# Presentation: 686366

## Probing fundamental physics using a superfluid interferometer

**Richard E. Packard, UC Berkeley, DMR 0244882**

- Matter-wave interferometers can reveal otherwise inaccessible yet fascinating phenomena of the natural world. The fundamental theory of superfluids predicts a linear gradient in a quantum phase proportional to the speed of the flowing liquid. We have recently observed and quantitatively verified this phase gradient for the first time using the superfluid quantum interferometer (shown on the left).

- the phase difference using our quantum interferometer.
- In doing so, we have solidified the conceptual bridge between the quantum mechanical picture of a superfluid (as a macroscopic wavefunction) and the classical hydrodynamic one.

- One of the many strange phenomena exhibited by superfluid helium is its tendency to flow *towards* a source of heat. Using this property, we set up a known superflow in the channel shown to the right and measured

**Notes:**
I. Superfluids:
Superfluidity is a phase of matter characterized by the complete absence of viscosity. An example is helium-4, the most abundant isotope of helium, which becomes superfluid at temperatures below 2.17 K (−270.98 °C or about -456 °F). A superfluid is an example of a macroscopic quantum system wherein almost all the molecules are in the same quantum state. Superfluids exhibit many exotic properties, one of which is the so-called "Josephson Effect". 

II. Josephson effect for Helium 4:
The superfluid Josephson Effect refers to the strange phenomena that occur when two volumes of liquid are connected by a passage whose dimensions are a few tens of nanometers. If a pressure differential is applied across the apertures to force the liquid from one side to the other, the resultant motion is a flow oscillation at a frequency which increases in direct proportion to the applied pressure head. Rather than pass through the aperture like an ordinary fluid, the average superfluid current is zero. This “quantum whistle” was discovered in our laboratory. 

III. Interference:
Each oscillating "weak link" is analogous to a coherent source of light in optics. Just as two coherent light waves characterized by a phase difference interfere with each other to produce an optical interference pattern, superfluid matter-waves  can interfere to produce an acoustic interference pattern that can be detected (and heard) using a microphone. 

IV. Interferometer:
The two interfering waves possess a property called relative phase which determines to what extent the waves are in sync or out of sync with each other. It determines the amplitude (loudness) of the whistle which is simply the amplitude of two waves added together. An instrument that can measure such phase differences is called an interferometer. 

Previously we used this interferometer for a proof-of-principle experiment as a gyroscope to measure of the rotation of the Earth. We now use it to probe fundamental physics; specifically the phase gradient associated with a superfluid flow, which was directly observed for the first time using our interferometer in a recent experiment (Physical Review Letters, May 2007). A heat current set up along a channel leads to  superflow. Connecting the interferometer between two points along the flow channel samples the phase difference between those points.  

V. Flux locking:
Our ability to introduce heat-induced phase differences in the interferometer permits us to “flux lock” our superfluid gyroscope, thus eliminating  nonlinearities and creating a large dynamic range for rotation measurements.

## Superfluid Helium-4 Interferometer

**Richard E. Packard, UC Berkeley, DMR 0244882**

**Education**
- In the past year, two graduate students - Yuki Sato and Aditya Joshi have worked on the basic research supported by this NSF grant.
**More information**
- The group website (continually updated) contains further details presented at the undergraduate level with supporting animations and sound clips.

**Outreach**
- The Principal Investigator and one of the students presented invited lectures around the country and abroad to disseminate the results of our research among a wider audience. These included lectures at Los Alamos, Cornell, Berkeley, AAAS San Francisco, Israel, Turkey and Japan.

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- http://www.physics.berkeley.edu/research/packard/

**Notes:**
Education:
A graduate student researcher in our group is involved with all aspects of his/her experiment, from the development of the experimental apparatus (including design and fabrication) to data acquisition and the analysis and interpretation of the final data. Such training is intended to develop a well rounded scientist who has many of the skills expected in an experimental physicist. 

Group members are encouraged to attend and present results at domestic and international conferences not only to disseminate information but also to develop and nurture professional speaking skills. For instance, one of our graduate students (Yuki Sato) delivered an invited talk describing our work at the International Symposium on Quantum Fluids and Solids held recently (August 2006) in Kyoto, Japan. Such events are a great opportunity to interact with professional colleagues working in similar fields resulting in a vibrant exchange of ideas. 

The PI, Professor Richard Packard delivered invited lectures at several venues in the past year, including the Annual Meeting of the AAAS in San Francisco (February 2007), the SUR (superfluids under rotation) workshop in Jerusalem, Israel (April 2007) and department colloquia at Cornell University, UC Berkeley Los Alamos, and U. of Istanbul 

Practical applications:
The modest requirements for the interferometer (2 K cryogenics and fabrication of apertures at the level of 100nm) and its potential resolution suggest that, when engineering challenges such as vibration isolation are met, superfluid Helium-4 interferometers could be used in a variety of real world milieus:
1)Seismology: transverse waves acquire a rotational component due to anisotropies in the Earth’s crust. At present there is no instrument that can record this motion.
2)Geodesy: determining small changes in the length of day and Earth orientation are required for the GPS system and to understand processes within the deep Earth.
3)Inertial navigation: submarines rely on gyroscopes and accelerometers to determine position. Superfluid gyroscopes may increase precision and lower costs for these instruments.
4)Detection of underground nuclear explosions: rotational seismic signals associated with underground explosions may provide new diagnostic information.

The newly developed flux locking technique brings us ever closer to a practical way of realizing these applications.