The Quantum Measurement Problem

Viewpoint

The Quantum Measurement Problem

A. J. Leggett
Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.

Despite the spectacular success of quantum mechanics (QM) over the last 80 years in explaining phenomena observed at the atomic and subatomic level, the conceptual status of the theory is still a topic of lively controversy. Most of the discussion centers around two famous paradoxes (or, as some would have it, pseudoparadoxes) associated, respectively, with the names of Einstein, Podolsky, and Rosen (EPR) and with Schrödinger's cat. In this Viewpoint, I will concentrate on the paradox of Schrödinger's cat or, as it is often known (to my mind somewhat misleadingly), the quantum measurement paradox.

Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.

Quantum Field Theory Resources

Quantum Field Theory Resources

‘Science is the organized skepticism in the reliability of expert opinion.’ - R. P. Feynman (quoted by Smolin, The Trouble with Physics, 2006, p. 307).

‘Science n. The observation, identification, description, experimental investigation, and theoretical explanation of phenomena.’ - http://www.answers.com/

String theory predictions are not analogous to Wolfgang Pauli’s prediction of neutrinos, which was indicated by the solid experimentally-based physical facts of energy conservation and the mean beta particle energy being only about 30% of the total mass-energy lost per typical beta decay event: Pauli made a checkable prediction, Fermi developed the beta decay theory and then invented the nuclear reactor which produced enough decay in the radioactive waste to provide a strong source of neutrinos (actually antineutrinos) which tested the theory because conservation principles had made precise predictions in advance, unlike string theory’s ‘heads I win, tails you lose’ political-type, fiddled, endlessly adjustable, never-falsifiable pseudo-‘predictions’. Contrary to false propaganda from certain incompetent string ‘defenders’, Pauli correctly predicted that neutrinos are experimentally checkable, in a 4 December 1930 letter to experimentalists: ‘... Dear Radioactives, test and judge.’ (See footnote on p12 of this reference.)

note:
neutrino: an elementary particle with zero charge and zero mass;
checkable 的网络定义:Able to be checked;
antineutrino 的网络定义:The anti-particle of a neutrino.
fiddled: Verb. commit fraud and steal from one's employer; n. bowed stringed instrument that is the highest member of the violin family; this instrument has four strings and a hollow body and an unfretted fingerboard and is played with a bow;
endlessly :continuing forever without end;
incompetent :someone who is not competent to take effective action;
propaganda:information that is spread for the purpose of promoting some cause;

Nobel laureate goes to Washington?

Dec 11, 2008
Nobel laureate goes to Washington?
Steven Chu

The Nobel-prize-winning physicist Steven Chu will be Barack Obama’s nomination for Secretary of the Department of Energy (DOE), according to reports from the US President-elect’s transition team.

Chu, 60, is currently the director of the Lawrence Berkeley National Laboratory in California and professor of biophysics at University of California, Berkeley. If his nomination is confirmed by the US senate next year, Chu would be the first working scientist to run the DOE, which has a budget of about $25bn and is one of the largest sources of funding of scientific research in the US. Since 2005, the department has been headed by Samuel Bodman, a former professor of chemical engineering who spent many years as a venture capitalist before joining the DOE.
Unlike some in the outgoing Bush administration, Chu is a firm believer that humans are damaging the Earth’s climate. Indeed, he believes that climate change scenarios laid out in 2007 by the Intergovernmental Panel on Climate Change may be on the conservative side.

Science for a better environment

Chu also believes that science can play an important role in reducing emissions of greenhouse gases. He has been involved in the Berkeley-based Energy Biosciences Institute — a $500m facility, sponsored by oil giant BP, that aims to develop new energy sources from biomass, including biofuels.

Born in St Louis, Missouri, Chu did a PhD in physics at Berkeley and shared the 1997 Nobel Prize in Physics with Claude Cohen-Tannoudji and William Phillips "for development of methods to cool and trap atoms with laser light" — an accomplishment that has led to a renaissance in the study of the quantum mechanics of many-body systems.

In an interview with Physics World magazine earlier this year, Chu expressed his conviction that scientists could work together to save the environment: “Just as in the Second World War, when there were scientists who worked on radar or the bomb because they felt there was an emergency, so there are scientists today who want to work on the energy problem”.

About the author
Hamish Johnston is editor of physicsworld.com

Physics of Information Group at IBM Research

Physics of Information / Quantum Information Group at IBM Research Yorktown

We are in the midst of an information revolution, so much so that even lay people know the basic facts about information—how it can be encoded in bits 0 and 1, stored, retrieved, transmitted, and processed using logic gates like AND and NOT. This revolution is based on our ability to treat information in an abstract way, largely independent of its physical embodiment, which may be as diverse as a hole in a punch card, a voltage in a wire, or the magnetization of a speck of iron oxide. The field of Information Physics treats ways in which it nevertheless fruitful to reintegrate physical laws and principles into the science of information. These include:

Thermodynamics: Processing information consumes energy and generates waste heat, and the amount turns out to depend both on hardware and the nature of the logic operations being performed. The founder of our group, the late Rolf Landauer, during his long career at IBM Research, continually emphasized the connection between information processing and physics, and discovered the connection between logical irreversibility and heat generation now known as Landauer's principle.

Quantum effects: Quantum phenomena like entanglement and interference were neglected in the classical theory of information processing developed by Shannon, Turing, von Neumann and their contemporaries. In retrospect this was a mistake. Including quantum effects, and indeed abstracting them away from any particular physical embodiment, leads to a more coherent and powerful theory of information processing, as well as making possible information-processing feats unachievable with conventional “classical” information, notably quantum cryptography and quantum computational speedups. In place of bits the new quantum information theory has qubits, which are capable of entanglement and superposition, and interact with one another via quantum gates.

Fault-tolerance: Any real physical information processing apparatus, whether man-made or biological, is subject to errors. To make computing systems scalable in the presence of errors, a fault-tolerant architecture is required. This old problem has become acute in the case of quantum computers, where a considerable gap remains to be closed between experimentally available error rates and the thresholds at which fault tolerant architectures would take hold.

Physical Complexity: How can various mathematical notions of complexity, such as time/space complexity, parallel complexity, and algorithmic information, be used to characterize the complexity of physical states, phase transitions, and the behavior of systems at and away from thermal equilibrium. Are there physical systems or dynamics that are uncomputable in the mathematical sense?

Physical Authentication: Can our understanding of the computational complexity be used to authenticate physical objects and evolutions as genuine, rather than forged or simulated?

© Physics of Information Group at IBM Research

Superconducting qubits get entangled

Superconducting qubits get entangled
http://physicsworld.com/cws/article/news/25845

Physicists in the US have taken another step towards the dream of a quantum computer by entangling two superconducting quantum bits (or qubits) for the first time. Circuits made from superconducting elements are promising candidates for a real quantum computer because they are compatible with conventional methods for making integrated circuits (Science 313 1423).

In the weird world of quantum mecahnics, particles can be "entangled" so that they have a much closer relationship than allowed by classical physics. For instance, two photons can be created in an experiment such that if one is polarized in the vertical direction, then the other is always polarized horizontally. By measuring the polarization of one of the pair, we immediately know the state of the other, no matter how far apart they are.

This "spooky action at a distance", which has no classical analogue, could allow multiple bits of information to be processed at the same time in a quantum computer. Such a device could therefore outperform a classical computer by many orders of magnitude. There are currently many rival ways of entangling particles, for example by trapping ions at ultra-low temperatures and manipulating their internal energy states with lasers

However, demonstrating entanglement is hard. In particular, the particles, or qubits, have to be sufficiently isolated from the environment so that the fragile entangled state exists for long enough to allow a calculation to be carried out. Various other conditions also have to be met -- together known as the "DiVincenzo criteria" -- such as being able to measure both qubits at the same time.

Now, however, a team from the University of California, Santa Barbara, has successfully entangled two superconducting qubits for the first time. Electrical circuits made from superconductors are promising candidates for a working quantum computer because they can be made from thin films using conventional microchip fabrication technology. Coupling can be achieved simply by electrical connections between qubits – far easier than the trapped ion approach, where ions need to be shuttled about so they can interact.

Matthias Steffen and colleagues at Santa Barbara were able to entangle two qubits, each made from a Josephson tunnel junction, that meet the DiVincenzo criteria completely with a precision of 87% of theoretical values. The researchers used a delicate method known as "quantum state tomography" to confirm the entanglement, whereby a series of different parameters are measured for the two particles and used to reconstruct the quantum state, much as image “slices” are captured and combined into a three-dimensional picture in tomographic medical imaging.

Although physicists have been able to entangle up to eight ions at the same time -- whereas the present work entangles just two quibits -- Steffen insists superconducting qubits are a viable approach towards quantum computing. "Substituting some of the materials in the fabrication process should translate to a straightforward improvement of our results and in the long run, continued materials research should also help improve qubit performance," he says.

The work was done by Santa Barbara's Quantum Computation research group, which is led by John Martinis.

Microchip ‘bus’ links up quantum bits

Microchip ‘bus’ links up quantum bits
http://physicsworld.com/cws/article/news/31300

Two independent groups in the US have created “buses” for transferring information between two microchip-based qubits. The buses could allow a number of qubits to be joined together to make powerful quantum computers using standard chip manufacturing processes.

The basic unit of information in a quantum computer is the qubit, which can take the value 0, 1 or — unlike a classical bit — a superposition of 0 and 1 together. When many of these qubits are combined or “entangled” together a quantum computer can process them simultaneously, enabling it to work exponentially faster than its classical counterpart for certain operations.

To achieve this entangling feat, quantum computers need to link remote qubits via a bus that can transmit their states to and fro. Although buses have already been created for trapped ions and atoms — two of the many realizations of qubits — a bus for a superconducting qubit had yet to be realized. Superconducting qubits are particularly promising for practical quantum computers because the whole system can potentially be printed onto a circuit in a similar way to those in present-day computers.

Raymond Simmonds and colleagues from the National Institute of Standards and Technology (NIST) have fabricated two superconducting “phase” qubits — each a super-cooled insulating barrier sandwiched by a pair of tiny metal grains — separated by a bus in the form of a cavity that contains a standing wave. They first prepare one of the qubits in the desired 0, 1 or superposition state with a microwave pulse that changes the quantum oscillations of the phase difference between the barrier’s electrodes. The researchers then use an external field to briefly tune the energy difference across the barrier so that the qubit resonates with the cavity and transfers its state to the standing wave, where it can be stored for up to 10 ns. At the other end of the cavity, the same tuning process transfers the state to the other qubit (Nature 449 438).

Meanwhile, Robert Schoelkopf and colleagues from Yale University used microwaves to prepare superconducting “charge” qubits, which are physically similar to phase qubits but have states defined by the number of paired-up electrons that have tunnelled across the barrier. Their cavity has no initial standing wave, instead relying on the electron-pair tunnelling itself to emit a virtual photon into the cavity and create a combined superposition state between the qubits. This is a similar technique to work performed by the Yale group last week, in which they showed that a superconducting qubit could be used as a single photon source (Nature 449 443).

The three operations — information transfer, storage and combined superposition — by the NIST and Yale groups could eventually underlie the gate operations required to perform calculations in a microchip-based quantum computer. However, this will require many superconducting qubits to be linked together, and in a much more reliable way than the current devices.

Berry’s phase seen in solid-state qubit

Berry’s phase seen in solid-state qubit
http://physicsworld.com/cws/article/news/31942

An international team of physicists is the first to show how information can be stored and manipulated in a solid-state quantum bit using “Berry’s phase” – an esoteric geometrical property of a quantum system.

The team controlled the Berry’s phase of paired electrons in a tiny piece of superconductor by exposing it to pulses of microwave radiation. The breakthrough could help physicists overcome a major barrier to practical quantum computing – the tendency of quantum bits to lose their quantum information content rapidly over time.

If a classical particle such as a stone undergoes a cyclic process – it is heated slightly and then cooled to its original temperature, for example – there is no way of telling from the cooled stone how it was heated, or even if it was heated at all. However, the same does not apply to quantum particles such as electrons, which retain some “memory” of the path taken in a cyclic process. This memory is in the form of a difference in phase between the initial and final quantum states and was first proposed by Michael Berry in 1984.

Berry’s phase is a “geometrical” effect that occurs in the abstract space defined by the orthogonal quantum states of a system. A key property of Berry’s phase is that it is not dependent on the path taken through this space, but only on the area enclosed by the loop. It turns out that this could be very useful to those designing quantum computers in which data are stored and processed in terms of quantum bits (or qubits) of information.
This is because phase plays an important role in quantum information, and manipulating the phase of a qubit corresponds to performing a logical operation. By cycling the system around a closed loop, Berry’s phase, and hence geometry, can be used to perform calculations.

A shaky hand

The manipulation of any qubit requires contact with the outside world. No matter how carefully this is done, the qubit is subjected to small amounts of noise, which could eventually destroy the quantum nature of the qubit, rendering it useless. However, the effect of noise on manipulating the Berry’s phase of a qubit can be likened to drawing a circle with a shaky hand – as long as the curve joins up and encloses the correct area, the manipulation of the qubit will be sound.
Quantum operations based on Berry’s phase have already been achieved in nuclear magnetic resonance and trapped-ion systems. However, these are large and unwieldy technologies and many physicists believe that it will be difficult to assemble them into practical quantum computers. However, solid-state qubits based on superconductors could, in principle, someday be miniaturized and mass-produced.

Now, Peter Leek and colleagues at ETH Zürich along with researchers in Canada and the US have demonstrated the first solid-state qubit based on Berry’s phase. Their results are reported today in Sciencexpress .

Aluminium qubit

The team’s qubit is a micrometre-sized piece of aluminium, which is a superconductor at very low temperatures. In its lowest energy state the superconductor contains a certain number of paired electrons and its first excited state contains that number plus one pair.
The aluminium was placed at the centre of a millimetre long superconducting wire that functioned as a microwave resonator. The resonant frequency of the resonator was set to be very different from the transition frequencies of the qubit – which served to isolate the qubit from its surrounding environment. However, the resonant frequency changes slightly depending upon the state of the qubit, allowing the researchers to monitor the state of the qubit by injecting a single microwave photon into the resonator.

The team then applied a number of different microwave signals to the resonator. Each signal caused the qubit to follow a path that enclosed a different area. This resulted in a number of different angles of Berry’s phase, which were measured using the single-photon technique.

“This is important research and I am very happy that it has been done”, said Jiannis Pachos of the UK’s University of Leeds, who is a proponent of Berry’s phase for quantum computing. Pachos told physicsworld.com that the next step is for physicists to build a solid-state two-qubit logic gate based on Berry’s phase – something that has already been done for ion-trap qubits.

This, however could prove difficult: “The control of solid-state two-qubit systems is lagging behind other systems”, said Pachos. Peter Leek told physicsworld.com that the team “are thinking of ways to do this at the moment”.