QNM-12: Monday * October 2012
The workshop on quantum noise and measurement we are organizing in Dresden is up and running. Initially I wanted to make a collective blog for all participants but after thinking a while I decided to blog my personal impressions: emphasis is on personal, the account on the talks presented is by no means accurate and was not checked with the speakers as it should be for publication in more representative media. I’ll present my reports with a delay of couple of days.
The workshop is held at Max Planck Institute for Physics of Complex Systems (mpi-pks). Frank Jülicher, managing director of the institute, has welcomed us giving some general info about the place that makes science, runs 15-20 workshops per year and collaborates with other two MP institutes in the city, those are in chemistry and biology.
Wolfgang Belzig, our main organizer, has started the workshop providing necessary technical info.
Markus Buttiker discussed the distribution of waiting times between the events of electron transfers and its unobvious relation to full counting statistics. He discussed it first for simple electron counters and single particle emmiters where it can be easily accessed experimentally. He presented theoretical resuts for distribution of waiting times for the generic case of quantum point contact and denonstrated the crossover from wigner-dyson distribution for high transparencies to poissonian one expected at low transparencies.
Leonid Levitov has actually given two talks in one. In the first part he discussed the noiseless electron sources where a special form of control pulse produces states with “no splash” : electron is transferred without extra electron-hole pairs generated. For a quantum point contact, such pulses are Lorentian.In the second part, he talked about spin qubits and nuclear spin effect in there that determine decoherence of the practical qubits. He concentrated on a long-standing experimental puzzle of ultra-slow oscillations, much slower than heart beats, observed in the qubits but only in the vertical dots. He has developed a minimal model to solve the puzzle. 3 crucial ingredients are: nuclear spin diffusion, nuclear polarisation inside the barrier separating the dots, energy dependence of triplet decay rates.
Robert Schoelkopf shortly reviewed the research on transmon qubits (slightly non-linear josephson junctions) in Yale and summarized new results. Althouth the number of qubits in the device still does not follow Moore’s law as function of time, there is almost exponential progress in combatting decoherence sources: T1, T2 have been improved with 3d cavity resonantors and filterinng techniques to 100 microsec range and are still improving. The main attraction of circuit QED setups is the simplicity, everything is described with Jaynes–Cummings Hamiltonian. Operation in strongly dispersive regime enables QND measurement of photon states in the cativy and provides a toolbox for manipulating light. Robert has concentrated on Kerr effect: frequency shift proportional to the square of photon number. The device has two cavities: one for storage and another for qubit readout. They produce coherent states and measure those doing Wigner tomography. Kerr effect provides a spectacular evolution of the states that is revealed by the measurements. Ideas for future have been discussed: to entangle the qubit and the photons in the cavity.
Jukka Pekola talked about experimental progress in investigating fluctuation relations in driven electron tunneling. He oulined fluctuation theorem that has been investiated recently in ETH with double-dot electron counter and related it to the entropy production. He outlined Jarzynski inequality als well as the problems related to the defintion of work. Experiment was made with a split SE box where charge has been monitored. Jarzynski relation is fullfilled with 3 per cent accuracy, the error is perhaps due to non-equilibrium conditions (difference of phonon and electron temperature). Better accuracy can be achived for the general equality, or integral fluctuation theorem that is known in markovain statistics and is valid for classical systems. What about quantum? There is no consistent definition of work. Rather we will talk about calorimetry quantifying fluctuations of energy flows. The plan is to study a simple two-level system and LZ tunneling. The simulation shows that the calorimetry can work in this setup. The experiment is under way.
Klaus Mølmer gave a rather review-like talk entitled “Quantum state control by quantum measurements or what you see is what you get”. He indicated he belongs to AMO (Atomic molecular optics) community and soon proved that this community is a way more jovile and friendly than condensed matters. His rather traditional introduction of interpretation and collapse of the quantum state he has ended with a dithyrambe to quantum dynamics noting that the dithyrambe also provide a perfect description of his wife (thinking back, I recognized that we have probably married twins). He has attributed the discovery of second quantization to Shakespire who has been constantly talking of “a dagger” and switched to Glauber and Kraus and repetative measurements and heterodyne detection of quantum noise. y the end we all got convinced that measurement provides better entanglement than any interaction: a point that actually never came to my non-AMO mind.
Jan van Ruitenbeek talked about his experiments on inelastic noise in nanowires that he attributed to a two-electron effect. His wires were rather short upon my taste: typically, a two-atom molecule hanging in a gap in a metallic break junction. They have resonant mechanical modes. Jan has demonstrated good characterization and understanding of the processes forming I-V curves in the wires in various conductance regimes, from tunnel to transparent. Most important feature is the threshold of inelastic process at voltage matching the resonant mode frequency. The results on noise in this regime are fresh and intriguing: Jan said they do not match any theory. Since I was also involved with one theory he has mentioned I felt a bit disrupted. Yet the experiment is still rather fresh…
Steven Girvin gave a colloquium talk: a special talk that is intended for a larger MaxPlanck community and should be accessible for biologist. Basically, it was a compherensive introduction to quantum-limited amplifiers starting from the basics of Heisenberg uncertainty. Actually many things and particular details of the Steve’s interpretation looked pretty new and fascinating to me: it’s like as a gifted piano player discovers new sounds in a thousand-times-heard piece of classical musics. Steve has finished the talk with a copy of a postcard to Bohr from Gerlach that reported this famous experiment. He mentioned that the interpretation of the experiment was wrong by the time.
And we got to dinner and poster session.
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