Anti-Zeno purification of spin baths by quantum probe measurements

At a Glance

Metadata	Details
Publication Date	2022-12-06
Journal	Nature Communications
Authors	Durga Bhaktavatsala Rao Dasari, Sen Yang, Arnab Chakrabarti, Amit Finkler, Gershon Kurizki
Institutions	Weizmann Institute of Science, Max Planck Institute for Solid State Research
Citations	15
Analysis	Full AI Review Included

Executive Summary

This research introduces a fundamentally new quantum control paradigm for purifying spin baths (environments) using selective measurements of a coupled probe qubit, achieving persistent low-entropy states critical for quantum technologies.

• Novel Control Paradigm: The method shifts control from the system (qubit) to the environment (spin bath) by utilizing selective (conditional) measurements of the probe.
• Purification Mechanism: Bath purification is achieved by performing a sequence of selective measurements (Conditional Trajectories, CTs) at intervals corresponding to the Anti-Zeno Effect (AZE) regime of the system-bath exchange.
• Coherence Enhancement: The purified bath state dramatically increases the probe qubit’s coherence autocorrelation time, extending its effective lifetime by approximately 10³ times compared to the unpurified bath.
• On-Demand Regime Control: By selecting different CT sequences (e.g., M_0,m vs M_n,m), engineers can enforce, on demand, either Quantum Zeno Effect (QZE, coherence slowdown) or AZE (coherence oscillation) dynamics on the probe qubit.
• Persistence: The low-entropy purified bath state persists long after the measurement sequence is completed, enabling subsequent coherent operations on other qubits immersed in the same bath.
• Experimental System: The protocol was successfully demonstrated using a Nitrogen-Vacancy (NV) center electron spin (probe) coupled to a nuclear spin bath in a diamond crystal at 4.2 K.

Technical Specifications

Parameter	Value	Unit	Context
Experimental Temperature	4.2	K	Continuous-flow liquid helium cryostat operation.
Probe System	Low-strain NV center	Diamond	Qubit (S) coupled to nuclear spin bath (B).
Measurement Interval (τ)	600 or 1200	ns	Intervals chosen to conform to the Anti-Zeno Effect (AZE) regime.
Bare Decoherence Time (T2)	~100	µs	Relevant timescale for determining the AZE regime.
Bath Relaxation Lifetime (T1)	> 10	s	Ultimate limit for the persistence of the purified bath state.
Autocorrelation Improvement	~103	Times	Increase in qubit coherence autocorrelation time after purification (CT M0,4).
Single-Shot Readout (SSRO) Fidelity	≥95	%	Average fidelity achieved during the measurement sequence.
CT Success Probability (M0,4)	~0.1	%	Probability of obtaining the desired sequence of 4 positive outcomes (can be optimized).
AZE Oscillation Frequency (M4,0)	313.5	kHz	Observed frequency of underdamped oscillation in the AZE regime (for τ = 600 ns fit).
Purified Bath Spectrum	Single or Double	Peak	Determined by the choice of Conditional Trajectory (CT).

Key Methodologies

The experiment utilized a low-strain NV center in diamond at cryogenic temperatures (4.2 K) to implement the Anti-Zeno purification protocol:

1. System Preparation:

   • The NV electron spin (probe qubit, S) was initialized to the |0> state via resonant optical pumping (A₁ transition).
   • A microwave transition then prepared the probe in a superposition state (|+) = (|+1> ± |-1>)/√2) for coherence tracking.

2. Conditional Trajectory (CT) Execution:

   • The probe was allowed to acquire phase due to interaction with the nuclear spin bath (B) over a fixed interval τ (600 ns or 1200 ns), chosen to be in the AZE regime (τ < T₂).
   • A projective measurement was performed by mapping the population back to the ancillary state |0> and performing single-shot readout (Ex excitation).

3. Selective Measurement Sequence:

   • The procedure was repeated m times (e.g., m=4) to form a Conditional Trajectory (CT).
   • Only specific sequences of outcomes (e.g., M_0,4, corresponding to four consecutive “positive” outcomes, or M_4,0, corresponding to four consecutive “negative” outcomes) were selected, forcing the spin bath into a low-entropy purified state.

4. Verification of Purification:

   • Coherence Decay (FID): The probe’s Free Induction Decay (FID) was measured before and after purification. A successful M_0,4 CT resulted in a 4-times slower decay rate (QZE regime).
   • Bath Spectrum (ODMR): Optically Detected Magnetic Resonance (ODMR) was used to measure the spin noise spectrum of the probe. Purification resulted in a significantly narrower, well-defined spectral peak, confirming noise reduction in the bath.

5. Persistence Measurement:

   • The lifetime of the purified bath state was measured using the qubit-state autocorrelation function C(t)C(t + t_d), where the waiting time t_d was varied up to the millisecond range.

Commercial Applications

The ability to purify and control the quantum state of a spin environment has profound implications for solid-state quantum technologies:

• Quantum Computing: Provides a mechanism for fault-tolerant quantum computation by protecting computational qubits from environmental decoherence. The persistent nature of the purified bath allows for long-duration coherent operations.
• Solid-State Quantum Memories: The demonstrated 10³-fold increase in coherence autocorrelation time enables the exploitation of spin baths as reliable, long-lived quantum memories, compatible with quantum network operations.
• Quantum Sensing and Metrology: Low-entropy purified spin baths are expected to substantially increase the signal-to-noise ratio of quantum sensors (e.g., NV magnetometers and thermometers), enhancing nanoscale magnetic imaging and chemical resolution.
• Quantum Network Infrastructure: The protocol supports the reliable, long-term storage and high-fidelity transfer of quantum information, which is essential for building distributed quantum networks.

Tech Support

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Original Source

• DOI: https://doi.org/10.1038/s41467-022-35045-3

References

1. 2002 - The Theory of Open Quantum Systems
2. 2022 - Thermodynamics and Control of Open Quantum Systems
3. 2010 - Quantum Computation and Quantum Information

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