Events at Physics |
Events on Friday, May 16th, 2025
- Thesis Defense
- Noise characterization for Spin and Charge qubits
- Time: 10:00 am - 12:00 pm
- Place: 5280 CH
- Speaker: Deepak Mallubhotla, Physics PhD student
- Abstract: Quantum computing offers substantial advantages over classical computers for certain problems, but a fundamental challenge persists: qubits suffer from environmental noise causing decoherence. To achieve scales required for practical applications, error correction techniques must overcome these noise limitations. Studying various noise types is essential for developing effective mitigation strategies.
This dissertation first analyzes evanescent-wave Johnson noise (EWJN) near small metallic devices. Caused by thermal and quantum charge motion in conductors, this noise creates field fluctuations beyond the conductor's surface. Noise correlations B(x, t) B(x', t') are calculated when device size is smaller than material skin depth, yielding closed-form solutions via multipole expansion.
Next, EWJN is examined near BCS superconductors using a half-space geometry where superconductor dimensions exceed qubit distance. Superconductors generate less noise than normal conductors at temperatures well below critical temperature, for both magnetic and electric fields. A Hebel-Slichter peak with enhanced noise appears just below critical temperature, dependent on qubit orientation.
Finally, this dissertation discusses 1/f charge noise.
It has been hypothesized that this noise is caused by fluctuating two-level systems (TLSs). We show that measurements of noise power spectral density do not fully determine TLS parameters exactly, and present a Bayesian technique of assigning likelihoods to different parameter ranges instead. This allows for partial, statistical information to be extracted, giving predictions of TLS size and density. Two recent measurements are analyzed, giving predictions consistent with both each other and with measurements in the literature obtained through other techniques. - Host: Maxim Vavilov/Bob Joynt
- Theory Seminar (High Energy/Cosmology)
- Quantum Entanglement Theory and Its Generic Searches at the Colliders
- Time: 1:30 pm - 3:00 pm
- Place: Sterling B343
- Speaker: Tianjun Li, Beijing, Inst. Theor. Phys.
- Abstract: We propose a new formalism for quantum entanglement, and study its generic searches at the colliders. For a general quantum system with N particles, we show that the quantum space (the total spin polarization parameter space) is complex projective space, and the classical space (the spin polarization parameter space for classical theory) is the cartesian product of the complex projective spaces. Thus, the quantum entanglement space is the difference of these two spaces. For the ff, AA, Af, fff, and ffA systems, we calculate their discrimants \Delta_i. The corresponding classical spaces are the discrimant locus \Delta=0 for ff system, and intersections of the discriminant loci \Delta_i=0 for AA, Af, fff, and ffA systems in the quantum space. We show that our criterion \Delta\not=0 is equivalent to the original Peres-Horodecki criterion for ff system. At the colliders, we can reconstruct the discriminants from various measurements, and probe the quantum entanglement spaces at exact level. This provides a fundamental approach to test the quantum entanglement. In addition, for the specific approach, we propose a generic method to calculate the quantum range and classical range for the expectation value of any physics observable, and can probe the quantum entanglement spaces which the previous way cannot test for some cases. Furthermore, we define the quantum non-locality tests as the tests for quantum entanglement space via the space-like separated measurements, which can be done at colliders as well. Event recording:
- Host: Vernon Barger