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QFarm

QFarm Quantum Seminar Series

Q-Farm Quantum Seminar Series presents Two Speakers

Topic: 
Universal Programmable photonic architecture for quantum information processing; Piezo-optomechanical transduction for microwave to optical quantum frequency conversion
Abstract / Description: 

Universal Programmable photonic architecture for quantum information processing
Ben Bartlett, Stanford University

Photonics provides a range of unique advantages as a platform for quantum information processing, but presents intrinsic challenges which make compact deterministic devices difficult to implement. In this talk, we describe an architecture for a photonic integrated circuit which can be dynamically programmed to implement any quantum operation, in principle deterministically and with perfect fidelity. Our architecture consists of a lattice of beamsplitters and phase shifters, which perform rotations on path-encoded photonic qubits, and embedded quantum emitters, which use a two-photon scattering process to implement two-qubit controlled gates deterministically. We discuss how to program the device and we show how machine learning techniques can be used to automatically implement highly compact approximations to desired quantum circuits.


 

Piezo-optomechanical transduction for microwave to optical quantum frequency conversion
Wentao Jiang, Stanford University

Efficient modulation of the optical properties of a material or a device has long been pursued for both classical and quantum applications. Mechanical deformation strongly perturbs the refractive index compared to other methods and has the potential to be orders of magnitude more energy efficient. In this talk, we compare different approaches for microwave to optical quantum frequency conversion and show how existing optomechanical converters with gigahertz-frequency mechanical mode suffer from some combination of low optical quality factor, low electrical-to-mechanical transduction efficiency, and low optomechanical interaction rate. We consider all these design challenges and demonstrate a piezo-optomechanical converter that combines an efficient piezoelectric transducer with an optimized optomechanical crystal, and improves the conversion efficiency by nearly three orders of magnitude.

Date and Time: 
Wednesday, November 20, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Entanglement phase transition in random unitary circuits with measurements"

Topic: 
Entanglement phase transition in random unitary circuits with measurements
Abstract / Description: 

Parametric resonance of linear harmonic oscillators is a well known phenomenon. Experimental advances in the recent past now make it possible to study parametric phenomena in a wide array of classical and quantum systems. Spanning single to many-body systems, we will show how parametric resonance can be harnessed for novel force sensing applications as well as the realisation of new phases of matter including time crystals.

Date and Time: 
Wednesday, November 6, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Entanglement phase transition in random unitary circuits with measurements"

Topic: 
Entanglement phase transition in random unitary circuits with measurements
Abstract / Description: 

Open quantum systems can display rich dynamics of quantum information: the information may be scrambled within the system, leaked to an environment, or shared between the system and the environment. In this talk, we discuss the dynamics of quantum information in a generic open system, modeled by local random unitary circuits that are interspersed by projective measurements. The interplay between unitary information scrambling and measurements leads to a sharp phase transition: at sufficiently high rates of measurements, any coherent information in the system is completely lost, while at sufficiently low rates, an extensive amount of information is robustly protected and survives --- this is a consequence of the natural quantum error correction enabled by scrambling dynamics. We will elaborate on how these two phases can be characterized from a variety of complementary perspectives based on the average entanglement entropy within the system, the Fisher information in measurement outcomes, and the quantum channel capacity of the open dynamics. Furthermore, we will present an original method to analyze the dynamics of quantum information in the open system by mapping random unitary circuits with measurements into well-known classical statistical mechanics models. This mapping reveals that the phase transition in a certain limit belongs to the universality class of the bond percolation in the 2D square lattice. We will discuss the implications of our results on characterizing near-term quantum devices.

Date and Time: 
Wednesday, November 6, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents "Universality in the dynamics of quantum information"

Topic: 
Universality in the dynamics of quantum information
Abstract / Description: 

The far-from-equilibrium dynamics of closed quantum systems has become a central topic in condensed matter physics, due to incredible experimental advances in cold atomic and other systems. Concepts of quantum information have taken center stage in this context, with entanglement, in particular, playing a central role in the emergence of thermodynamics. Predicting the behavior of these quantities, however, is notoriously hard as most existing analytical and numerical tools, designed for systems in equilibrium, do not carry over to the dynamical setting. In my talk I will discuss how studying a set solvable minimal models has allowed us to partially conquer this problem and uncover universal features of quantum dynamics. In particular, I will describe some simple hydrodynamic models of how quantum information spreads in time, and discuss a recent prediction for entanglement growth that is particularly relevant for cold atom experiments.

Date and Time: 
Wednesday, October 23, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series

Topic: 
TBA
Abstract / Description: 

Anna will give an overview of the process we use to model and design large-scale superconducting quantum circuits at Rigetti. I will introduce the different stages of the iterative design flow focusing on our modeling approach which combines classical lumped-element circuit models with circuit quantization to calculate the lossy eigenmodes of the system. Using this circuit theoretic technique, we studied multiplexed qubit readout schemes for scalable circuit architectures, for example coupling multiple resonators to a single Purcell filter to protect the qubit from radiative decay channels. I will present our findings on loss and mode hybridization as a function of the Purcell filter geometry and coupling capacitances.

Date and Time: 
Wednesday, September 25, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents TWO TOPICS

Topic: 
A quantum annealer with fully programmable all-to-all coupling via Floquet engineering -and- Spin squeezing in free-space atomic sensors
Abstract / Description: 

A quantum annealer with fully programmable all-to-all coupling via Floquet engineering
Peter McMahon

Quantum annealing is a promising approach to heuristically solving difficult combinatorial optimization problems. However, the connectivity limitations in current devices lead to an exponential degradation of performance on general problems. We propose an architecture for a quantum annealer that achieves full connectivity and full programmability while using a number of physical resources only linear in the number of spins. We do so by application of carefully engineered periodic modulations of oscillator-based qubits, resulting in a Floquet Hamiltonian in which all the interactions are tunable; this flexibility comes at a cost of the coupling strengths between spins being smaller than they would be had the spins been directly coupled. Our proposal is well-suited to implementation with superconducting circuits, and we give analytical and numerical evidence that fully-connected, fully-programmable quantum annealers with 1000 qubits could be constructed with Josephson parametric oscillators having coherence times of 500 microseconds, and other system-parameter values that are routinely achieved with current technology. Our approach could also have impact beyond quantum annealing, since it readily extends to bosonic quantum simulators and would allow the study of models with arbitrary connectivity between lattice sites. Describes work performed jointly with Tatsuhiro Onodera and Edwin Ng.


Spin squeezing in free-space atomic sensors
Zheng Cui

The compatibility of cavity-generated spin-squeezed atomic states with atom-interferometric sensors that require freely falling atoms is demonstrated. An ensemble of hundreds of thousands of spin-squeezed atoms in a high-finesse optical cavity with near-uniform atom-cavity coupling is prepared, released into free space, and measured using one of two different methods. The first method consists of recapturing the atoms in the cavity and probing them with the same QND measurement used to generate the initial entanglement among the atoms. Up to 9.8^{+0.5}_{-0.4} dB of metrologically-relevant squeezing is retrieved for few-hundred microseconds free-fall times, and decaying levels of squeezing are mapped out up to 3 milliseconds free-fall times. This protocol suffers of atom loss and atom-cavity coupling inhomogeneity after recapture. Fluorescence population spectroscopy is an alternative method when longer free-falls times are required. This method allows for the atom ensemble to free fall for up to 4 milliseconds without significant loss of squeezing nor quantum coherence. When operating as a microwave atomic clock with a 3.6 millisecond Ramsey time, a single-shot fractional frequency stability of 8.4(0.2)x10^{-12} is reported, 4.1(0.2) decibels below the quantum projection limit. The ability of the clock to utilize the maximum squeezing available is limited by microwave amplitude and phase noise, and external magnetic field fluctuations in the system. Free fall times of up to 8 milliseconds are also achieved, but at a loss of state coherence.

Date and Time: 
Wednesday, June 19, 2019 - 12:00pm
Venue: 
Y2E2 300

Q-Farm Quantum Seminar Series presents Entanglement Wedge Reconstruction and the Information Paradox - & - Large Momentum Transfer Clock Atom Interferometry

Topic: 
Entanglement Wedge Reconstruction and the Information Paradox | Large Momentum Transfer Clock Atom Interferometry
Abstract / Description: 

The Q-FARM seminar series is designed to bring together the various groups in the university interested in quantum science and engineering. Seminars take place every other Wednesday at lunch time, with two speakers per session giving short talks on their research. The primary goal of these seminars is to strengthen the community and increase collaboration. Theoretical and experimental talks are balanced so that the whole community may participate.


Entanglement Wedge Reconstruction and the Information Paradox, Geoff Penington

I will review the ideas of entanglement wedge reconstruction and holographic quantum error correction in AdS/CFT and show how they can be used to resolve the black hole information paradox.

Large Momentum Transfer Clock Atom Interferometry, TJ Wilkason

Atom interferometry is a promising candidate for future precision sensors with a diverse science impact, ranging from laboratory tests of general relativity and the equivalence principle, to searches for dark matter and detection of gravitational waves. I will present efforts towards increasing the sensitivity of atom interferometers through the use of narrow-line clock transitions in alkaline earth atoms such as strontium. These narrow-line transitions promise to circumvent existing constraints encountered in conventional atom interferometers, enabling increased space-time area through large momentum transfer (LMT) atom optics and reducing the sensitivity to laser phase noise. I will show recent results on LMT interferometry on the 689nm intercombination line of strontium, which demonstrate for the first time an LMT interferometer based on sequential single-photon transitions, a critical requirement for gravitational wave detection using clock atoms. We study spontaneous emission and other losses at large pulse area for this new type of atom interferometer and characterize the rejection of common laser phase noise in a gradiometer configuration.

Date and Time: 
Wednesday, May 22, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

Q-Farm Quantum Seminar Series presents Two Talks

Topic: 
Noise-resilient quantum circuits - AND - Spin squeezing in free-space atomic sensors
Abstract / Description: 

A noisy quantum computer can simulate any noiseless quantum computation with an overhead that scales very modestly with the size of the computation, in the asymptotic limit in which the size of the computation becomes large. However, the overhead in practice is still too large even for state-of-the-art quantum computing devices. In order to circumvent this problem, we propose a specialized algorithm that remains robust in the presence of error even without error correction. Even though the size of the circuit increases with the problem size, the accumulated error on the answer is stabilized at a level comparable to the physical noise rate. This is possible because the errors introduced at earlier times are judiciously diluted more by the design of the circuit. This algorithm can dramatically speed up the existing (classical) computational methods to study strongly interacting quantum many-body systems. One may thus optimistically hope to accurately predict the properties of such systems with a noisy quantum computer, provided that the noise rate in these devices continues to get lower.

- and - 

The compatibility of cavity-generated spin-squeezed atomic states with atom-interferometric sensors that require freely falling atoms is demonstrated. An ensemble of hundreds of thousands of spin-squeezed atoms in a high-finesse optical cavity with near-uniform atom-cavity coupling is prepared, released into free space, and measured using one of two different methods. The first method consists of recapturing the atoms in the cavity and probing them with the same QND measurement used to generate the initial entanglement among the atoms. Up to 9.8^{+0.5}_{-0.4} dB of metrologically-relevant squeezing is retrieved for few-hundred microseconds free-fall times, and decaying levels of squeezing are mapped out up to 3 milliseconds free-fall times. This protocol suffers of atom loss and atom-cavity coupling inhomogeneity after recapture. Fluorescence population spectroscopy is an alternative method when longer free-falls times are required. This method allows for the atom ensemble to free fall for up to 4 milliseconds without significant loss of squeezing nor quantum coherence. When operating as a microwave atomic clock with a 3.6 millisecond Ramsey time, a single-shot fractional frequency stability of 8.4(0.2)x10^{-12} is reported, 4.1(0.2) decibels below the quantum projection limit. The ability of the clock to utilize the maximum squeezing available is limited by microwave amplitude and phase noise, and external magnetic field fluctuations in the system. Free fall times of up to 8 milliseconds are also achieved, but at a loss of state coherence.

Date and Time: 
Wednesday, May 8, 2019 - 12:00pm
Venue: 
Physics/Astrophysics (Varian II) Building, Room 102/103

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