Frequently Asked Questions (FAQ) about Trapped Ion Quantum Computing

General public Physics students Scientists Physicists

Q: What is an ion?

A: An ion is a single atom that has a net charge.  In our experiments, this happens when the atom loses an electron.  Because the lost electron leaves the nucleus with one un-paired proton, the net charge of the atom becomes positive.  This positively charged atom can now be trapped by electric fields.

Q: How is a quantum computer similar to a classical (standard) computer?

A: At its core, a classical computer (like the one on your desktop) takes inputs and applies logical operations to produce an output. The inputs and outputs are different electronic signals and the operations are called logic gates. A quantum computer does the same thing: it takes quantum inputs and applies a quantum logic gate to produce quantum output.

Q: How can an ion act as a quantum computer?

A: Single atoms and ions obey the rules of quantum mechanics which means that an individual ion is in a quantum state. Using lasers we can manipulate the quantum state of the ion to produce a different state. So, we can use ions as our quantum inputs and outputs and special lasers to produce quantum logic gates for a quantum computer.

Q: Will I ever have a quantum computer on my desktop?

A: That is a hard question to answer right now.  The state of the art in trapped ion quantum computing right now is comparable to the vacuum tube technology in the 1940s for classical computers.  Simple computers used to occupy big rooms, but there was a big breakthrough (the semiconductor transistor) that changed everything for the classical computer and we now have enormous computing power at our fingertips.  Will the same thing happen for the quantum computer?  We have to wait and see.

Q: Why is there an interest in a quantum computer?

A: A large quantum computer could solve some kinds of problems exponentially faster than any possible classical computer. The problem that attracts the most interest is factoring a large number into its composite primes, a problem that is very hard to calculate on a classical computer. Current encryption schemes rely on this difficulty to prevent unauthorized decoding of information. If someone built a large quantum computer they could break today's most commonly used codes.

Q: How do the detection lasers work?

A: The narrow atomic resonance is similar to a driven harmonic oscillator, like a mass on a spring.  In the case where the laser frequency is resonant with the atomic frequency, the atom absorbs and re-emits the laser light very rapidly.  Unlike the spring problem, though, the atom re-emits the laser light in all directions.  We collect a few percent of this light and it tells us that the ion is present.

Q: How do we measure the quantum state of an ion?

A: We use state-dependent fluorescence to measure the quantum state of an ion. If we use either of two isotopes of cadmium ion (111 or 113) then the two states that act as our qubit levels are separated by a large frequency difference (~14.5 GHz). That means that if we shine laser light that is resonant with one of the states the ion will fluoresce a large number of photons whereas the laser will be off-resonant with the other state and so the ion will fluoresce almost no photons. By counting the photons we can easily distinguish between the two quantum states of the ion.

Q: What is the difference between an ion trap and a neutral atom trap?

A:  Ion traps can confine their charged "atoms" by applying either radiofrequency electric fields (rf Paul trap) or a combination of static electric and magnetic fields (Penning trap).  This method has the benefit of leaving the electronic levels undisturbed. On the other hand, the neutral atoms can be confined using their magnetic moment (i.e. spin of the valance electron) in an external magnetic field gradient, strong optical dipole traps (using AC Stark shifts from a focused off-resonant laser beam), and many more trapping schemes that often couple the internal electronic state with an external force.

Q: What is the difference between cold ion and cold neutral atom research?

A: Technology: The laser wavelengths needed to excited the valence electron in most ions are in the ultraviolet range, as opposed to the visible range for neutrals, making laser-manipulation of ions more challenging (and expensive).  Confinement:  Ions are confined much more strongly in their traps than neutral atoms.  Is is nearly impossible to knock a trapped ion out of the trap, unless you kill the power.  Furthermore, ions are routinely confined to a space of under 10 nanometers, allowing great control of its motion.  At the same time, ions do not get close to one another, due to their strong Coulomb repulsion.  Thus, quantum statistical properties (deriving from the indistinguishability of atoms) do not play a significant role in the dynamics of trapped ions, and there is no BEC of trapped ions. Ions and neutrals are somewhat complementary: neutrals come in large numbers and weakly interact with the surroundings (and eachother), while cold ions interact so strongly with one another that they form crystals of typically small numbers of ions.

Q: What is the relevance of heating measurements of the motion of trapped ions?

A: Motional heating of trapped ion motion can affect the fidelity of entangling gates, depending on which particular scheme is used to entangle the ions.  There are several protocols (the Cirac-Zoller and Molmer-Sorensen, for example) that depend on an initially pure quantum state, or at least a state where the ions are localized to much less than the optical wavelength (Lamb-Dicke limit).  In these cases, heating can give gate errors. There are other schemes (such as the Duan or Garcia-Ripoll) that lessen the dependence on motional coherence..

Q: Is heating from the ground state of motion the same thing as decoherence?

A: Yes.  When starting from the ground state of motion, heating to excited states involves a evolution in the vibrational state population.  However, because the initial pure quantum state evolves into a mixed state, where it is not possible to speak of a wavefunction anymore, this is indeed decoherence.  While in the basis of vibrational states there are never any coherences (eg, off-diagonal matrix elements of the density matrix), the fact remains that this is a question of density matrix basis.  In other bases, the initial ground state can be expressed as superpositions of eigenstates -- such as the Gaussian wavepacket in position space, and the same process of incoherent heating will certainly destroy any coherences in this representation.  Decoherence does not depend upon the particular basis chosen to represent the quantum state!