Playing Time Backwards
The Rubidium Quantum Sequencer
When an atom absorbs a photon, the usual scenario is that an electron is lifted from a lower energy level to what’s known as an excited state. How long it stays there depends on the particular atom and the state but can range from nano seconds to minutes and even hours. What excites quantum physicists in this process is the ability to effectively store a photon in such a transition then release it later in the opposite process known as a photon echo. But in order to be useful in devices such as quantum memories and encryption systems, the coherence of the original photons must be maintained.
This is what distinguishes a true quantum memory from a simple detect and re-emit memory. You could simply count the number and timing of photons hitting a detector, store that information perhaps on a computer then regenerate the pulse using a laser diode. The problem would be that although the timing and amplitude of the pulses would be right, the phase information would be lost. This means that if the photon stored in the memory were one of a coherent pair, after it were released it would no longer generate an interference pattern with its twin photon.
In a true quantum memory all of the information about the incoming pulse is stored. So a photon from a laser stored there will create an interference pattern with a second, un-stored photon from the same laser. And this is the kind of memory that’s useful in quantum information processing.
Mahdi Hosseini, Dr Ben Buchler and Professor Ping Koy Lam from the ANU Department of Quantum Science are amongst a number of scientists around the world working on various types of quantum memory. In a recently released paper in the prestigious journal Nature, they describe a new type of quantum memory based on rubidium vapour. This new memory can not only stores pulses with their coherence information intact, it is able to split a single pulse into multiple parts, stretch and compress the length of the pulses and re-sequence the pulses so they can be recalled in any order. Recalling the pulses in any order could be useful as a random access memory for quantum information.
At the heart of the system is a sealed glass vessel with an optical window at each end, containing rubidium vapour and housed in a magnetic field gradient. Rubidium was chosen because its atomic structure has a hyperfine splitting in its ground state. In effect this means that there are two minimum energy electron configurations.
“It is possible to generate a photon echo using just a two level atomic transition, but this limits you to storage times related to the lifetime of the excited state. With the three level rubidium system we can in effect, move the excited electron to a second very long-lived storage state using a bright control laser. This also means that if we turn that control laser off, the electronic state is effectively frozen.” Dr Buchler explains.
When the laser pulse enters the chamber the rubidium atoms are excited and the pulse is absorbed. The atoms then begin to “spin”. Each atom spins with a different speed that depends on the initial frequency of the atom, which is controlled by the magnetic field. Because the atoms all spin at different speeds its statistically almost impossible for them ever to realign to the point they were all at when the pulse entered. It’s a little bit like setting the combination on a safe then spinning the dials at different rates. No matter how much you keep spinning them, they’re never likely to coincidentally realign and open the safe.
“What we would really like to do to get the pulse out, is to turn time backwards,” Dr. Ben Buchler explains, “But of course we can’t do this. What we can do though, is reverse the external magnetic field. This sets the spins into reverse and after a given time they’ll be right back where the absorption occurred.”
When this happens the dipoles are all aligned in the exact same way they were during absorption and the moving charges generate the right electromagnetic field for a photon. The net result is that a photon is re-emitted with exactly the same phase frequency and amplitude as the one absorbed. And it has retained all it’s quantum mechanical coherence information. It’s almost like spinning the safe combination dials backwards in exactly the way they went forwards. After a set time you hit the right combination and the safe can be opened.
However, in the case of the rubidium gas chamber, the photon release requires both the right spin combination and the control beam to be on. If the control beam is off the electrons have no way of getting back into the appropriate ground state so no photon is created even if the spins are aligned. In terms of the safe, spinning the locks backwards will get you the right combination, but it is the control beam that provides the energy to open the door.
“This gives us the ability to send a sequence of pulses into the gas chamber, absorb them all, then by switching the control beam on and off as we reverse the field, select which ones are re-emitted and which are not. And of course we can then flip the field and run the spins forward again, then backwards and so on. In this way we can store the pulses and re-emit them in any order or sequence.” Dr Buchler says.
The optical table on which the memory runs is a maze of hundreds of components but the basic principle is more ingenious than complex. “Actually the experiment was shockingly simple. We had the idea at our group discussion in the morning and because the lab was largely set up, we were seeing the first results that afternoon.”
These latest experiments are the PhD work of Mahdi Hosseini who began working in the ANU quantum optics group 1 year ago. He expects to push the technique even further in the next two years by investigating the storage of single photons and other quantum states of light.