Category Archives: Quantum physics and information

Quantum computing is exciting and important—really!

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Quantum computing, say its champions, promises prodigious power. Its basic currency, the qubit, exists in an on/off limbo until it’s read out, so if you could operate on k qubits, a potentially vast space of 2k values opens up for computation. The fundamental operation on qubits is a rotation. Combine the rotations, and you have logic gates. Combine the logic gates, and you have algorithms. In principle, these algorithms can perform calculations far beyond classical computing’s conceivable reach.

But to wield that power, you need an actual quantum computer, and building one has proved impossible. Qubits live in small, cold enclaves within the classical macroworld. When heat and other environmental disturbances inevitably intrude, they rob a quantum system of its coherence, its entanglement, and its ability to compute.

So beguiling is the potential of quantum computers that rather than putting people off, the difficulty of building one has assumed the qualities of a mythical quest. Like Jason’s for the Golden Fleece, the quest for a quantum computer is hard and long. To sustain it, the champions of quantum computing appeal not to Olympian gods but to terrestrial funding agencies. Not surprisingly, quantum computing has acquired an aura of hope—and hype. Researchers have made steady progress, though. Physicists have fashioned qubits from superconducting Josephson junctions, trapped ions, semiconducting quantum dots, and other systems. They’ve even built working logic gates.

Still, scaling up a handful of logic gates, whose physical embodiments could require a roomful of lasers, cryopumps, and other finicky equipment, to an actual computer remains out of reach. Rolf Landauer, the IBM physicist who pioneered the notion that information is intrinsically physical, was famously skeptical of quantum computing. All papers on the topic, he said, should come with a disclaimer, and if you didn’t have one, he was happy to offer his own:

This proposal, like all proposals for quantum computation, relies on speculative technology, does not in current form take into account all possible sources of noise, unreliability and manufacturing error, and probably will not work.

Landauer’s skepticism could prove justified in the end, but it would be a pity if research in quantum computing stopped now. Much of it continues to be worthwhile. At NIST’s lab in Boulder, Colorado, for example, David Wineland and his collaborators have applied the techniques they developed for atomic clocks to build logic gates based on trapped ions. Thanks to their work on logic gates, they developed new, entanglement-based clocks of unprecedented precision.

In making qubits out of gallium arsenide quantum dots, Jason Petta, who is now at Princeton University, and his collaborators at Harvard measured the tiny fluctuating magnetic field of 106 gallium and arsenic nuclei inside a quantum dot—a remarkable feat.

Results have been just as impressive on the theoretical front. The work of Microsoft’s Alexei Kitaev and others on topological quantum computation has spawned rich and fruitful explorations of the mathematical similarities of field theory, knots, and the fractional quantum Hall effect. Princeton’s Robert Calderbank has applied the theory of quantum error correction to understand radar polarimetry, and Ignacio Cirac and Frank Verstraete of the Max Planck Institute for Quantum Optics outside Munich have used the entangled states that crop up in quantum information theory to analyze networks of coupled spins.

Do all these advances, and others, represent milestones on a longer, ultimately successful journey or the ends of truncated trips? I don’t know. But they’re exciting and important—really.

This essay by Charles Day first appeared on page 104 of the March/April 2007 issue of Computing in Science & Engineering, a bimonthly magazine published jointly by the American Institute of Physics and IEEE Computer Society.

My Nobel wish list

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My record for predicting the winners of Nobel prizes is mixed. The last time I made a public prediction was two years ago. I correctly picked Konstantin Novoselov and Andre Geim as winners, but I thought their work on graphene—by analogy with the work of Robert Curl, Harold Kroto, and Richard Smalley on buckyballs—would earn the pair the chemistry prize. The only prize I got completely correct was Mario Vargas Llosa’s for literature.

This year, rather than make predictions, I’ve decided to identify who I hope will win the prizes I care about the most: physics, chemistry, physiology or medicine, and literature.

Physics

One of the topics of enduring interest to physicists is the boundary between the realms of quantum and classical behavior. In 2004 I wrote a news story about an ingenious experiment that explored that boundary. Markus Arndt, Anton Zeilinger, and their colleagues at the University of Vienna sent buckyballs through a pair of closely spaced slits.

When the molecules were cold, they behaved like quantum objects and formed interference fringes after passing through the slits. But when the molecules were hot, the coherent fringes disappeared. Evidently, the molecules’ temperature and emission of thermal photons—not their size or mass—demarked the quantum–classical boundary.

That story was my first direct encounter with research on how the environment influences quantum behavior. The second came in 2009 when I wrote about a calculation that resolved a 82-year-old quantum paradox: Why is a chiral molecule found in either its left-handed or right-handed isomeric forms and not in a superposition of the two?

To reach their answer, Klaus Hornberger and Johannes Trost of Ludwig-Maximilians University calculated the most probable states of a deuterated dihydrogen dilsulfide molecule in the presence of helium atoms. At room temperature, once the pressure exerted by the He atoms exceeded 1.6 × 10−5 mbar, the He atoms would kick the D2S2 molecule out of a mix of superpositions and into either its left-handed or right-handed form.

As I noted in my story, that a calculation could precisely locate a quantum–classical boundary is both mundane and profound—mundane, because the calculation made use of standard, unadulterated quantum mechanics; profound, because it demystified the quantum–classical boundary.

The physicist who has done the most to advance the notion that the environment, when fully and properly accounted for, drives the quantum–classical boundary is Los Alamos National Laboratory’s Wojciech Zurek. I hope he’s awarded the physics prize.

Chemistry

The discovery, published in February 2008, of superconductivity in a compound that contains iron and arsenic touched off an explosion of research that continues to this day. Several branches of the family of iron-based superconductors have since been discovered.

Although no family member’s critical temperature can yet match the highest of the cuprates, the iron-based superconductors are significant because their superconductivty, like that of the cuprates, is mediated by electron–electron interactions. Evidence is building that the pairing symmetry is not d-wave, as in the case of the cuprates, but is a form of s-wave.

The iron-based superconductors, therefore, demonstrate that high-temperature superconductivity is not limited either to the cuprates or to the precise form it takes in the cuprates. Other chemical families, as yet undiscovered, could have still higher critical temperatures.

Hideo Hosono of the Tokyo Institute of Technology made the discovery. I hope he is awarded the chemistry prize.

Physiology or medicine

The last time pharmacology was honored with a Nobel Prize was in 1988, when James Black, Gertrude Elion, and George Hitchings shared the award “for their discoveries of important principles for drug treatment.” This year, I hope that Ravinder Maini and Marc Feldmann of Imperial College London are rewarded for identifying tumor necrosis factor as a potential (and now effective) drug target for treating inflammatory diseases, such as rheumatoid arthritis.

Literature

The Wikipedia entry on William Trevor, whose photo appears above, begins like this:

William Trevor, KBE (born 24 May 1928) is an Irish author and playwright. One of the elder statesmen of the Irish literary world, he is widely regarded as one of the greatest contemporary writers of short stories in the English language.

If the Swedish Academy can suspend its habitual political posturing and instead reward sensitivity, sympathy, and skill, then it might just bestow the literature prize on Trevor. Doing so would honor not just him, but two great writers whose work inspired him and who weren’t awarded Nobel prizes: Anton Chekhov and James Joyce.

From basic to applied in 83 years

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In June 1929 a paper by the 23-year-old Nevill Mott appeared in the Proceedings of the Royal Society of London. As Mott noted in his introduction, theoretical arguments and empirical evidence supported the notion that electrons have an intrinsic magnetic moment, or spin. “The question arises,” he wrote, “has the free electron ‘really’ got a magnetic moment, a magnetic moment that we can by any conceivable experiment observe?”

Mott’s question is subtler than it might first appear. If you turn to the paper’s appendix, you’ll find what Niels Bohr told Mott: The uncertainty principle forestalls any attempt to distinguish an electron’s intrinsic magnetic moment from the magnetic field that arises from its motion. But, as Mott (shown here) demonstrates in his paper, it is possible to quantify the intrinsic magnetic moment because it turns out the probability that an electron scatters off an atomic nucleus in a given direction depends on the orientation of the electron’s spin.

Forty-two years after Mott’s paper was published, Mikhail Dyakonov and Vladimir Perel of the Ioffe Institute in Leningrad found a similar effect in semiconductors. According to their theoretical analysis, an electric field applied along a strip of semiconductor drives electrons to scatter off impurities in a spin-dependent way: Those with up spins veer to one side, while those with down spins veer to the other side.

Dyakonov and Perel’s paper did not attract much attention. Indeed, in 1999 Jorge Hirsch reproduced the analysis without either himself or—one assumes—his paper’s reviewers being aware of its Russian antecedent. He called the phenomenon the spin Hall effect.

The effect that Dyakonov, Perel, and Hirsch predicted depends on the presence of extrinsic impurities. But in 2003 two groups of theorists—Shuichi Murakami, Naoto Nagaosa, and Shou-Cheng Zhang; Jairo Sinova, Allan MacDonald, and their collaborators—independently proposed that a spin Hall effect could arise intrinsically when spin–orbit coupling of electrons to the lattice atoms acts with the applied electric field to change the semiconductor’s band structure.

Controlling electron spins through electric fields is technologically enticing. Murakami, Nagaosa, and Zhang wrote in the 2003 Science paper:

Principles found here could enable quantum spintronic devices with integrated information processing and storage units, operating with low power consumption and performing reversible quantum computation.

What’s more, because electric fields ultimately arise from static charges, they constitute a finer, faster, and more convenient means to control spins than do magnetic fields, which ultimately arise from moving charges.

Weak then strong

I did not become acquainted with the spin Hall effect’s history through deep, broad study of 20th-century physics. Rather, in 2005 I wrote a news story about the effect’s experimental verification. David Awschalom, his graduate students Yuichiro Kato and Roberto Myers, and Art Gossard detected the effect through a spin-dependence in the polarization of reflected light. Independently, Jörg Wunderlich, Bernd Kästner, Sinova, and Tomas Jungwirth looked instead for the circularly polarized light emitted by an LED when spin-polarized electrons and holes recombine.

Both experiments were tours de force of ingenuity and execution. They needed to be. The spin Hall effect in gallium arsenide, the material the two groups used, is weak—too weak, perhaps, form the basis of a industrially significant technology. Nevertheless, the spin Hall effect has been one of the past decade’s most fruitful areas of research. There’s now a quantum spin Hall effect, a spin Hall effect for light, and an inverse spin Hall effect.

Now comes a paper that reports a spin Hall effect of technologically interesting magnitude. In the 4 May issue of Science, Cornell University’s Robert Buhrman and his collaborators report the discovery of what they call a massive spin Hall effect in the brittle, semimetal β phase of tantalum. The effect, which works at room temperature, is strong enough to flip the spins in an adjacent ferromagnet.

That spin-flipping ability could form the basis of nonvolatile computer memory—that is, memory that isn’t wiped clean whenever you turn off the power. In fact, to demonstrate β-tantalum’s promise, Buhrman’s team built nanodevices whose active components consisted of few-nanometer-thick layers of β-tantalum, cobalt iron boron (a ferromagnet), and magnesium dioxide (to improve the ferromagnet’s performance).

The prototype device did indeed work, at room temperature, as a magnetization switch. The switching current Ic is higher than that of magnetic tunnel junctions (MTJs), which use spin-polarized currents to perform the switching. However, Buhrman anticipates that with routine optimization Ic could be reduced to the point that spin Hall devices would compete with MTJs.

Mott’s 1929 paper, his first, is characteristic of his early interest in atomic and nuclear physics. In 1933 he took up a professorship at the University of Bristol, and from then on he devoted himself to the field in which he received a Nobel prize: condensed matter physics. Mott died in 1996 at the age of 90—too soon, unfortunately, to learn what his theoretical paper in atomic physics had begotten 83 years later.

Edward Condon’s reflections on the first 60 years of quantum physics

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On 2 December 1960 Edward Condon stood in the auditorium of the Natural History Museum in Washington, DC, to address the 1500th meeting of the Philosophical Society of Washington. The topic of his talk was another scientific milestone. Sixty years before, at the 19 October meeting of the German Physical Society in Berlin, Max Planck presented his radiation formula for the first time; quantum mechanics made its public debut.

Edward Condon

Condon (shown here) was well qualified to survey the history and progress of quantum physics. After earning his PhD in physics in 1926 at the University of California, Berkeley, he moved to Goettingen to work with Max Born. That same year he published what is perhaps his most famous contribution to physics: His quantum mechanical extension of James Franck’s semiclassical description of vibronic transitions in molecules. In 1929 he and Philip Morse wrote Quantum Mechanics, the first English-language textbook on the topic.

Besides witnessing and participating in the establishment of quantum mechanics, Condon had another early experience that I think prepared him for delving into the subject’s history. Between leaving high school and attending university, he spent three years as a reporter for the Oakland Inquirer and other newspapers.

A reporter’s curiosity and tenacity are evident in Condon’s Washington talk, which appeared in written form in the October 1962 issue of Physics Today. Fascinated by how Lord Rayleigh and other great old physicists of the time struggled to accommodate Planck’s formula within their classical worldviews, he dug into their papers and memoirs and quoted them extensively. Even Planck himself had difficulty, as evidenced from the excerpt that Condon quoted from Planck’s autobiography:

My futile attempts to fit the elementary quantum of action somehow into the classical theory continued for a number of years [actually until 1915] and they cost me a great deal of effort. Many of my colleagues saw in this something bordering on a tragedy. But I feel differently about it, for the thorough enlightenment I thus received was all the more valuable. I now knew for a fact that the elementary quantum of action played a far more significant part in physics than I had originally been inclined to suspect, and this recognition made me see clearly the need for the introduction of totally new methods of analysis and reasoning in the treatment of atomic problems.

In all, Condon devoted five of nine pages of his Physics Today article to his inquiries into the acceptance of quantum mechanics among physicists. That editorial choice, plus his emphasis on his own fields of study, atomic and nuclear physics, left him little room to cover the application of quantum mechanics to condensed matter and field theory. Still, I urge you to read the fascinating article.

And if you want to learn more about Condon, I recommend another Physics Today article. In “Edward Condon and the cold war politics of loyalty,” which appeared in December 2001, historian Jessica Wang discussed the groundless political persecution that Condon faced during his distinguished and productive career.

Is weak measurement more than an experimental tool?

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“You cannot measure a quantum particle without disturbing it. Or can you? Weird ‘weak measurements’ are opening new vistas in quantum physics.”

Thus reads the dek that tops Adrian Cho’s excellent news story in last week’s issue of Science. Adrian brought me and other readers up to date on the applications and implications of weak measurement, a concept that debuted in a 1988 paper in Physical Review Letters by Lev Vaidman, Yakir Aharonov, and David Albert.

My first encounter with weak measurement came in 2008 when I wrote about a paper in Science by Onur Hosten and Paul Kwiat. The paper described the observational confirmation of a spin Hall effect for light, which arises when a linearly polarized beam (purple in the figure) refracts at a boundary. Interaction between the beam’s polarization or spin degree of freedom and its geometric or orbital degree of freedom causes its right-hand (red) and left-hand (blue) circularly polarized components to separate.

SpinHallEffectLight.jpg

The separation that Hosten and Kwiat sought was just 70 nm. To measure it, they adapted Vaidman, Aharonov, and Albert’s weak measurement approach, which I introduced in the following way:

The three theorists considered the case of a Stern–Gerlach experiment whose beams and magnets are too weak to segregate up and down spins. Ordinarily, such a setup would yield not the two well-separated spots of Otto Stern and Walther Gerlach’s famous experiment but a peanut-shaped blob.

That unhappy situation would change, the three theorists argued, if you used two polarizing filters placed before and after the magnets. Orienting the filters’ axes at 90∞ would cut off all transmission, of course. But setting them just off perpendicular would have a surprising effect: The wavefunctions of those few atoms that made it through would interfere and boost the spin-dependent displacement by orders of magnitude.

Weak measurement does not provide a free lunch, however. In graphical terms, it pushes a peaked signal further away from the origin, making its displacement easier to determine. But the technique also reduces the peak’s amplitude, making it harder to see anything at all. If an experiment’s resolution is limited only by the statistics of photon counting, the two effects cancel. That was far from the case for Hosten and Kwiat’s experiment, in which systematic errors, not a paucity of photons, limited resolution.

My story was about the spin Hall effect of light, not weak measurement. Indeed, if Hosten and Kwiat had been able to overcome jitter in their laser’s pointing direction, wobbles in the optical table, and other sources of blur, they need not have used weak measurement at all. Moreover, the experiment can be analyzed using the 19th-century physics of Augustin-Jean Fresnel and James Clerk Maxwell.

But as Adrian recounts in his story, weak measurement could be more than just a useful tool for determining small values. By barely perturbing a particle’s wavefunction—at least during the weak interaction phase—weak measurement appears to provide a means of circumventing quantum mechanics’ prohibition on tracking individual particles as they fly through an interferometer.

What’s more, in a feature article that appeared in last November’s Physics Today, Aharonov, Sandu Popescu, and Jeff Tollaksen proposed that weak measurements can be understood in terms of an alternative formulation of quantum mechanics—one in which wavefunctions propagate forward and backward in time symmetrically.

Not surprisingly, weak measurement as an interpretation of quantum mechanics has proven controversial. Aharonov, Popescu, and Tollaksen’s article provoked four letters in Physics Today, all of which took issue with it. Adrian also encountered the controversies when he reported his story.

It’s difficult for a nonexpert like me to evaluate the broader claims of weak measurement. For one thing, Aharonov, Popescu, and Tollaksen assert that their interpretation is “completely equivalent to standard quantum mechanics in so far as their predictions are concerned.” And some of the arguments, pro and con, hinge on interpretation, rather than experimentally determined facts or unimpeachable mathematical analysis.

Aharonov, Popescu, and Tollaksen suggest that their time-symmetric formulation of quantum mechanics could readily accommodate new, as-yet undiscovered physics. Even if that doesn’t turn out to be the case, it’s beyond dispute that weak measurement has proven useful in the lab.

Charles Day

QM in SF

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Although writers of science fiction can be reasonably accused of flouting the laws of physics, they nevertheless incorporate real physics—or extrapolations of real physics—in their work.

Quantum mechanics especially tempts writers. Despite being more than a century old, the theory seems perpetually modern, like Bauhaus architecture, twelve-tone music, or imagist poetry. Moreover, quantum mechanics is difficult, subtle, and weird.

There is, however, a problem with incorporating entanglement, complementarity, and other quantum concepts into a novel: Quantum mechanics is most apparent and influential in systems that are small, cold, and isolated. Novelists tend to prefer their stages large, their action hot, and their characters interactive.

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That mismatch is overcome in Ian McDonald’s 2007 novel Brasyl. Set in the Amazon jungle in 1732, Rio de Janeiro in 2006, and São Paulo in 2032, the novel hinges on Hugh Everett’s many-worlds interpretation of quantum mechanics.

Everett devised the interpretation in 1957 to circumvent the unpalatably probabilistic nature of the orthodox “Copenhagen” interpretation. According to Everett, observing a system doesn’t force it into one of its many possible states. Rather, observing a system forces it into all of its many possible states, creating for each of them a new and parallel universe.

Brasyl prominently features another manifestation of quantum mechanics: quantum computation, which by 2032 has become not only possible but also dangerous. (I won’t say why, lest I spoil the plot for you.)

I suspect McDonald found the many-worlds interpretation attractive because, unlike other quantum manifestations, it plays out in the macroworld, if only an imaginary one. Quantum computation, however, remains confined to small isolated systems. But to work, it must scale up—by 2032, if McDonald is correct.

Charles Day