Latest Research on Quantum Computing 33
zeristor writes "The The Economist is running a story
about the latest progress in Quantum computing. It seems that what has been glossed over in Physics as a minor detail, the decoherence of the superposition of states, is actually quite fundamental to Quantum computing.
The decoherence can be measured by something called the Loschmidt echo (is this esoteric or am I just thick? This sounds like a bad episode of Star Trek.)
Also goes on to explain how entanglement can be prolonged. All in all very interesting developments."
What, no Duke Nukem Forever joke? (Score:2, Offtopic)
Re:What, no Duke Nukem Forever joke? (Score:1)
Re:What, no Duke Nukem Forever joke? (Score:2, Funny)
Of course, most physicists know they need only refer to their email in-boxes to find several offers for products which claim to help maintain entanglement, suppress decoherence, avoid spontaneous emission, and increase their ability to perform bang-bang pulses at regular int
This is both a FP and not a FP (Score:5, Insightful)
We won't know until somebody reads the article and actually understands what it means.
I found it interesting that something that sounds quite fundamental to quantum physics has been passed over for so long with a 'And then something happens'.
Re:This is both a FP and not a FP (Score:2, Funny)
In case of Slashdotting... (Score:4, Informative)
The same cannot be said of many properties in quantum physics, such as the spin of an atomic nucleus (loosely speaking, which way it is pointing) or the position of an electron orbiting such a nucleus. At a small scale, such properties can have more than one value at once. In 1994, Peter Shor, a mathematician then at AT&T's Bell Laboratories in New Jersey, realised that a computer that used such quantum properties to represent information could factorise large numbers extremely quickly. This is an important problem, because much of modern cryptography is based on the difficulty of factorising large numbers--so being able to do so quickly would render many modern codes easily breakable. Then, in 1996, a colleague of Dr Shor's at Bell Labs, Lov Grover, showed that such a quantum computer would be able to search through an unsorted database much faster than an ordinary computer--another important application.
Computer technology
With these insights, quantum computing, which had first been thought of as a possibility in the early 1980s, became a hot topic of research. It was clear to many physicists that using "qubits"--which, unlike ordinary bits, can exist in a "superposition" of the values 0 and 1 simultaneously--might yield an exponential improvement in computing power. This is because a pair of qubits could be in four different states at once, three qubits in eight, and so forth. What Dr Shor and Dr Grover showed was that the improvement, if the technological hurdles could be overcome, would be not hypothetical, but real, and useful for important problems.
The technology necessary to manipulate qubits, in their various incarnations, is challenging. So far, nobody has managed to get a quantum computer to perform anything other than the most basic operations. But the field has been gathering pace, and was the topic of much discussion among the scientists gathered in Montreal for the annual March meeting of the American Physical Society, the largest physics conference in the world.
There are currently several different approaches to quantum computing, all of which rely on fundamentally different technologies, including ultra-cold ions that are cooled by lasers, pulses of laser light, nuclear-magnetic resonance and solid-state devices such as superconducting junctions or quantum dots (which are confined clouds of electrons). What all these technologies have in common is that they can be used to invoke and exploit the bizarre phenomenon of superposition.
Superposition is not simple. Though a qubit may, for a while, be in a state of superposition between 0 and 1, it must eventually choose between the two. And in even the best quantum computers, that choice, or "decoherence", happens in a fraction of a millisecond. Just how the choice is made, and how to prolong the preceding period of "coherence" that allows quantum computations to be made, constitute a long-unexplained gap at the heart of modern physics. For nearly 80 years, since the inception of quantum theory in the 1920s, most physicists were content to gloss over the process. What is perhaps surprising is that the technological challenge of quantum computing is now a driving force behind efforts to understand the most abstract and philosophical underpinnings of quantum mechanics.
Echoes of the future
Until a qubit interacts with the macroscopic world, which follows the classical laws of physics, it behaves according to the laws of quantum mechanics, which are well understood, at least by physicists. However, the interaction with the classical world--decoherence--and hence exactly where the divide between the quantum and classical worlds lies, are not well understood. When decoherence is deliberately provoked, the process is known as measurement. Before a qubit is measured, for example, it could have a 90% chance of being 1, and a 10% chance of being 0. After the measurement, it takes on one of these two values. But the details of how it chooses between the two are something that, until the advent of quantum computing, most physicists chose to remain agnostic about. Some even quipped that the answer to that question was beyond the realm of physics. Measurement was thought to occur instantaneously, and its effects were added to the theory ad hoc.
But decoherence, though it occurs on a short time scale, happens gradually, unlike the instantaneous idea of measurement. It can thus be investigated. Wojciech Zurek, of America's Los Alamos National Laboratory in New Mexico, discussed his research group's efforts to investigate decoherence. They recently proved that the rate at which decoherence occurs can be measured by something called the Loschmidt echo. The Loschmidt echo, named after a 19th-century German physicist, is an observable experimental measure of the sensitivity of a quantum system to changes in the energy of the system. (The exact form it takes depends on what sort of physical system is being considered.) Dr Zurek expects that the link between decoherence and the Loschmidt echo should aid theoretical understanding of decoherence.
In a less abstract tack, several papers presented at the conference showed how decoherence could actually be combated. Although the naive expectation is that any interaction between the qubits of a quantum system and the outside world will provoke decoherence, it turns out that the right kind of external signals can in fact prolong the period of coherence.
Chikako Uchiyama of Yamanashi University, in Japan, discussed how, in the general case, the application of very short pulses, poetically known as bang-bang pulses, at regular intervals can serve not only to suppress decoherence, but also to maintain entanglement--the quantum coupling between several qubits which allows computations to get done. It turns out that, in the absence of such pulses, disentanglement happens even faster than decoherence, so there is even more of a need to suppress it. The specific form of the pulses, Dr Uchiyama says, depends on the quantum-computing technology in question--in nuclear magnetic resonance, the pulses could be of the magnetic field, while for quantum dots, it would be the electric field that is pulsed.
Kaveh Khodjasteh of the University of Toronto looked at a related question, focusing on decoherence rather than disentanglement. He showed how a quantum error-correcting code which introduced only one extra qubit for error correction would create a robust system for quantum computation which had tolerance for faults caused by spontaneous emission, another bizarre quantum effect. (Error-correcting codes are used to ensure the integrity of quantum calculations.) Many quantum error-correcting codes have been proposed before, but most require a large overhead of qubits--some need up to eight times as many qubits as those necessary for a computation, while Mr Khodjasteh's code needs only one extra qubit, no matter how many are being used for the computation.
Several other speakers at the conference focused on detailed descriptions of how decoherence occurs in specific systems, such as superconducting junctions, or quantum dots. The promise of quantum computation, spurred on by the insights of Dr Shor and Dr Grover, is inciting physicists to probe, experimentally and theoretically, the junction between the quantum and the classical. They seem to be finding that the process of decoherence is more gradual, quantifiable and open to investigation than was previously suspected. Though a useful quantum computer is probably still many years away, the field of quantum computing is well on its way to solving its first problem.
Re:In case of Slashdotting... (Score:1)
Just a quick layman question on the above. It is my understanding that subatomic particles exist in two or more states until an intrusive event happens (such as observation) and it is that event that forces the particle to "choose". Is this correct or am I mis-reading?
Re:In case of Slashdotting... (Score:1)
If it's not it is undecided prior to observation in what energy state it is, but it will collapse into one of the eigenvalues.
The particle must be in an eigenvalue to produce a standing wave solution to the Schrodinger equation, or else the particle would vanish.
So yes, in layman terms, the particle chooses one energy state when you observe it.
Re:In case of Slashdotting... (Score:2)
Another, in some ways more rational way to explain it, is that a qubit is in both states, and that the qubit is always in both states. It's just that *you* are also in both states, and when a "meaurement" happens that gives a single value for the qubit what happens is that you in one state see one result for the qubit, and you in another state see
Re: My Explanation of Quantum Physics (Score:1)
Re: My Explanation of Quantum Physics (Score:2)
I just thought of an actual use for that code. Assume sig points to an audio port. The output tone would then indicate the load on the system. Owwwwww, owwww, pain, pain, lol.
As for Quantum weirdness, it kinda makes sense to me. It's hard to describe what I picture, but I'll take a stab at it. Our entire perceived reality would just be a single point or surface in a much larger system or reality. When it seems a quantum value has more than one value, it really has both
Advanced Craps cheating (Score:4, Interesting)
The question is, what do we use as 'bang-bang' pulses in order to keep the dice from decohering until we can coerce them into making our point?
1. ???
2. Go shoot craps
3. Profit!
Re:Advanced Craps cheating (Score:2)
Gnarly, d00d! (Score:4, Funny)
Why don't you send me a quantum echo from the future when this is all running nicely...
Re:Gnarly, d00d! (Score:3, Funny)
You don't need to understand grammer.. (Score:2)
-psy
Re:You don't need to understand grammer.. (Score:2)
It's quantum [wikipedia.org] .
Re:You don't need to understand grammer.. (Score:2)
-psy
Re:You don't need to understand grammer.. (Score:4, Informative)
- on Slashdot,
- with an ellipse [sinc-ic.org] at the beginning,
- and mispelled grammar [reference.com]
in the process? Are you trolling for "funny" points or something?Anyway, since you put "Quantum" at the beginning of your sentence, where it would have been capitalized anyway, it wasn't at all clear that it was capitalization that you were talking about. Sorry.
Re:You don't need to understand grammer.. (Score:2, Offtopic)
-psy
Re:You don't need to understand grammer.. (Score:2)
Even the dyslexic can use a spell checker, sport, and you'd be wise to do so - especially when you're attacking somebody else's writing.
Re:You don't need to understand grammer.. (Score:2)
Re:You don't need to understand grammer.. (Score:1)
Re:You don't need to understand grammer.. (Score:2)
Re:You don't need to understand grammer.. (Score:1)
Re:You don't need to understand grammer.. (Score:2)
Decoherence is no detail (Score:5, Insightful)
Re:Decoherence is no detail (Score:1)
By, like recently, do you mean since 1995? [arxiv.org] Quantum error correction (or more generally, the theory of fault-tolreant quantum computation) is one of the most surprising discoveries of the last decade. The next few years are the years where these ideas finally get tried out in the lab. The coming of the quantum machines has begun.
Re:Decoherence is no detail (Score:2)
Eigenvalues (Score:3, Informative)
Eigenvalues and eigenstates have meaning when in terms of an operator which represents the perturbation to or observation of the system.
Every operator has a characteristic set of eigenvectors. Every quantum system is described by a wavefunction and prior to a pertubation/observation this wavefunction can be described as a linear combonation of the eigenvectors of the operator.
Following a perpurbation/observation decribed by the operator, the quantum system will be described by a one and only one eigenvector of the operator.
Of course, the probability of a particular eigenvector being chosen is represented by the square of the eigenvalue of the eigenvector.
for more see wikipedia [wikipedia.org]
The Loschmidt Echo (Score:1)
This Loschmidt echo thing seems to be buried quite deep, I have not found it referenced in my Quantum Mechanics books.