Physicists make discovery in quantum mechanics

September 23, 2009,

(Santa Barbara, Calif.) -- Physicists at UC Santa Barbara have made an important advance in quantum mechanics using a superconducting electrical circuit. The finding is reported in this week's issue of the journal Nature.

The researchers showed that they could detect the quantum correlations in the results of measurements of entangled quantum bits, using a superconducting electrical circuit. The correlations are stronger than can be obtained using classical (non-quantum mechanical) physics, and according to the physicists, this illustrates that the oddities of clearly extend to macroscopic systems. The work is part of an ongoing collaboration between the UCSB laboratories of John Martinis and Andrew Cleland.

The results of measurements in quantum mechanics are intrinsically unpredictable, according to the theory of quantum mechanics, and yet still contain very strong correlations, in contradiction with classical physics. In particular, measurements of "entangled states," such as a pair of particles with opposite spins, allow stringent tests of the predicted discrepancy between quantum and , as described by the "Bell inequalities." Measuring such a discrepancy is known as a "Bell violation."

According to , Bell violations should be detectable using "qubits," superconducting quantum bits, but measuring these violations is technologically challenging. Martinis, Cleland, and their colleagues have overcome these challenges, and report a clear violation of Bell's inequality with two entangled superconducting qubits. Thus, they have demonstrated that this macroscopic electrical circuit is a quantum system.

The measurement of a Bell violation in a superconducting circuit was recently stated to be the next primary challenge for the superconducting qubit community, according to Martinis.

Martinis said: "This experiment has met this challenge, achieved by performing a very demanding measurement on a pair of Josephson qubits, a measurement that requires excellent control over qubit state preparation, entanglement, and very high fidelity single-shot state measurements of the entangled qubits. It directly proves that quantum mechanics is the only possible description for the behavior of a macroscopic electrical circuit."

More information: Violation of Bell's inequality in Josephson phase qubits, Nature, 461, 504-506 (24 September 2009) doi:10.1038/nature08363

Source: University of California - Santa Barbara (news : web)

Explore further: Researchers make milestone discovery in quantum mechanics

Related Stories

Pushing quantum mechanics to higher levels

August 11, 2009

Scientists at UC Santa Barbara have devised a new type of superconducting circuit that behaves quantum mechanically -- but has up to five levels of energy instead of the usual two. The findings are published in the August ...

Fock states could hold clues to quantum memory components

December 23, 2008

( -- “Fock states will play a role in the future of quantum computing,” Andrew Cleland tells “We have completed the first experimental measurement of the time decay of Fock states in a superconducting ...

Entanglement without Classical Correlations

August 27, 2008

Quantum mechanics is full of counterintuitive concepts. The idea of entanglement – when two or more particles instantaneously exhibit dependent characteristics when measured, no matter how far apart they are – is one ...

Yale scientists bring quantum optics to a microchip

September 8, 2004

A report in the journal Nature describes the first experiment in which a single photon is coherently coupled to a single superconducting qubit (quantum bit or "artificial atom"). This represents a new paradigm in which quantum ...

Measuring single qubits

June 20, 2006

“In a quantum system,” explains Alexander Korotkov at the University of California, Riverside, “the result of a measurement can change the system so that it moves in the same direction as the result.” Testing this ...

Recommended for you

How community structure affects the resilience of a network

June 22, 2018

Network theory is a method for analyzing the connections between nodes in a system. One of the most compelling aspects of network theory is that discoveries related to one field, such as cellular biology, can be abstracted ...

The pho­to­elec­tric ef­fect in stereo

June 22, 2018

In the photoelectric effect, a photon ejects an electron from a material. Researchers at ETH have now used attosecond laser pulses to measure the time evolution of this effect in molecules. From their results they can deduce ...

Water can be very dead, electrically speaking

June 21, 2018

In a study published in Science this week, the researchers describe the dielectric properties of water that is only a few molecules thick. Such water was previously predicted to exhibit a reduced electric response but it ...


Adjust slider to filter visible comments by rank

Display comments: newest first

1.1 / 5 (23) Sep 23, 2009
otto1923, your right that entanglement is real, but it cannot be used for communication, unfortunately.
2.1 / 5 (10) Sep 23, 2009
Entanglement is a verified and testable phenomenon, it's just poorly interpreted. What's sad is people accept maligned explanations from popular media sources, if you are going to make a definite statement about something mathematical or scientific in nature you better do your homework first.
2.1 / 5 (8) Sep 24, 2009
If the properties of an entangled qbit cannot be communicated, how can you determine that entanglement exists? If you measure the spin state of one of a pair of entangled qbits and therefore know the state of the partner, do you not have to measure the state of the partner for verification? Do you move the partner qbit into your testing appartus and if so, how is that confirming any action at a distance? If on the other hand you can measure the state of the partner with apparatus remote from the origin then communication occurs.

No communication means that entanglement is not really physical property and that entanglement is likely just an artifact of a flawed experimental processes, having been designed to achieve a predetermined outcome.
3 / 5 (4) Sep 24, 2009
So what was the answer? Is it so very hard to explain that it can't be done in 1000 characters or less? I've been seeing talk about the wonders of quantum communications for years, but not one list of the practical features. PLEASE enlighten us.
2.5 / 5 (2) Sep 24, 2009
An analogy is imagine a pair of people spinning coins on earth and on Pluto. The result is random, but if one gets heads, so the other, and if one gets tails so does the other. Their results are the same, but how does that let you send a message?
1.5 / 5 (4) Sep 24, 2009
Entanglement has been proven many times. Bell inequalities sets the correlation limits. This allows sending information across the universe instantly, or making calculations such as factoring much faster than our present day methods, or also perfect encryptions. Yes, there really are applications.
not rated yet Sep 24, 2009
An analogy is imagine a pair of people spinning coins on earth and on Pluto. The result is random, but if one gets heads, so the other, and if one gets tails so does the other. Their results are the same, but how does that let you send a message?

BUild a probe with a photonic quantum computer to control it... At prep time entangle the bits... Launch... and at a basic level, flip heads on earth flips heads on Pluto... now if that was a basic level instruction to the probe it should by all means be 'instantaneous' to receive that instruction.
Entangle the
not rated yet Sep 24, 2009
Thus, they have demonstrated that this macroscopic electrical circuit is a quantum system.

If I understand correctly, this is the gist of the article. A pretty nice feat of technology, bringing us one step closer to quantum computing and also upping the ante for QM. The scales at which it's being applied are getting bigger and bigger. There was an NS article recently about a virus(the bio type) being put in superposition.
1.6 / 5 (7) Sep 24, 2009
Spinning coins on earth and on Pluto is hardly a valid example. Both photons (or applicable quanta) must be entangled at a common source. Remote entanglement generation without a physical link has never been demonstrated let alone even suggested. So i entangle two photons with a known spin state. i then transport one of the entangled photons to a remote location, at of course less than c AND TRANSPORT the INFORMATION at less than c.

kinda defeats the whole premise.

What entanglement then suggests is that by altering the spin state of either photon (the qbit)the state of the partner qbit is altered.

How can you know the speed of the entanglement link without associated communication occuring? How do we know it is ftl? Can we know of ftl events?

It is like a stage illusionist (a magician) with just a sleight of hand the impossible seems to occur. It is though - just a trick and because we are not sure how it happens we are deceived and believe something unusual has occured.
1 / 5 (19) Sep 24, 2009
@out7x, I should have said, entanglement cannot be used for faster than light communication, as was implied by otto1923.
5 / 5 (2) Sep 24, 2009
Entanglement is a well known and well proven property of QM. Google "double slit experiment" and "delayed choice". Even though *quantum* information is most definitely transmitted FTL, it can't be decoded until you compare *classical* measurements carried out at both ends, which can't occur FTL. So, you can't have FTL actual communication, but, with the delayed choice experiment, you can most definitely prove that *something* was transmitted, not just FTL, but the "message" was received before it was sent, if you set it up right. NO! You CANNOT use this to get tomorrow's lottery numbers, stock quotes, or tomorrows obituary to find out if you're still alive tomorrow. Sorry.

Some invalid questions are frequently asked:
"How fast is the quantum information sent?"
"If it's instantaneous, then in which inertial reference frame is it instantaneous and why would there be a preferred frame when relativity shows there's no such thing?"

To respond to both questions:
2.3 / 5 (6) Sep 24, 2009
To respond to both questions:

The quantum information doesn't "travel" from one to the other. One way to think of it is that both quantum particles already have their final state at the moment of the entanglement... even if that state is triggered by a *later* event, such as the delayed choice of measurement in the delayed choice experiment. The choice at the end of the experiment dictated the quantum state of both particles from the moment of entanglement through the later moment of choice.

The problem with trying to use this technique to send classical information FTL (or even backwards in time) is that the receiving particle measurements appear random. You can neither tell when information is being attempted to be sent, nor when it ends, nor what it is, while it's being received (if it's being received). You essentially receive a random string of measurements during the entire time you choose to "read".
5 / 5 (2) Sep 24, 2009
Only later, after you record the quntum changes created at the "sender" and compare to the "receiver", can you determine that, "hey, there actually WAS a correlation". Of course, by the time you've done that, it's slower than light and forward in time, just as any normal "real" communication with radio signals or carrier pigeon.

This is a VERY popular topic on the physics newsgroups that people (beginners to these ideas) are constantly trying to explain how they can use this technique to communicate FTL or even backwards in time. Unfortunately, you just can't get around the inability, at the receiver station, to determine when a signal is being sent or what it is because of the seemingly random nature of the measurements. You *HAVE* to make recordings at the sender stating and at the receiver station, then later bring them together and compare.
(continued... (next post is last in series)
5 / 5 (2) Sep 24, 2009
(continued... part 4 of 4)
This "bringing together" is done classically, wasting away any time benefits of the quantum coolness, with the one exception of saying, "Hey, there actually WAS a correlation"... well AFTER the fact and equal to or slower than c.

BTW, the quantum properties are usually opposites of each other, so the flipping coin scenario would be more like, flip and receive heads at the transmitter, you're guaranteed to get tails at the receiver station. Minor point though. But, as you can see, with that coin example, there's no way to know what the message is or even if one is being transmitted.

not rated yet Sep 24, 2009
I still can't understand why this can't be used for communication. As people have said, make a space probe and fill a "special" containment with entangled particles with their sister particles left on Earth in a box.
Send the probe to pluto at normal chemical rocket speeds. 11 years maybe to get there.
Now use the box on earth and start sending 1 thousand particles through a horizontal polarizer. Consequently particles coming out of the box on pluto will show 1000 vertically polarized particles. This could be the Start Of Message indicator. Then start oscillating the particles on earth using polarized filters, and hey presto the opposite polarization occurs on Pluto, and YES, the effect is instantaneous. Sure it took the probe 11 years to get their, but once there we could communicate with it instantly.

If this is wrong WHY?????
And if the equipment takes a while to decode, who cares???? Surely it would be quicker than the time it takes light to travel to Pluto!!!!!
1 / 5 (19) Sep 25, 2009
CSharpner answered it well.
For the sender to communicate to a receiver, (s)he would have to cause an effect to the receivers equipment, but there is no causal connection going on here. The wavefunction is to be considered a single entity, not two , one effecting the other.
1 / 5 (19) Sep 25, 2009
The problem is that, if the sender arranged the entangled particles to go through ‘horizontal polarizers’ before being sent, (s)he would know the state before sending them. This would kill the entanglement, as that would be considered a measurement. The sender cannot ‘set the dial’ in any way like this, (s)he must send an unknown entangled state to the receiver.
As I pointed out above, there is no classical causal connection here. The sender cannot cause an effect to the receiver, because to do so, the sender would have to know what (s)he is sending. If the sender knew there would be no entangled state, so the sender is in the same boat as the receiver. Because there is no causal connection the sender and receiver cannot be differentiated.
1.1 / 5 (20) Sep 25, 2009
No valenarris, not "(yet)",.... never!

I see I misread Ober's post,.. he didn't mean send polorized particles.
1.2 / 5 (20) Sep 25, 2009
,.... but in general my point still stands because the sender can't 'set' the polarization, he can only perform a measurement, so how can he tell the receiver which particles made it. through the polarizer and which didn't?
1.2 / 5 (20) Sep 25, 2009
Oh, ok, I see in what sense you mean 'yet' now.
1 / 5 (10) Sep 25, 2009
Any proposals, theory or experimental results that proport to demonstrate causal violation are simply flawed in some way, whether the error is understood or known is irrelevant. In our reality cause and effect is never violated.

To suggest that ftl transmission of state between entangled objects occurs demands that you measure the velocity of the event. Yet as repeatedly stated that measurement requires information transfer at less than c.

Once you start to introduce subjective terms such as choice and give particles, entangled or not, the free will to decide then the science is lost.

It is the realm of QM to provide statisical data on the probabilty of effects but let us not confuse that with providing actual physical causes for the observed data.

Everything happens for a reason.

1 / 5 (19) Sep 25, 2009
@bluehigh; QM does NOT "provide actual physical causes for the observed data". QM as is currently formulated is fundamentally acausual. It's a presumption of classical science and apparently yours as well, that casuality is an essential aspect of Reality. Causality is all about probability even in the realm of classical physics. The is no analytic link between cause and effect, only a constant conjunction of observed events, so it does not imply deterministic conclusions, only probabilistic ones. Such concepts are a means of ordering reality for mind, causality is an a-priori intuition,.. not an entity of iself ingrained in nature apart from a conceptualization of it. In fact the lack of applicability to QM of causality prevents a rational understanding of the underlying reality, ('making predictions' not being considered 'understanding')

Many still think that eventually a theory will provide an old fashioned classical understanding of QM realm, but I think never.
1 / 5 (19) Sep 25, 2009
,... the reason is because the intellectual faculties we inherently use to form the conceptual framework in which to experience reality are subject dependent, and not necessarily compatible with Reality at all scales, and I believe this is manifest in QM.
1 / 5 (4) Sep 25, 2009
Don't be silly. Causality is NOT a presumption, it is a known and demonstrable fact of our existence. More so, causality may be necessary for our existence. Direct links that can be analysed exist within ALL events and would always be determistic if we had enough data.

QM is simply a way to statisically represent effects that are difficult to quantify because the cause is complex or unknown.

As Voltaire famously said,
Chance (read probability)- is the known effect of every unknown cause.
Sep 26, 2009
This comment has been removed by a moderator.
1.4 / 5 (21) Sep 26, 2009
QED is formulated without regard to causality or an arrow of time. See state vector reduction where the wavefunction collapses to a observable very much undeterministically. I would refer you to Bell inequality tests, which are empirical proof that QM cannot be explained by some hidden unknown variables. It's not that the 'cause is unknown' at the moment as you suggest, nature is fundamentally acausal.

Now if what Velanarris suggests, that a extra dimension works in adding another degree of freedom to make sense of it all then maybe you faith will have promise. An extra dimension would require physical justification though IMO, to bridge the gap to classical physics.
5 / 5 (1) Sep 26, 2009
cause and effect have NEVER been violated

How can you be so sure? Have you experienced all of time to state that with such certitude?

Here's a rewording: if someone could only see the colour red, wouldn't that person dismiss the very concept of colour as absurd and inconceivable?

Even if someone could show you an example of causality being violated, you, and us for that matter, couldn't process it as such, having to reduce it to causality in order to make sense of, or verify it. Read Flatland.
Sep 26, 2009
This comment has been removed by a moderator.
not rated yet Sep 26, 2009
Assuming that the spin direction states of two entangled electrons are undecided until either is measured, is there nothing anyone could do to influence the outcome before collapse? Is there no field that could be put near one elctron to affect its outcome so that when it collapses its spin is in a given direction. (I can't imagine measuring a spin direction for a single particle as I wouldn't know whether I was looking at it from the front or from the back .. but I may be taking "spin" too literally in a common sense meaning.)

The influence would not need to be exact or even very great as you could have hundreds of electrons and just count the number of + and - spin direction outcomes and play the winner as your either "+" or "-" message to send to the holder of the partner electons.

So are we powerless to affect the sin direction of an electron. It must be a 0/1 property which is decided on collapsing independent of the environment it is in?
1 / 5 (18) Sep 26, 2009
He's asking for an example of causality violation. I stated that QM cannot be formulated in classical terms (of causality, time asymmetric, etc). Two different statements. He thinks that nature at the qm realm is deterministic, 'it's just that we don't understand it yet'. This is wrong, I referenced the Bell experiments. He can look up the uncertainty principal and see why the electron is never in the nucleus.
1 / 5 (3) Sep 26, 2009
@Kasen - indeed it is unlikely that we could process causal violation. I contend as mentioned previously that causality is essential to our reality and existence.

How is it blind faith to belive that causal violation has never been documented. Surely the onus is on the proponents of an acausal reality to show evidence of causality violation. You can knock yourself out trying. I can on the other hand consistently demonstrate causality. I repeat - just because QM fails to identify causes does not mean that a cause is absent. It defies logic.

In any case this all gets off topic. This is a thought experiment and has no basis in reality. The cat is without doubt EITHER dead OR alive. To have any real superposition you would have to show and tell. To say - oh we can not do that because as soon as we measure or observe the state changes(waveform collapse) is simply a way to avoid being seen as living in a fantasy.
Sep 27, 2009
This comment has been removed by a moderator.
1 / 5 (4) Sep 27, 2009
Once an electron's spin (eg) has been measured, its previous superimposed states will have collapsed never to be accessible again? Since many particles (eg protons?) have been around such a long time one might think that there should be none left unobserved/unmeasured. But electrons can be created in complementary pairs and thus renewing their opportunity to exist in QM states? What about the long lasting protons .. can they be refreshed for use in QM experiments after a measurement?

And will, as a consequence, a quantum computer only be able to use ROM memory, not RAM?
1 / 5 (5) Sep 27, 2009
If the photons remain entangled, the distance is an illusion
not rated yet Sep 27, 2009
An analogy is imagine a pair of people spinning coins on earth and on Pluto. The result is random, but if one gets heads, so the other, and if one gets tails so does the other. Their results are the same, but how does that let you send a message?

Sure, you are saying a single result though and that it's random. What if we can determine what the coin toss result is and we can do so 100 times. We make 10 heads, 1 tails, 5 heads, 1 tails, then 3 heads, then the body of a bit coded message, then end with the same pattern in reverse.

Everyone keeps saying it has to be like a coin toss, and only 1 toss... maybe it is random, but again people used to say that flying was impossible, going to the moon was impossible, breaking the sound barrier was impossible, invisibility was impossible, etc.
1 / 5 (19) Sep 27, 2009
It's not impossble because it's hard to do, it's impossible in principal. It's not an engineering problem. For FTL communication to work there must be a distinction between a sender and receiver, but there is none, because both can only make a measurement, and neither can 'set the dial' as they wish.
3 / 5 (2) Sep 27, 2009
Using capitalization for emphasis;

You can't control what the RESULT of the MEASURE will be. You can't MAKE it heads on earth and get tails on pluto. What you measure is what you get, it is not KNOWN ahead of time or CONTROLLABLE. No MEANINGFUL information could ever be sent.

If we ever COULD find a way to control the outcome of entangled particles, we could only send PREDETERMINED messages. Even in THAT case you could NOT create NEW information and be able to send it.
1 / 5 (6) Sep 27, 2009
Hmm...Here's a wild proposition. What if the communication wasn't done as we are used to, with language and code, but something more like feelings, emotions, intuitions and other mushy, fuzzy stuff. Entangled brains as some sort of FTL telepathy?

Seeing as consciousness and cognitive functions in general are emergent from a large number of particles pretty close to the quantum scale, would it be that ludicrous to assume that quantum entanglement could affect those processes significantly?

Mind you, the quantum effect needn't occur at subatomic levels, as the above article demonstrated(ok, it's a bit of stretch, but still). I doubt it's something that could ever be proven scientifically, but who knows, if someone got it down to a form of trainable 'mental art', it might be useful.

I'd definitely prefer a more Dune-ish approach to future technologies. Mentats are way cooler than technological singularities.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.