Category: Top Posts

Guest Post: John Preskill on Individual Quantum Systems

By Sean Carroll | October 10, 2012 6:12 pm

In the last post I suggested that nobody should come to these parts looking for insight into the kind of work that was just rewarded with the 2012 Nobel Prize in Physics. How wrong I was! True, you shouldn’t look to me for such things, but we were able to borrow an expert from a neighboring blog to help us out. John Preskill is the Richard P. Feynman Professor of Theoretical Physics (not a bad title) here at Caltech. He was a leader in quantum field theory for a long time, before getting interested in quantum information theory and becoming a leader in that. He is part of Caltech’s Institute for Quantum Information and Matter, which has started a fantastic new blog called Quantum Frontiers. This is a cross-post between that blog and ours, but you should certainly be checking out Quantum Frontiers on a regular basis.

When I went to school in the 20th century, “quantum measurements” in the laboratory were typically performed on ensembles of similarly prepared systems. In the 21st century, it is becoming increasingly routine to perform quantum measurements on single atoms, photons, electrons, or phonons. The 2012 Nobel Prize in Physics recognizes two of the heros who led these revolutionary advances, Serge Haroche and Dave Wineland. Good summaries of their outstanding achievements can be found at the Nobel Prize site, and at Physics Today.

Serge Haroche developed cavity quantum electrodynamics in the microwave regime. Among other impressive accomplishments, his group has performed “nondemolition” measurements of the number of photons stored in a cavity (that is, the photons can be counted without any of the photons being absorbed). The measurement is done by preparing a Rubidium atom in a superposition of two quantum states. As the Rb atom traverses the cavity, the energy splitting of these two states is slightly perturbed by the cavity’s quantized electromagnetic field, resulting in a detectable phase shift that depends on the number of photons present. (Caltech’s Jeff Kimble, the Director of IQIM, has pioneered the development of analogous capabilities for optical photons.)

Dave Wineland developed the technology for trapping individual atomic ions or small groups of ions using electromagnetic fields, and controlling the ions with laser light. His group performed the first demonstration of a coherent quantum logic gate, and they have remained at the forefront of quantum information processing ever since. They pioneered and mastered the trick of manipulating the internal quantum states of the ions by exploiting the coupling between these states and the quantized vibrational modes (phonons) of the trapped ions. They have also used quantum logic to realize the world’s most accurate clock (17 decimal places of accuracy), which exploits the frequency stability of an aluminum ion by transferring its quantum state to a magnesium ion that can be more easily detected with lasers. This clock is sensitive enough to detect the slowing of time due to the gravitational red shift when lowered by 30 cm in the earth’s gravitational field.

With his signature mustache and self-effacing manner, Dave Wineland is not only one of the world’s greatest experimental physicists, but also one of the nicest. His brilliant experiments and crystal clear talks have inspired countless physicists working in quantum science, not just ion trappers but also those using a wide variety of other experimental platforms.

Dave has spent most of his career at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. I once heard Dave say that he liked working at NIST because “in 30 years nobody told me what to do.” I don’t know whether that is literally true, but if it is even partially true it may help to explain why Dave joins three other NIST-affiliated physicists who have received Nobel Prizes: Bill Phillips, Eric Cornell, and “Jan” Hall.

I don’t know Serge Haroche very well, but I once spent a delightful evening sitting next to him at dinner in an excellent French restaurant in Leiden. The occasion, almost exactly 10 years ago, was a Symposium to celebrate the 100th anniversary of H. A. Lorentz’s Nobel Prize in Physics, and the dinner guests (there were about 20 of us) included the head of the Royal Dutch Academy of Sciences and the Rector Magnificus of the University of Leiden (which I suppose is what we in the US would call the “President”). I was invited because I happened to be a visiting professor in Leiden at the time, but I had not anticipated such a classy gathering, so had not brought a jacket or tie. When I realized what I had gotten myself into I rushed to a nearby store and picked up a tie and a black V-neck sweater to pull over my levis, but I was under-dressed to put it mildly. Looking back, I don’t understand why I was not more embarrassed.

Anyway, among other things we discussed, Serge filled me in on the responsibilities of a Professor at the College de France. It’s a great honor, but also a challenge, because each year one must lecture on fresh material, without repeating any topic from lectures in previous years. In 2001 he had taught quantum computing using my online lecture notes, so I was pleased to hear that I had eased his burden, at least for one year.

On another memorable occasion, Serge and I both appeared in a panel discussion at a conference on quantum computing in 1996, at the Institute for Theoretical Physics (now the KITP) in Santa Barbara. Serge and a colleague had published a pessimistic article in Physics Today: Quantum computing: dream or nightmare? In his remarks for the panel, he repeated this theme, warning that overcoming the damaging effects of decoherence (uncontrolled interactions with the environment which make quantum systems behave classically, and which Serge had studied experimentally in great detail) is a far more daunting task than theorists imagined. I struck a more optimistic note, hoping that the (then) recently discovered principles of quantum error correction might be the sword that could slay the dragon. I’m not sure how Haroche feels about this issue now. Wineland, too, has often cautioned that the quest for large-scale quantum computers will be a long and difficult struggle.

This exchange provided me with an opportunity to engage in some cringe-worthy rhetorical excess when I wrote up a version of my remarks. Having (apparently) not learned my lesson, I’ll quote the concluding paragraph, which somehow seems appropriate as we celebrate Haroche’s and Wineland’s well earned prizes:

“Serge Haroche, while a leader at the frontier of experimental quantum computing, continues to deride the vision of practical quantum computers as an impossible dream that can come to fruition only in the wake of some as yet unglimpsed revolution in physics. As everyone at this meeting knows well, building a quantum computer will be an enormous technical challenge, and perhaps the naysayers will be vindicated in the end. Surely, their skepticism is reasonable. But to me, quantum computing is not an impossible dream; it is a possible dream. It is a dream that can be held without flouting the laws of physics as currently understood. It is a dream that can stimulate an enormously productive collaboration of experimenters and theorists seeking deep insights into the nature of decoherence. It is a dream that can be pursued by responsible scientists determined to explore, without prejudice, the potential of a fascinating and powerful new idea. It is a dream that could change the world. So let us dream.”

CATEGORIZED UNDER: Guest Post, Science, Top Posts

Nobel Prize to Haroche and Wineland

By Sean Carroll | October 9, 2012 8:32 am

Nobody comes to these parts (at least, they shouldn’t) looking for insight into atomic physics, quantum optics, and related fields, but hearty congratulations to Serge Haroche and David Wineland for sharing this year’s Nobel Prize in Physics. Here are helpful stories by Alex Witze and Dennis Overbye.

One way of thinking about their accomplishments is to say that they’ve managed to manipulate particles one at a time: Haroche with individual photons, and Wineland with trapped ions. But what’s really exciting is that they are able to study intrinsically quantum-mechanical properties of the particles. For a long time, quantum mechanics could be treated as a black box. You had an atomic nucleus sitting there quietly, not really deviating from your classical intuition, and then some quantum magic would occur, and now you have several decay products flying away. The remoteness of the quantum effects themselves is what has enabled physicists to get away for so long using quantum mechanics without really understanding it. (Thereby enabling such monstrosities as the “Copenhagen interpretation” of quantum mechanics, and its unholy offspring “shut up and calculate.”)

These days, in contrast, we can no longer refuse to take quantum mechanics seriously. The experimentalists have brought it up close and personal, in your face. We’re using it to build things in ways we wouldn’t have imagined in the bad old days. This prize is a great tribute to physicists who are dragging us, kicking and screaming, into a quantum-mechanical reality.


Bias, Bias Everywhere

By Sean Carroll | October 8, 2012 2:00 pm

Admitting that scientists demonstrate gender bias shouldn’t make us forget that other kinds of bias exist, or that people other than scientists exhibit them. In a couple of papers (one, two), Katherine Milkman, Modupe Akinola, and Dolly Chugh have investigated how faculty members responded to email requests from prospective students asking for a meeting. The names of the students were randomly shuffled, and chosen to give some implication that the students were male or female, and also whether they were Caucasian, Black, Hispanic, Indian, or Chinese.

And the inquiries most likely to receive positive responses were the ones that came from … white males! You should pause a minute to collect yourself after hearing this shocking news. Here are the fractions of students who didn’t even get a response to their emails, and the fractions who were turned down for a meeting. (Biases aside, can you believe that over half of the prospective students who asked for a meeting were turned down?)

The results pretty much speak for themselves, and help to highlight the kinds of invisible biases that are impossible to detect directly but can end up exerting a large influence on the course of a person’s career. As previously noted, the first step to eradicating (or at least lessening) these kinds of distortions is to recognize that they exist. (Although a quick perusal of our comment sections should suffice to convince skeptics that the biases are very real, and oftentimes proudly defended.)

Interestingly, the studies didn’t only look at scientists, but at academics from a broad variety of disciplines, with dramatically different results. Read More

Digging Up the Early Universe

By Sean Carroll | October 2, 2012 8:31 am

I wrote another column for Discover (the actual magazine), which is now available online. It’s about how far back in cosmological time we can push our knowledge on the basis of actual data, not mere theory.

Of course we literally look back in time every time we peer into a telescope, since it takes time for light to travel to us from distant objects. But there’s an earliest moment we can possibly see using light — the moment of recombination, about 380,000 years after the Big Bang, when electrons hooked up with protons and other nuclei to form atoms. Earlier than that, the electrons were floating around freely, bumping into photons, and generally making the universe opaque.

So we have to be a bit more clever. And we have been: using the fact that the early universe was a nuclear fusion reactor, and observing the surviving abundances of light elements to pin down what conditions were like at that time. This technique gets us within seconds of the Big Bang. But if things break just right — the dark matter turns out to be a weakly-interacting particle, whose properties we can study here on Earth — we might be able to push the data-informed era much earlier back than that.

Think about what that means: Sitting here on Earth, cosmologists extrapolated our understanding back 13.7 billion years, to a few seconds after the universe began. We used that understanding to make predictions about the current universe—and we were right. We may not know for sure whether it will rain tomorrow, but we do know exactly how protons and neutrons bounced around like Super Balls in the nuclear inferno of the Big Bang. This will surely go down as one of the most impressive accomplishments of the human intellect.

And yet cosmologists want to do better still. The goal is to discover relics that predate even Big Bang Nucleosynthesis. At the moment that’s not quite possible, but there is one promising candidate: dark matter, the dense but unseen stuff that holds galaxies together.

Roughly speaking, if we get lucky, we could learn about conditions in the universe about 1/10,000th of a second after the Big Bang. We’d like to go even much earlier than that, but let’s not forget to be impressed at how well we’ve already done.


Guest Post: Joe Polchinski on Black Holes, Complementarity, and Firewalls

By Sean Carroll | September 27, 2012 2:55 pm

If you happen to have been following developments in quantum gravity/string theory this year, you know that quite a bit of excitement sprang up over the summer, centered around the idea of “firewalls.” The idea is that an observer falling into a black hole, contrary to everything you would read in a general relativity textbook, really would notice something when they crossed the event horizon. In fact, they would notice that they are being incinerated by a blast of Hawking radiation: the firewall.

This claim is a daring one, which is currently very much up in the air within the community. It stems not from general relativity itself, or even quantum field theory in a curved spacetime, but from attempts to simultaneously satisfy the demands of quantum mechanics and the aspiration that black holes don’t destroy information. Given the controversial (and extremely important) nature of the debate, we’re thrilled to have Joe Polchinski provide a guest post that helps explain what’s going on. Joe has guest-blogged for us before, of course, and he was a co-author with Ahmed Almheiri, Donald Marolf, and James Sully on the paper that started the new controversy. The dust hasn’t yet settled, but this is an important issue that will hopefully teach us something new about quantum gravity.


Thought experiments have played a large role in figuring out the laws of physics. Even for electromagnetism, where most of the laws were found experimentally, Maxwell needed a thought experiment to complete the equations. For the unification of quantum mechanics and gravity, where the phenomena take place in extreme regimes, they are even more crucial. Addressing this need, Stephen Hawking’s 1976 paper “Breakdown of Predictability in Gravitational Collapse” presented one of the great thought experiments in the history of physics. Read More

CATEGORIZED UNDER: Guest Post, Science, Top Posts

Let the Universe Be the Universe

By Sean Carroll | September 25, 2012 11:37 am

My article in the Blackwell Companion to Science and Christianity, which asks “Does the Universe Need God?” (and answers “nope”), got a bit of play last week, thanks to an article by Natalie Wolchover that got picked up by Yahoo, MSNBC, HuffPo, and elsewhere. As a result, views that are pretty commonplace around here reached a somewhat different audience. I started getting more emails than usual, as well as a couple of phone calls, and some online responses. A representative sample:

  • “Sean Carroll, servant of Satan…”
  • “God has a way of bring His judgement to those who mock Him… John Lennon stated “Christianity will end, it will disappear.” Lennon was shot six times after saying that… Marilyn Monroe said to Billy Graham after Graham said the Spirit of God had sent him to preach to her: “I don’t need your Jesus”. A week later she was found dead in her apartment.”
  • “See you in hell.”
  • “Maybe GOD is just a DOG that you will meet when you are walking on the Beach trying to figure out how to get sand out of your butt crack.”

I admit that last one is a bit hard to interpret. The others I think are pretty straightforward.

A more temperate response came from theologian William Lane Craig (a fellow Blackwell Companion contributor) on his Reasonable Faith podcast. I mentioned Craig once before, and here we can see him in action. I’m not going to attempt a point-by-point rebuttal of his comments, but I did want to highlight the two points I think are most central to what he’s saying. Read More

CATEGORIZED UNDER: Religion, Top Posts

Scientists, Your Gender Bias Is Showing

By Sean Carroll | September 19, 2012 3:44 pm

Nobody who is familiar with the literature on this will be surprised, but it’s good to accumulate new evidence and also to keep the issue in the public eye: academic scientists are, on average, biased against women. I know it’s fun to change the subject and talk about bell curves and intrinsic ability, but hopefully we can all agree that people with the same ability should be treated equally. And they are not.

That’s the conclusion of a new study in PNAS by Corinne Moss-Racusin and collaborators at Yale. (Hat tip Dan Vergano.) To test scientist’s reactions to men and women with precisely equal qualifications, the researchers did a randomized double-blind study in which academic scientists were given application materials from a student applying for a lab manager position. The substance of the applications were all identical, but sometimes a male name was attached, and sometimes a female name.

Results: female applicants were rated lower than men on the measured scales of competence, hireability, and mentoring (whether the scientist would be willing to mentor this student). Both male and female scientists rated the female applicants lower.

This lurking bias has clear real-world implications. When asked what kind of starting salaries they might be willing to offer the applicants, the ones offered to women were lower.

I have no reason to think that scientists are more sexist than people in other professions in the US, but this is my profession, and I’d like to see it do better. Admitting that the problem exists is a good start.

CATEGORIZED UNDER: Top Posts, Women in Science

Faith and Epistemological Quicksand

By Sean Carroll | September 17, 2012 9:49 am

Alvin Plantinga and Thomas Nagel are well-known senior philosophers at Notre Dame and NYU, respectively. Plantinga, a Christian, is known for his contributions to philosophy of religion, while Nagel, an atheist, is known (nevertheless) for his resistance to purely materialist/naturalist/physicalist theories of the mind (e.g. in his famous article, “What is it like to be a bat?“).

Now Nagel has reviewed Plantinga’s most recent book in the NYRB, giving it a much more sympathetic reading than most naturalists would offer. (For what it’s worth, Plantinga is a supporter of Intelligent Design, and Nagel has often spoken of it approvingly, while not quite buying the whole sales pitch.) Jerry Coyne offers a reasonable dissection of the review.

I wanted to home in on just one particular aspect because it was instructive, at least for me. There is a long-standing claim that “faith” is a way of attaining knowledge that stands independently of other methods, such as “logic” or “empiricism.” I’ve never quite understood this — how do we decide what to have faith in, if not by the use of techniques such as logic and empiricism? Read More

CATEGORIZED UNDER: Philosophy, Religion, Top Posts

Electrical Resistance

By Sean Carroll | September 13, 2012 11:31 am

A little while back, an anecdote was being passed around by liberal folks on Facebook that made Ann Romney look pretty bad. Apparently she said that a woman in the workforce “should be happy just to be out there in the working world and quit complaining that she’s not making as much as her male counterparts.” Even by the relatively relaxed standards that are rightfully applied to the families of political candidates rather than the candidates themselves, that sounded a little tone-deaf to me. So I checked on and, indeed, found out that the story was completely false. It was made up by a humor site, and then picked up by people who don’t like Romney, who were willing to take it at face value. As ridiculous as any particular claim may be, confirmation bias nudges us toward greater credulity when we are faced with stories that we want to believe are true.

Which brings us to the Chevy Volt, the electric car from General Motors. One of the blogs I generally read is Outside the Beltway, which is a group of conservatives who are more than willing to decry the worst excesses of conservatives as well as liberals. I generally don’t agree with them (except for the decrying), but they say a lot of interesting things. Doug Mataconis, one of the bloggers there, fell quite a bit short of that standard in a recent post about the Volt.

Mataconis, relying on an equally silly Reuters article, tells us that GM loses $50,000 every time it sells a Volt. The attitude of the post is simple — “maybe I’m no fancy businessman, but even I know that it’s not a good strategy to keep building cars and selling them at a tremendous loss!”

Well, that would be a bad strategy. So bad, in fact, that it might be advisable to pull back a bit and ask if that’s what’s actually happening. Read More

New Life for Gravitational Waves in Space?

By Sean Carroll | September 11, 2012 8:42 am

Last year we brought the bad news that NASA had pulled back from the LISA project, an ambitious proposal to build a gravitational wave detector in space. The science reach of LISA would be amazing, teaching us a great deal about black holes, general relativity, and cosmology.

Fortunately, the European Space Agency did not give up on the idea, and has kept it in the queue of possibilities without actually saying they will do it. They began to design a somewhat down-scaled mission, now dubbed NGO for “New Gravitational wave Observatory.” (Hey, nobody said NASA had a monopoly on dopey acronyms.) NGO was put into the hopper along with two other proposals as part of a selection process to decide on the ESA’s next large-scale mission, dubbed L1 (“L” for “large”), as part of the Cosmic Vision program. It lost out to JUICE, a mission to Jupiter’s moons with admittedly a much cooler acronym as well as some very good science behind it.

But if there is an L1, that implies that someday there might be an L2, and NGO is still in the running to be Europe’s next big mission in astrophysics from space. Read More


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