Quantum Entanglement at a Distance

We have a new paper in the latest issue of Nature, about remote entanglement of two ions. Here’s a couple press releases: Michigan press release, and Maryland press release. We were also on Slashdot. You can read the paper here (pdf). Unfortunately, I’m not on this paper—this was the other project in the lab.

So, what’s this all about? Well, suppose we had two computers, and we wanted to transfer information from one to another. Typically, this would be done by sending bits across the internet (or wireless network, etc.). Now, what if we wanted to do this with a quantum computer? Well, now we have two types of information: classical and quantum. We could just read out the information of one computer, and send it to the other. The read-out process collapses the quantum information into regular classical bits. We could easily send this to the other computer easily and work with it (The classical bits could trigger the control logic of the other quantum computer).

Now what if what the other computer needed was the quantum information? We need a way to transmit this quantum information. It turns out one way of transferring this information is through quantum teleportation. This requires quantum entanglement.

Suppose I had three qubits, q0 q1 q2, where q1 and q2 are entangled. I give you qubit q2, and want to transfer the information in q0 to you. I perform a joint measurment on both qubits that I have (q0 and q1). When I tell you the result of this measurement, then you can perform a quantum operation on q2 to recover the information in q0. The two entangled qubits are like a quantum information bus. But we still need classical information to determine how to recover the information.

What we have acheived in the lab is a means of entangling these two qubits without needing them to be right next to each other. In my description above, I initially put q1 and q2 into an entangled state, and gave you one of them. Now, you could have your own qubit and live in Michigan, and I have my own in Maryland. We can then ‘open up’ the quantum channel by entangling the two qubits, allowing transfer of quantum information.

How is this done physically? First, we have a single ytterbium ion in an ion trap, and excite it to an excited state. The ion has two decay channels. If it decays to the qubit state |0>, it emits a photon of one color (say ‘blue’). If it decays to qubit state |1>, it emits a photon of a different color (’red’). In this method, we have generated a single photon entangled with a single ion. So, we have both ions do this, giving us a photon entangled with the first ion (q1), and a photon entangled with the other ion (q2).

We then interfere these photons on a beamsplitter. If a photon hits a beamsplitter, it can go one of two ways. We have detectors at each output port to detect the photon. Now, if the two photons interfere on the beamsplitter, we will only coincidence detection if the photons are of a different color. So we detect a ‘red’ photon and a ‘blue’ photon. But we don’t know which ion emitted which photon.

What we do know is that one ion is in |0> and the other is in |1>. That’s the key. These two ions are now entangled. And the quantum communication channel is open.

Random Quantum Optics Question

Consdier a coherent photon state |alpha> interacting with a two-level system.  Now, in the large photon limit, we ought to be able to recover the semi-classical interaction: i.e. a classical EM field interacting with a two-level system.  Conceptually, this should be the case.  Has anyone shown this?  From what I can tell, the interaction with a coherent state gives rise to Rabi flopping that decays, and has revivals.