Tag Archives: time

Quantum computer as detector shows space is not squeezed

 Robert Sanders


Ever since Einstein proposed his special theory of relativity in 1905, physics and cosmology have been based on the assumption that space looks the same in all directions – that it’s not squeezed in one direction relative to another.

A new experiment by UC Berkeley physicists used partially entangled atoms — identical to the qubits in a quantum computer — to demonstrate more precisely than ever before that this is true, to one part in a billion billion.

The classic experiment that inspired Albert Einstein was performed in Cleveland by Albert Michelson and Edward Morley in 1887 and disproved the existence of an “ether” permeating space through which light was thought to move like a wave through water. What it also proved, said Hartmut Häffner, a UC Berkeley assistant professor of physics, is that space is isotropic and that light travels at the same speed up, down and sideways.

“Michelson and Morley proved that space is not squeezed,” Häffner said. “This isotropy is fundamental to all physics, including the Standard Model of physics. If you take away isotropy, the whole Standard Model will collapse. That is why people are interested in testing this.”

The Standard Model of particle physics describes how all fundamental particles interact, and requires that all particles and fields be invariant under Lorentz transformations, and in particular that they behave the same no matter what direction they move.

Häffner and his team conducted an experiment analogous to the Michelson-Morley experiment, but with electrons instead of photons of light. In a vacuum chamber he and his colleagues isolated two calcium ions, partially entangled them as in a quantum computer, and then monitored the electron energies in the ions as Earth rotated over 24 hours.

As the Earth rotates every 24 hours, the orientation of the ions in the quantum computer/detector changes with respect to the Sun’s rest frame. If space were squeezed in one direction and not another, the energies of the electrons in the ions would have shifted with a 12-hour period. (Hartmut Haeffner image)
As the Earth rotates every 24 hours, the orientation of the ions in the quantum computer/detector changes with respect to the Sun’s rest frame. If space were squeezed in one direction and not another, the energies of the electrons in the ions would have shifted with a 12-hour period. (Hartmut Haeffner image)

If space were squeezed in one or more directions, the energy of the electrons would change with a 12-hour period. It didn’t, showing that space is in fact isotropic to one part in a billion billion (1018), 100 times better than previous experiments involving electrons, and five times better than experiments like Michelson and Morley’s that used light.

The results disprove at least one theory that extends the Standard Model by assuming some anisotropy of space, he said.

Häffner and his colleagues, including former graduate student Thaned Pruttivarasin, now at the Quantum Metrology Laboratory in Saitama, Japan, will report their findings in the Jan. 29 issue of the journal Nature.

Entangled qubits

Häffner came up with the idea of using entangled ions to test the isotropy of space while building quantum computers, which involve using ionized atoms as quantum bits, or qubits, entangling their electron wave functions, and forcing them to evolve to do calculations not possible with today’s digital computers. It occurred to him that two entangled qubits could serve as sensitive detectors of slight disturbances in space.

“I wanted to do the experiment because I thought it was elegant and that it would be a cool thing to apply our quantum computers to a completely different field of physics,” he said. “But I didn’t think we would be competitive with experiments being performed by people working in this field. That was completely out of the blue.”

He hopes to make more sensitive quantum computer detectors using other ions, such as ytterbium, to gain another 10,000-fold increase in the precision measurement of Lorentz symmetry. He is also exploring with colleagues future experiments to detect the spatial distortions caused by the effects of dark matter particles, which are a complete mystery despite comprising 27 percent of the mass of the universe.

“For the first time we have used tools from quantum information to perform a test of fundamental symmetries, that is, we engineered a quantum state which is immune to the prevalent noise but sensitive to the Lorentz-violating effects,” Häffner said. “We were surprised the experiment just worked, and now we have a fantastic new method at hand which can be used to make very precise measurements of perturbations of space.”

Other co-authors are UC Berkeley graduate student Michael Ramm, former UC Berkeley postdoc Michael Hohensee of Lawrence Livermore National Laboratory, and colleagues from the University of Delaware and Maryland and institutions in Russia. The work was supported by the National Science Foundation.

Source: UC Berkeley

Clocks are essential for us to keep track of our time. In the image, spectators gather on the grounds in front of the countdown clock during a space shuttle launch. Credit: NASA

Atomic timekeeping, on the go

New approach may enable more stable and accurate portable atomic clocks.

By Jennifer Chu


CAMBRIDGE, Mass. – What time is it? The answer, no matter what your initial reference may be — a wristwatch, a smartphone, or an alarm clock — will always trace back to the atomic clock.

Clocks are essential for us to keep track of our time. In the image, spectators gather on the grounds in front of the countdown clock during a space shuttle launch. Credit: NASA
Clocks are essential for us to keep track of our time. In the image, spectators gather on the grounds in front of the countdown clock during a space shuttle launch. Credit: NASA


The international standard for time is set by atomic clocks — room-sized apparatuses that keep time by measuring the natural vibration of atoms in a vacuum. The frequency of atomic vibrations determines the length of one second — information that is beamed up to GPS satellites, which stream the data to ground receivers all over the world, synchronizing cellular and cable networks, power grids, and other distributed systems.


Now a group at MIT and Draper Laboratory has come up with a new approach to atomic timekeeping that may enable more stable and accurate portable atomic clocks, potentially the size of a Rubik’s cube. The group has outlined its approach in the journal Physical Review A.


While chip-sized atomic clocks (CSACs) are commercially available, the researchers say these low-power devices — about the size of a matchbox — drift over time, and are less accurate than fountain clocks, the much larger atomic clocks that set the world’s standard. However, while fountain clocks are the most precise timekeepers, they can’t be made portable without losing stability.


“You could put one in a pickup truck or a trailer and drive it around with you, but I’m guessing it won’t deal very well with the bumps on the road,” says co-author Krish Kotru, a graduate student in MIT’s Department of Aeronautics and Astronautics, and a Draper Lab Fellow. “We have a path toward making a compact, robust clock that’s better than CSACs by a couple of orders of magnitude, and more stable over longer periods of time.”


Kotru says such portable, stable atomic clocks could be useful in environments where GPS signals can get lost, such as underwater or indoors, as well as in militarily “hostile environments,” where signal jamming can block traditional navigation systems.


Co-authors of the paper include Justin Brown, David Butts, Joseph Kinast, and Richard Stoner of Draper Laboratory.


A shift in time


The team came up with the new atomic timekeeping approach by making several “tweaks” to the standard method.


The most accurate atomic clocks today use cesium atoms as a reference. Like all atoms, the cesium atom has a signature frequency, or resonance, at which it oscillates. Since the 1960s, one second has been defined as 9,192,631,770 oscillations of a cesium atom between two energy levels. To measure this frequency, fountain clocks toss small clouds of slow-moving cesium atoms a few feet high, much like a pulsed fountain, and measure their oscillations as they pass up, and then down, through a microwave beam.


Instead of a microwave beam, the group chose to probe the atom’s oscillations using laser beams, which are easier to control spatially and require less space — a quality that help in shrinking atomic clock apparatuses. While some atomic clocks also employ laser beams, they often suffer from an effect called “AC Stark shift,” in which exposure to an electric field, such as that produced by a laser, can shift an atom’s resonant frequency. This shift can throw off the accuracy of atomic clocks.


“That’s really bad, because we’re trusting the atomic reference,” Kotru says. “If that’s somehow perturbed, I don’t know if my low-quality wristwatch is wrong, or if the atoms are actually wrong.”


To avoid this problem, most standard fountain clocks use microwave beams instead of lasers. However, Kotru and his team looked for ways to use laser beams while avoiding AC Stark shift.


Keeping time, in miniature


In laser-based atomic clocks, the laser beam is delivered at a fixed frequency and intensity. Kotru’s team instead tried a more varied approach, called Raman adiabatic rapid passage, applying laser pulses of changing intensity and frequency — a technique that is also used in nuclear magnetic resonance spectroscopy to probe features in individual molecules.


“For our approach, we turn on the laser pulse and modulate its intensity, gradually turning it on and then off, and we take the frequency of the laser and sweep it over a narrow range,” Kotru explains. “Just by doing those two things, you become a lot less sensitive to these systematic effects like the Stark shift.”


In fact, the group found that the new timekeeping system suppressed the AC Stark shift by a factor of 100, compared with a conventional laser-based system. Unlike fountain clocks, which shoot atoms more than a meter upwards in order to measure a single second, the team’s apparatus measures time in intervals of 10 milliseconds — an approach that is less accurate than fountain clocks, but much more compact.


“That’s fine, because we’re not trying to make the world’s standard — we’re trying to make something that would fit in, say, a Rubik’s cube, and be stable over a day or a week,” Kotru says.


The stability and accuracy of the system, he says, should be comparable to that of microwave-based atomic clocks on today’s GPS satellites, which are bulky and expensive.


Going a step further, the team tested the system’s response to physical forces. “Let’s say one day we got it small enough so you could put it in your backpack, or in your vehicle,” Kotru says. “Having it be able to operate while you’re moving across the ground is important.”


Just short of physically shaking the system, the group “created a displacement between the atoms and the laser beam,” moving the laser beam from side to side as it probed the cloud of atoms. Even under such simulated shaking, the system was able to measure the atoms’ resonant frequency, with a high degree of sensitivity.


The team is now working to reduce the size of other components of the system, including the vacuum chamber and electronics.


“Additional miniaturization could ultimately result in a handheld device with stability orders of magnitude better than compact atomic clocks available today,” Kotru says. “Such a device would satisfy requirements for more technologically intensive applications, like the synchronization of telecommunications networks.”


This research was sponsored by Draper Laboratory.

Source: MIT News Office