Latest News
Benefits of Frustration
Scientists have demonstrated a new way to control quantum interactions that makes it possible to fine-tune the way in which the magnetic properties of trapped atoms couple to, and are "entangled" with, those of their neighbors -- a development with potentially important applications in quantum computing and condensed-matter simulations.
Spin Frustration in Three Ions
Spin Frustration in Three IonsThe ability to selectively control the quantum states of a large array of objects is a necessary prerequisite to practical quantum information-processing, just as the ability to control connections among groups of transistors is at the heart of conventional electronics. However, any eventual quantum computer will almost certainly rely on a phenomenon unknown to classical physics: entanglement, the condition in which the states of two or more objects become so inextricably connected that none of them can be described separately.
Trapped ions are considered to be one of the most promising platforms for inducing, sustaining and managing entanglement among numerous objects. Now physicists at the Joint Quantum Institute and the University of Michigan have devised a way* to entangle the spin states of three adjacent ions and to control the nature of the spin couplings.
"Spin" is a property of atoms and subatomic particles that causes them to behave like minuscule bar magnets, with a polarity conventionally described as either "up" or "down." Thus correlating the spins of two confined ions is relatively straightforward: Both spins can be up, both can be down, or one can be up and one down; and each of those conditions entails a slightly different energy level.
The case of three ions, however, poses a challenging problem because there are three competing interactions (between Ions 1 and 2, Ions 2 and 3, and Ions 1 and 3) which produce four possible outcomes: all spins up, all spins down, or two in one direction and one in the opposite direction. It is this last possibility that is problematic. In general, two of the three ions will find it energetically favorable to "anti-align" -- that is, to end up with their spins in opposite directions like two bar magnets laid side-by-side with the north pole of one next to the south pole of the other. But in that situation, the third ion has no preferred orientation. As a result, the entire system is caught between two equally energetically unfavorable alternatives, a condition called "frustration."
The very uncertainty of that condition, however, makes it potentially exploitable as a means of creating entanglement among groups of ions and even ensembles of groups.
The JQI team set out to understand and control the behavior of such competing magnetic interactions by trapping ytterbium ions in a vacuum chamber with electric fields generated from nearby electrodes. The ions come to rest and form a linear chain with 2 to 5 micrometers space between each ion. The researchers then direct two laser beams at the chain from opposite sides of the chamber. Each ion responds to the laser beam in a way that depends on its spin orientation, oscillating very slightly in a direction perpendicular to the axis of the ion chain. This "transverse" motion changes the relative positions of the ions and provides a force between them through their electrical repulsion. As a result, the spin properties become strongly correlated through the transverse mode of motion, which is controllable by tuning the laser.
"All previous experiments with trapped ions have used axial modes, which is sort of like compressive motion of a bunch of masses connected by springs," says JQI Fellow and group leader Chris Monroe. "Think of the way a Slinky toy gets longer and shorter as you let it hang from your hand and then move it up and down. But the problem with axial modes is that their frequencies are all over the map. It's very hard to deal with them all. Sort of like hitting a drum -- no nice tone, but a bunch of chaotic notes that don't sound well together.
"The transverse modes, however, are like modes of a violin string, with all modes at nearly the same frequency. A beautiful tone indeed. This is important because in order to do our experiment eventually with 100 or 1000 ions, we don't want to deal with all of those dissonant modes and instead want them to be bunched up and happy together. If you add 1000 violinists to the orchestra, it's a lot louder but still sounds good. But put 1000 unruly teenagers yapping on their cell phones into a room, and it's cacophony."
After the team determined that the simple tuning of the laser frequency and its application time could cause the ions to move in ways that were dependent on -- but also affected -- the way the ions' spins were coupled, the next question was: Could they control it? They found that by carefully correlating the laser forces with spin configuration, they were able to identify which beam properties and time intervals produced a given pattern of spin alignments, and how to tune the beam to produce a desired condition.
That is equivalent to the kind of control provided by sequences of logic gates in electronic circuits, but with the added feature of entanglement among the three ions.
"The same system is scalable to lots more particles," Monroe says. "It should be ideally suited to quantum simulation of magnetism, a task at which classical computers fall flat."
Diamond Sparkle as Quantum Information
Diamond Sparkle as Quantum Information
College Park, MD, Sept. 10, 2009
Diagram of Diamond NV ExperimentCredit: JQI
The work,* by scientists at Harvard, JQI, Texas A&M, and MIT, including JQI Fellow Jake Taylor, involves a commonly occurring but extraordinarily useful defect found in diamond, known as a nitrogen-vacancy (NV) center.
Perfect diamond consists of a famously rigid lattice of carbon atoms. But sometimes an impurity occurs in which an atom of nitrogen (carbon's immediate neighbor on the periodic table) takes the place of one carbon atom. That substitution creates a tiny region in the lattice that energetically favors the presence of a "vacancy" - the absence of one carbon atom. The combined nitrogen atom and its adjacent vacancy make up the NV center.
These centers are of intense interest to quantum science because of their peculiar optical characteristics. When excited by light, electrons in the NV center fluoresce strongly, helping give diamond its "sparkle." More importantly, the wavelength and intensity of the emitted photons can vary substantially depending on the spin state of the electrons; and that state, in turn, depends critically on the presence of magnetic or electric fields, or on electromagnetic radiation applied to the site. This phenomenon makes NV centers exquisitely sensitive detectors of magnetic fields on a nanometer scale.
"The spin-dependent fluorescence of NV centers allows us to measure the local magnetic field, which gives unprecedented spatial resolution and high sensitivity, including the measurement of individual nuclear spins," says Taylor, who is now at the National Institute of Standards and Technology. "That is a crucial tool for understanding molecular dynamics and of particular interest in biological systems."
In addition, the electrons in NV centers have two advantages that make them ideal for use as qubits - the quantum equivalent of the "bits" in conventional computers. They retain their spin states for comparatively long time spans, and the states can be manipulated rapidly by microwave radiation.
Once the electron spins are placed in any desired state, they are irradiated by green laser light, and the nature of the emitted photons reveals the electron state. However, there is a major problem in taking repeated measurements: The laser light frequently "resets" the electron states before photons can be measured. As a result, there is large uncertainty in the electron spin state.
The scientists, who publish their results in the Sept. 10 on-line advance publication site of the journal Science (Science Express) were able to circumvent this problem by using microwave pulses to couple the electron spins to the nuclear spin of a nearby carbon-13 atom. The nuclear spin state is not affected by the 532 nm laser beam that is used to read out the electron spin state, and so it serves as a sort of memory device.
Of course, even this relatively long-persisting nuclear information will ultimately fade as the nucleus reverts to its ground state. So the team went one step further, linking the 13C nuclear spin to that of a yet another carbon-13 atom in the vicinity. This coupling is equivalent to the action of switches or "gates" in electronic logic circuits whereby each gate performs a specific operation on a unit of information. If the configuration is right, one can work backwards from the final state of the information to recover its original state. The corresponding linking of two atomic spins in diamond thus provides added memory stability by making the first atom's state recoverable by examining the second.
"With ideas from quantum information science, we are able to improve the collection of light by mapping the electron spin information into nuclear spin, then using the nuclear spin as a 'control' bit on the electron spin, and measuring the electron spin many times," Taylor says.
"Moreover, this example of a small quantum circuit also displays the potential for NV centers to serve as the basis for a room-temperature quantum computing device. In particular, by demonstrating good control and measurement of three quantum bits - that is, the electron state and two nuclear states - we have gotten much closer to the 'single-shot' read-out regime. At that point, we will eventually be able to deterministically measure a single electron spin, using the light emitted by the NV center."
* "Repetitive readout of a single electronic spin via quantum logic with nuclear spin ancillae," L. Jiang, J.S. Hodges, J.R. Maze, P. Maurer, J.M. Taylor, D.G. Cory, P.R Hemmer, R.L. Wadsworth, A. Yacoby, A.S. Zibrov, M.D. Lukin. Published on-line in Science Express at http://www.sciencemag.org/cgi/content/abstract/1176496.
More Articles...
Page 2 of 22
