Ten Years of Quantum Coherence and Decoherence

In 2010 I attended a meeting on Quantum Coherence and Decoherence in Benasque, Spain. I've reproduced part of my original blog post on the meeting below.

 

September 13, 2010  

Here are the slides for my talk today at Benasque: On the origin of probability in quantum mechanics.

At the end I took a poll of the workshop participants and found that over half agreed with the following statement. About 20 percent were strongly opposed. Note this is a meeting on quantum coherence and decoherence, so there are a lot of practical types here, including experimentalists.

It is plausible (but of course unproven) that unitary evolution of a pure state in a closed system can reproduce, for semi-classical creatures inside the system, all of the phenomenology of the Copenhagen interpretation.

As one insightful participant pointed out while I was taking the poll, this is really a mathematical question (if not entirely well-posed), not a physics question.

My recent paper with Roman Buniy: Macroscopic Superpositions in Isolated Systems answers the mathematical question about the dynamics of complex isolated systems under Schrodinger evolution. I had forgotten entirely about the poll in the intervening years (I only came across the blog post by accident recently), but the question persisted... Only in 2020 did I realize that von Neumann's Quantum Ergodic Theorem [1] [2] can be used to prove the result.

Video and slides explaining the paper, published in Foundations of Physics, volume 51, 85 (2021).

Some Benasque photos from 2010 :-)

Added from comments: 

There are really multiple issues here. Theorists will differ in their opinions on the following questions: 

 

1. (largely mathematical): Does the phenomenology of pure state evolution in a closed system (e.g., the universe) reproduce Copenhagen for observers in the system? 

This is a question about dynamical evolution: of the system as a whole, and of various interacting subsystems. It's not a philosophical question and, in my opinion, it is what theorists should focus on first. Although complicated, it is still reasonably well-posed from a mathematical perspective, at least as far as foundational physics questions go. 

I believe the evidence is strong that the answer to #1 is Yes, although the issue of the Born rule lingers (too complicated to discuss here, but see various papers I have written on the topic, along with other people like Deutsch, Zurek, etc.). It is clear from Weinberg's writing that he and I agree that the answer is Yes, modulo the Born rule. 

Define this position to be 

Y* := "Yes, possibly modulo Born" 

There are some theorists who do not agree with Y* (see the survey results above), but they are mostly people who have not thought it through carefully, in my opinion. I don't know of any explicit arguments for how Y* fails, and our recent results applying the vN QET strengthen my confidence in Y*. 

I believe (based on published remarks or from my personal interactions) that the following theorists have opinions that are Y* or stronger: Schwinger, DeWitt, Wheeler, Deutsch, Hawking, Feynman, Gell-Mann, Zeh, Hartle, Weinberg, Zurek, Guth, Preskill, Page, Cooper (BCS), Coleman, Misner, Arkani-Hamed, etc. 

But there is a generational issue, with many older (some now deceased!) theorists being reticent about expressing Y* even if they believe it. This is shifting over time and, for example, a poll of younger string theorists or quantum cosmologists would likely find a strong majority expressing Y*. 

[ Social conformity and groupthink are among the obstacles preventing broader understanding of question #1. That is, in part, why I have listed specific high profile individuals as having reached the unconventional but correct view! ]

2. Does this make you confident that the other branches really "exist"? They are "real"? 

Here we get into philosophical questions and you will get a range of answers. 

Many of the Y* theorists (including me) might say:

a. MW is the only logically complete version of QM we have. Copenhagen is not well-defined and inadequate for cosmology (cf density perturbations from inflation and galaxy formation). 

b. I find the existence of the other branches rather extravagant, and I leave open the possibility that there might be some more fundamental modification of QM that changes everything. But I have no idea what that model looks like and there are strong constraints on its properties from Bell, causality, etc. Even a small amount of nonlinearity in the Schrodinger equation leads to lots of causality violation, etc. etc. 

 

c. I believe that any practical experiment that tries to check whether unitary evolution always holds (i.e., the other branches are *in principle accessible*) will always find it to be the case. In particular this means we will realize and manipulate more and more complicated superposition states over time, and this raises the question of why you and I cannot be in a superposition state right now... 

Note it is possible that only one single decoherent branch of the universal wavefunction is actually realized by Nature ("is real"), and that quantum randomness is an illusion. Hartle and Gell-Mann were sort of hedging this way in some of their last papers on this topic. But remember Gell-Mann even hedged about the reality of quarks before they were directly observed in deep inelastic scattering. 

An aspect to this problem that few theorists appreciate is that a quantum theory of gravity is, at the global level, "timeless": it should be a theory of quantum amplitudes describing an entire spacetime geometry and quantum trajectories of other degrees of freedom on that manifold. As such the many branches of the universal wavefunction are realized "all at once" and concepts like observers must be emergent -- they cannot be fundamental aspects of the theory itself. 

Most of the action in quantum gravity (i.e., strings or loop qg) has been "local" in nature: what are the stringy excitations, compactification, local vacua, etc. The global wavefunction of the universe was already considered by Wheeler and DeWitt but there are still lots of unresolved issues.

Ditchley Foundation meeting: China Today and Tomorrow

China Today and Tomorrow 

20 MAY 2021 - 21 MAY 2021 

This Ditchley conference will focus on China, its internal state and sense of self today, its role in the region and world, and how these might evolve in years to come. 

There are broadly two current divergent narratives about China. The first is that China’s successful response to the pandemic has accelerated China’s ascent to be the world’s pre-eminent economic power. The Made in China 2025 strategy will also see China take the lead in some technologies beyond 5G, become self-sufficient in silicon chip production and free itself largely of external constraints on growth. China’s internal market will grow, lessening dependence on exports and that continued growth will maintain the bargain between the Chinese people and the Chinese Communist Party through prosperity and stability. Retaining some elements of previous Chinese strategy though, this confidence is combined with a degree of humility: China is concerned with itself and its region, not becoming a global superpower or challenging the US. Economic supremacy is the aim but military strategy remains focused on defence, not increasing international leverage or scope of action. 

The second competing narrative is that China’s position is more precarious than it appears. The Belt and Road Initiative will bring diplomatic support from client countries but not real economic gains. Human rights violations will damage China abroad. Internally the pressures on natural resources will prove hard to sustain. Democratic and free-market innovation, combined with a bit more industrial strategy, will outstrip China’s efforts. Careful attention to supply chains in the West will meanwhile reduce critical reliance on China and curb China’s economic expansion. This perceived fragility is often combined though with a sense of heightened Chinese ambition abroad, not just through the Belt and Road Initiative but in challenging the democratic global norms established since 1989 by presenting technologically-enabled and effective authoritarian rule as an alternative model for the world, rather than just a Chinese solution. 

What is the evidence today for where we should settle between these narratives? What trends should we watch to determine likely future results? ...

[Suggested background reading at link above.] 

Unfortunately this meeting will be virtual. The video below gives some sense of the unique charm of in-person workshops at Ditchley.

See also this 2020 post about an earlier Ditchley meeting I attended: World Order Today

... analysis by German academic Gunnar Heinsohn. Two of his slides appear below.

1. It is possible that by 2050 the highly able STEM workforce in PRC will be ~10x larger than in the US and comparable to or larger than the rest of the world combined. Here "highly able" means roughly top few percentile math ability in developed countries (e.g., EU), as measured by PISA at age 15. 

[ It is trivial to obtain this kind of estimate: PRC population is ~4x US population and fraction of university students in STEM is at least ~2x higher. Pool of highly able 15 year olds as estimated by PISA or TIMMS international testing regimes is much larger than in US, even per capita. Heinsohn's estimate is somewhat high because he uses PISA numbers that probably overstate the population fraction of Level 6 kids in PRC. Current PISA studies disproportionately sample from more developed areas of China. At bottom (asterisk) he uses results from Taiwan/Macau that give a smaller ~20x advantage of PRC vs USA. My own ~10x estimate is quite conservative in comparison. ]

2. The trajectory of international patent filings shown below is likely to continue. 

London and Tallinn

I'll be speaking about AI and Health in Tallinn, after a stop in London to help the Tories prepare for the upcoming general election ;-)

Tallinn Digital Summit is where the frontrunners of digital nations drive the global conversation on digitalization.

Over the course of a day political leaders, policy innovators, thought-leaders, entrepreneurs and tech-community spotlight the most topical matters of digital transformation and tackle questions about its implications on economies, societies and governments. TDS is an annual meeting place for enhancing practical sharing of ideas and lessons to chase the opportunities of digital transformation for economy, e-governance development as well as societies. Also, to shape a more coherent approach to challenges brought by digital transformation.

Being one of the most digitally advanced countries, Estonia is an ideal location for the event. It has significant experience in building a digital society and economy, having built its digital core on secure distributed architecture. The country is also an outsized creator and exporter of startups, and possesses considerable cybersecurity expertise. Tallinn also hosts the HQ of NATO Cyber Defence Centre of Excellence and the European Agency for the Operational Management of Large-Scale IT Systems.

Ditchley Foundation Conference: The intersection of machine learning and genetic engineering

I'll be back in the UK soon for the meeting described below. Above, Ditchley House.

The intersection of machine learning and genetic engineering: what should be our check list for society and state as we blast off?

07 - 09 FEB 2019

Advances in machine learning and genetic engineering are combining to produce rapid advances in medicine, development of materials and genetic engineering. Parallel advances in robotics and automation have made the practical process of gene editing scalable. The possibility exists that advances in quantum computing could further accelerate progress on machine learning, bringing a second boost to this technological rocket.

This Ditchley conference will bring together an unusual mix of deep expertise and scientific renown in the disciplines; thinkers on religion, ethics and law; investors fueling innovation; and political leaders looking to shape the approach of society and state to fast emerging possibilities. We will attempt to establish sufficient common understanding of what the science promises and what it doesn’t and then explore the opportunities and risks that are likely to unfold at speed. This will be a first pass at preparation for potential blast off – what should be our moral, legal, economic and national security checklist as we wait on the launch pad of a new age?

The progress on machine learning is quite narrow in scope – deep learning using neural networks and other techniques on large data sets that now exist that didn’t previously and that are store-able and computable in a way that was not possible previously. But whereas progress towards general AI is often overstated, full general AI is not required to radically accelerate gene sequencing, editing and programming, with costs falling all the time and scale and speed increasing.

We will examine and try to come to preliminary conclusions on questions such as the following:

How should the most aggressive genetic engineering technologies be regulated?

How can societies best assess the ethical issues raised by these technologies to find an optimal balance between fostering genetic technologies for the common good while preventing abuse?

What are the implications for the global economy and economic cooperation and competition between states? Are we entering a period of bio-nationalism as well as AI nationalism? Should this be compared to the space race of the Cold War? How can we avoid competition between states driving abandonment of norms and moral standards? What will be the impact on the labour force of the new combined technologies of AI and bio-engineering? Within countries, will potential applications of the new technologies further intensify the concentration of wealth and power in a few hands?

What are the implications of rapid combined advances in AI and bioengineering for defence and national security? Will countries be tempted to pursue military applications either through bio-weapons or through the genetic improvement of military forces? What new materials will emerge and how will they affect the balance of power in warfare?

What are the implications for medicine and public health? If we are able to find targeted genetic cures for diseases like cancer then what will the impact be on the population? What are the implications for ageing or declining populations?

How should we handle the implications of deeper knowledge about the influence of our genes on our characteristics and on the characteristics of groups? How do we chart a course between remaining scientifically objective and providing material that could be misused to support racist conclusions by those tending to that view?

What opportunities and threats are there in the potential of these combined technologies for democracies and the equal value put on the view point of each citizen in the electoral system and the rule of law? More philosophically, how can we make sure the development of these technologies contributes to a positive sense of human progress and meaning, rather than to a sense of alienation and loss of purpose? How can we manage the tension between science and religion as human capability to shape the genetic world increases?

I'm only briefly in London on my way there, but might be able to squeeze in a few meetings :-)

See also:

The Future of IVF and Gene-Editing (Psychology Today interview)

The Future is Here: Genomic Prediction in MIT Technology Review

Genomic Prediction of Complex Disease Risk (bioRxiv)

ASHG 2018

See you in San Diego! American Society of Human Genetics.

My schedule is so booked with meetings that I wonder whether I will be able to attend any talks... (Everyone in the genomics world seems to be attending this event.)

Nevertheless, if you see me wandering around please say HI!

Illumina International Summit on Population Genomics 2018

Illumina International Summit on Population Genomics 2018

The aim of this meeting is to bring together individuals, initiatives, and projects that are shaping the future of genomics in healthcare. This meeting provides a forum for international networking, shared learning and collaboration to address the challenges in realising the potential of genomics to improve human health. The success and accumulated experience of local programmes provide potentially great synergies around the globe. We hope that you will be able to join us and offer your insights and passion for a future where genomics sits at the innovation centre of global healthcare.

I'm in the UK again for this conference and some secret meetings in London. See also London Calling.

LATTICE 2018 at MSU

The 36th Annual International Symposium on Lattice Field Theory begins tomorrow, hosted by MSU. My opening remarks are below. No peeking if you are an attendee!

LATTICE 2018 Opening Remarks 7/23/2018

Good morning. I’d like to extend my warmest welcome to all of you on behalf of Michigan State University. We are very pleased and honored to be the hosts for The 36th Annual International Symposium on Lattice Field Theory.

It is my opinion that even within Physics, and even within Theoretical Physics, Lattice Field Theory is underappreciated. The idea that we can constructively realize quantum field theories in silico, that we can perform precision calculations in the deepest models of fundamental physics, is really incredible. It has taken many decades to get to this point: to master strongly coupled quantum fluctuations, spacetime trajectories of quantum fields like quarks and gluons, advanced algorithms and hardware designs, matching to effective field theories, and many other conceptually beautiful but ultimately concrete things.

Along with some recent AI advances like AlphaGo, the precise ab initio calculation of physical quantities in lattice QCD must be considered among the most impressive computations performed by the human species. If some Alien visitors were evaluating the accomplishments of our civilization, I would want them to take into account the work of people here today.

I first became aware of lattice gauge theory from John Preskill’s lecture notes for Physics 234, a year-long Caltech course on advanced topics in QCD. I never imagined, back in the 1980s, the successes that all of you have achieved today. The important message to young people is that one should not be dissuaded from attempting difficult projects.

At MSU we made the decision a few years ago to invest in lattice physics. We went from no lattice researchers, to one of the larger groups in the US. One of the drivers for this decision was the hope that lattice simulations would one day connect QCD to the experimental results coming from FRIB -- the MSU / DOE Facility for Rare Isotope Beams. Today we can compute, from first principles, the properties of light hadrons. In the coming decades, I believe we will compute real time scattering amplitudes and nuclear forces from QCD itself.

DOE and MSU are investing, all told, roughly a billion dollars in FRIB. While it is the Experimentalists who build and run the machine, and deserve the main credit, we as Theorists have the responsibility to ensure that the results of the experiment inform our deeper understanding of nuclear physics and QCD. Physicists are not stamp collectors -- we do not measure things just to measure them. We measure things which are important and have deep implications.

To reach the long awaited goal of connecting nuclear physics directly to QCD, we depend on the lattice community, on all of you. May the next 30 years see as much progress as the last.

Thank you very much.

Action photos!

AI Lollapalooza in Stockholm

The Rise of AI (Bloomberg Hello World documentary)

Great profile of Geoff Hinton, Yoshua Bengio, etc., but covers many other topics.

Note to readers: I'll be at the 35th International Conference on Machine Learning (ICML 2018) in Stockholm, Sweden (July 10-15, 2018), giving a talk at the Reproducibility in ML Workshop.

Let me know if you want to meet up!

Farewell Asia, Hello Scifoo

Apologies for the lack of blog posts. I've been on the road in Asia and quite busy for the past week. I head back to the bay area for Scifoo this weekend. See you there!

Personalized Medicine in HK

I'm on my way to a Gordon Conference on Personalized Medicine at HKUST. It will be held in their Institute for Advanced Study, pictured above.

Lake Como and Florence photos

I was in Lake Como for a small meeting of ~30 people, including sovereign wealth, hedge, and pension fund heads, plus a few intellectuals and leading figures from government. Brexit made for extra excitement in our discussions. Hint to scaremongers: the smart money is not as scared as you have tried to make the public.

I can't really share many photos from that meeting, which was held at two large villas on the lake, one a hotel and the other a private estate. Most of the photos below are from Florence. In the first photo below I'm giving some after dinner remarks at the Como meeting.