# Josephson junction

#### Fizica7

Isn't the Josephson junction in effect a perpetual motion system ?

Related Atomic and Condensed Matter News on Phys.org

#### jamie.j1989

Whats the reasoning behind this? There's no energy lost in the tunnelling of a cooper pair across the junction, as thats the essence of quantum mechanical tunnelling, the probability of tunnelling across the junction is just resonant with the barrier height $E_j$, that being if the energy of the cooper pair is an integer multiple of $E_j$ then the tunnelling probability is greatest.

#### Fizica7

Well it seems like one of those "Did you know that..." type thingy... As I see it it's motion, of electricity, without any end in sight... Sure it's not scalable or of any real use but it is like a very rare thing which laughs in the face of the notion that there is no such thing as a perpetual system... It's like a very rare gem.
I mean if you create one and trow it into deep space, I'll reach the other side of the galaxy and still hold its tiny charge, no?

#### Fizica7

pl. look up the following and point out at which stage of the working of the J Junction you felt like "a perpetual motion system "?

I'm not going to read that cause it's above my pay grade(shhh-education).
I initially found out about the Josephson junction while browsing Wikipedia where it says :
" The Josephson effect is the phenomenon ofsupercurrent—i.e. a current that flows indefinitely long without any voltage applied—across a device known as a Josephson junction (JJ), ".
So i imagined it as a tiny loop of electricity that never stops going around the loop... isn't it so? Or maybe the wiki article needs better wording?

#### f95toli

Gold Member
First of all, you seem to be confusing a Josephson junction -which is a type of device which is typically made from an insulating layer sandwiched between two superconducting electrons - with the phenomenon of supercurrent which has to do with current flowing "without resistance" in a superconductor.
.
Now, if you make a loop of a superconducting material (no Josephson junction needed) and somehow induce a current (there are are various ways of doing this) in it, he current will indeed -ideally- flow forever since a perfect superconductor will have no dissipation at DC. This is how superconducting magnets are used when in "persistent" mode.

However, this is not perpetual motion in the usual sense (which tends to refer to phenomena which "creates" energy). If you try to use the energy "stored" in this flowing current (the inductive energy) for anything it will decrease and eventually stop flowing altogether.

Hence, it is no more perpetual motion than the earth circling the sun; you can store energy in such a system but it can't create "new" energy.

#### Fizica7

Yeah I might be mixing the 2 a little bit cause when I first heard that notion ( nothing lasts forever, there's no perpetual motion) it was at a very small age, maybe 5, and it stuck with me in the sense that it was like the definition of unachievable... Like just the notion itself, that it would be impossible to create something, place it on a table, give it a "go" and it would last forever..it was just so fascinating to read about the possibility that there is something than can be man made and will last forever. In my mind it never required to do any "work" or to have any practical use to qualify as a perpetual motion... just it's existence alone.

"Josephson junction"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving