Single electron waves traveling through a single electron gate on a superconductor.
In a sense, the electricity that we all depend on for applications ranging from pacemakers to microchips to cars is crude. It happens as current, like water flowing through pipes. Which is peculiar in a way. Electricity only needs a single particle to communicate a single bit—or quantum qubit—of information, yet achieving a single electron system, in which just a lone electron is passed between points, remains at the edge of theory, a sort of electronics/computing holy grail.
Once achieved, the single electron system could be invaluable to quantum computing, enabling researchers to utilize the weird quantum properties of electrons rather than their photon constituents. It’s the difference between a quantum optical system (photons) and a quantum system based on the electricity we all know and love. Christian Flindt, a theoritician at Genève University, wrote a piece for Nature describing experiments realizing this possibility via what’s known as a leviton, a quasiparticle. Last week I had the chance to ask him a bit about it (pun not fully intended).
Motherboard: What is a quasiparticle?
Christian Glattli: Quasiparticles typically occur in solid-state systems involving an (astronomically) large number of electrons and nuclei. Sometimes all the particles behave in a collective way and this collective excitation may effectively be described as something that behaves just as a particle, thus a quasiparticle.
There are many examples of quasiparticles, for instance a phonon, which is a vibration of the atomic lattice in a solid. This collective vibration of the nuclei can be thought of as a phonon moving through the solid.
In a simpler picture, think about waves on an ocean. What you see is in principle just lots of water molecules. However, they may move together in a way that we call a wave. And we can discuss properties of the wave, like how big is it, how fast is it moving, etc, even if we in principle are just looking a lot of water molecules.
So, quasiparticles are simple descriptions that emerge out of a complex system of many particles.
Is a leviton something that might occur naturally?
Levitons are already very hard to create in the lab. As [Lenonid] Levitov showed in his theoretical works, one has to use a carefully designed voltage pulse. So, although levitons in principle could occur naturally, I find it very unlikely.
Can any quasiparticles occur naturally?
Yes, certain quasiparticles occur naturally. The phonons (lattice vibrations) that I mentioned are a good example. They determine many physical properties of solids.
What is a single electron system?
Single electron systems are the ultimate lower limit on electronics. We are used to that electronics becomes smaller and smaller, but once we reach the single electron level, it can’t get smaller.
At the single electron level, we may hope to use some of the quantum properties of electrons to perform quantum computing and quantum information processing.
Am I correct in that in this system it's not an electron moving along a conductor, but rather a leviton moving along or through a surface of electrons? So rather than sending a cup of water across a sea in a boat, you're sending the same amount of water through the sea itself, as a wave ...
Yes, I agree with your description. The leviton is a wave on the surface of a sea of electrons and it (typically) carries just a single electron charge.
So levitons have a charge?
Yes, levitons carry charge, typically one or several electron charges. Interestingly, under certain circumstances that my even carry just a fraction of the electron charge.
How can we have a fraction of a charge?
OK, this is indeed tricky: There is a physical phenomenon called the fractional quantum Hall effect in which electrons exhibit a collective behavior which can be described by (yet another) type of quasiparticles called Laughlin quasiparticles. The charge of these quasiparticles is only a fraction of the charge of real electrons. If one would apply the carefully designed voltage pulses to a system of Laughlin quasiparticles, one might produce levitons that would carry only a fraction of the electron charge.
Can you explain a bit about how this might be applied to quantum computing?
In solid-state quantum computing, one would like to encode a quantum bit (qubit) in the spin of an electron. (Think of the spin as an arrow that can either point up or down, and up could represent 1 and down 0.) If one encodes a qubit in the spin of a leviton, the leviton can transmit quantum information through an electronic circuit. Basically, everything that has been done so far in quantum optics with a single or a pair of photons (for instance entanglement generation and detection, quantum teleportation, and quantum cryptography) can now in principle be tested with levitons.