state $ \tilde{p}_w $ is a better resource for RSP than the entangled state $ \tilde{p}_B $.

This raises the exciting prospect of a new approach to quantum computing that may not require the notoriously difficult preservation of entangled states,  giving hope that there may yet be a viable approach to quantum computing for the rest of us without requiring helium cooling infrastructure. Subsequently, quantum discord has become a research hot topic that spawned a dedicated site that helps keep track of the publications in this area.

At this point it is not obvious (at least to me) what impact these new insights on quantum discord will have in the long run, i.e. how do you develop algorithms to take advantage of this resource, and how will it, for instance, impact the channel capacity for quantum communication? (For a take on the latter see R.Tucci’s latest papers).

What seems clear, though, is that D-Wave has one more good argument to stress the inherent quantumness of their device.

There is a really poorly produced video lecture available on this subject by a co-author of the Quantum Discord paper. (If only the presenter stopped moving so that the camera would not have to be constantly and noisily adjusted). Possibly the point of this dismal production value is to illustrate headache-inducing discord. In that case the University of Oxford certainly succeeded spectacularly.

Diamonds are a Qubit’s Best Friend

A Nitrogen Vacancy in a diamond crystal isolates the nuclear spin yet makes it accessible via the hyperfine coupling.

A while ago, I looked into the chance that there would ever be a quantum computer for the rest of us. The biggest obstacle for this is the ultra-low temperature regime that all current quantum computing realizations require.  Although a long shot, I speculated that high temperature super-conductors may facilitate a D-Wave-like approach at temperature regimes that could be achieved with relatively affordable nitrogen cooling. Hoping for quantum computing at room temperature seemed out of the question. But this is exactly the tantalizing prospect that the recent qubit realization within a diamond’s crystal structure is hinting at – no expensive cooling required at all. So, ironically, the future quantum computer for the rest of us may end up being made of diamond.

A qubit requires a near perfectly isolated system i.e. essentially any interaction with the environment destroys the quantum information by randomly transitioning the pure qubit quantum state to a mixed ensemble state (a random superposition mixture of wavefunctions).  The higher the temperature, the more likely are these unwanted interactions via increased Brownian and thermal background radiation, a process know as decoherence.  Solid state qubit realizations are therefore always conducted at temperatures close to absolute zero, and require expensive Helium cooling.  Even under these conditions, qubits realized on superconducting chips don’t survive for very long. Their typical coherence time is measured in micro-seconds. On the other hand, ion-based systems can go for several minutes. While this is an obvious advantage, the challenge in using this design for quantum computing is the ability to initialize these systems into a known state, and the read-out detection sensitivity.  But great strides have been made in this regard, and in the same Science issue that the diamond results were presented, this article has been published that demonstrates a suitable system that exhibits quantum information storage for over 180 seconds.

All these coherence times are very sensitive to even the slightest temperature increase i.e. every millikelvin matters.  (This graph illustrates this for the first commercially available quantum computing design).

Key is to not have too many point defect.

It is in this context that the result of a successful qubit storage in a diamond lattice is almost breathtaking. A coherence time of over one second at room temperatureHarvard is not know for cutting edge experimental quantum computing research, so this result is surprising in more than one respect.

The diamond in question is artificially made and needs to contain some designer irregularites (but not too many of them):  These point defects replace a carbon atom in the diamonds crystal grid with a nitrogen one. If there are no other nitrogen vacancies nearby, the nuclear spin of this atom is very well isolated. Rivaling one would otherwise require close to absolute zero temperatures. On the other hand, this atom’s extra electron can readily interact with EM fields, and this is eactly what the researchers exploited. But there is more to it.

The really intriguing aspect is that this nuclear spin qubit in turn can be made to interact with the spin states of the excess electron, and the coherence times of both can be individually enhanced by suitably tuned laser exposure. The different coupling mechanisms are illustrated in the I came across this popular science write-up that does an excellent job in explaining this (long time readers know that I am rather critical of what usually passes as popular science, so I am delighted when I find something that I can really recommend).

The original paper concludes that additional coherence enhancing techniques could yield  jaw-dropping qubit storage of up to 36 hours at room temperature.

Of course, when everything else fails, physicists can always fall back on this novel approach, a song designed to scare a qubit to never come out of its coherent state:

More Lost Papers

Lost Papers Dropping from the Page of Time

As promised, the translation of the paper that contains Einstein’s last important contribution to modern physics has been completed.   Paul Terlunen graciously provided the initial translation, and my wife Sara helped immensely with the final editing.

Starting point for this effort was the news that Einsteins hand-written manuscript had been re-discovered.  This has been reported in various physics LinkedIn groups, and subsequently there was some interest to look at this paper in an English translation, but there was none readily available.

This is the second part of Einstein’s publications on this topic and I will now start on a translation for the first paper as well.

Nevertheless, if you already have some familiarity with quantum mechanics, you can read this last paper without having worked through the previous one.  It is intriguing to follow the author’s line of thought, and to share in his intrigue with the puzzling nature of the quantum statistics that were uncovered for the first time. To the modern reader, it is also interesting to see in hindsight where Einstein erred when speculating on the nature of the electron gas; At the time, he did not know that his statistics only applied to bosons, and that electrons would turn out to be fermions.



Vested in IT Security and Losing Sleep Over Quantum Computing?

If so, then this special edition of the IT security magazine hakin9 might be just what the doctor ordered.

Your humble blogger was asked to provide an article and so I contributed the “Who’s Afraid of the Big Bad Quantum Computer?” piece.

It is very smart of the editor staff of hakin9 to approach bloggers, as we already write material for free and an ample sample portfolio of articles can be readily perused. As with this blog I do not benefit in any way financially from this. For me this magazine is just another good venue to get the word out about this exciting technology that is now becoming an IT reality.

Regular Blogging Will Resume Shortly

Currently I am spending way too much time commenting on Scott Aaronson’s blog where the Joy Christian “Bell Inequality Disproof” controversy is still in full swing.  The latter also inpsired me to the new “QC Bet Tracker” page on this humble blog of mine.

Head over if you want to see a first class science imbroglio.